1. 1 Introduction
  2. 2 Privacy Enhancing Live Distribution Specification
    1. 2.1 Intent
      1. 2.1.1 Privacy on the Internet
      2. 2.1.2 Protection from data recovery after shutdown
      3. 2.1.3 Working on sensitive documents
      4. 2.1.4 Portability
      5. 2.1.5 Target user
      6. 2.1.6 Filling some empty room in the Tor distributions landscape
      7. 2.1.7 Summary
    2. 2.2 Threat model
      1. 2.2.1 The goal of the attacker
      2. 2.2.2 Capabilities, methods and other means of the attacker
    3. 2.3 Distribution
    4. 2.4 Operational requirements
      1. 2.4.1 Platform
      2. 2.4.2 Media
      3. 2.4.3 Virtual machines
    5. 2.5 Other considerations
      1. 2.5.1 Maintainability
      2. 2.5.2 Sustainability
      3. 2.5.3 Open-source transparency, easing peer review
      4. 2.5.4 Easy feedback
    6. 2.6 Implementation requirements
      1. 2.6.1 Kernel requirements
      2. 2.6.2 Network requirements
      3. 2.6.3 User interface and applications
      4. 2.6.4 Usability
    7. 2.7 The future
      1. 2.7.1 Memory recovery attacks
      2. 2.7.2 HTTP keepalive
      3. 2.7.3 Mounting of filesystems stored on removable devices
      4. 2.7.4 Miscellaneous
  3. 3 Implementation
    1. 3.0 Other Tails design documents
    2. 3.1 Download
    3. 3.2 Software
      1. 3.2.1 Core software
      2. 3.2.2 Other applications
    4. 3.3 Localization
      1. 3.3.1 Best supported (Tier-1) languages
      2. 3.3.2 Locales and spell checking
      3. 3.3.3 Input methods
    5. 3.4 Notification of security issues and new Tails releases
    6. 3.5 Feedback
    7. 3.6 Configuration
      1. 3.6.1 The Tor® software
      2. 3.6.2 IPv6
      3. 3.6.3 DNS
      4. 3.6.4 HTTP Proxy
      5. 3.6.5 SOCKS libraries
      6. 3.6.6 Network Filter
      7. 3.6.7 MAC address spoofing
      8. 3.6.8 Host system swap
      9. 3.6.9 Host system RAM
      10. 3.6.10 Host system disks and partitions
      11. 3.6.11 Filesystems stored on removable devices
      12. 3.6.12 Passwords
      13. 3.6.13 Tor Browser
      14. 3.6.14 Thunderbird
      15. 3.6.15 Pidgin
      16. 3.6.16 GnuPG
      17. 3.6.17 Persistent Storage
      18. 3.6.18 Installation on USB sticks and SD cards
      19. 3.6.19 Wireless devices handling
      20. 3.6.20 OpenSSH
      21. 3.6.21 Automatic upgrades
      22. 3.6.22 Panel applets and GNOME Shell extensions
      23. 3.6.23 DHCP hostname leaks
      24. 3.6.25 Application isolation
      25. 3.6.26 wget
      26. 3.6.27 APT
      27. 3.6.28 Electrum
      28. 3.6.29 Kernel hardening
      29. init_on_free=1
      30. slab_nomerge
      31. slub_debug=FZ
      32. vsyscall=none
      33. mce=0
      34. page_alloc.shuffle=1
      35. kASLR
      36. kernel.kexec_load_disabled = 1
      37. mds=full,nosmt
      38. randomize_kstack_offset=on
      39. net.core.bpf_jit_harden = 2
      40. spec_store_bypass_disable=on
      41. 3.6.30 Time zone
    8. 3.7 Running Tails in virtual machines
      1. 3.7.1 Current support
      2. 3.7.2 Security concerns
      3. 3.7.3 Running Tails inside a Windows session
    9. 3.8 Build process and maintenance
      1. 3.8.1 Build tools
      2. 3.8.2 Testing process
      3. 3.8.3 Upgrades
    10. 3.9 Hardware support
    11. 3.10 Caveats
    12. 3.11 Fingerprint
  4. 5 Bibliography

1 Introduction

In this document we present a specification of a Privacy Enhancing Live Distribution (PELD) as well as an actual implementation of it called Tails.

By writing this document we intend to help third-parties do security analyses of any given PELD and specifically of Tails. We also wish to help establish best practices in the field of PELD design and implementation, and thus raise the baseline for all similar projects out there.

This document is heavily based on preliminary work that was done as part of Incognito 2008.1-r1 Documentation. The Bibliography section has pointers to other inspiration and reference sources.

2 Privacy Enhancing Live Distribution Specification

Note: the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

2.1 Intent

Let us introduce what the PELD goals are: the features provided by the PELD, the kind of user the PELD targets, and the (empty) room we think the PELD fills in the Tor distributions landscape.

2.1.1 Privacy on the Internet

The Privacy Enhancing Live Distribution (or PELD for short) aims at providing a software solution providing the user with the technological means for using popular Internet technologies while maintaining their privacy, in particular with respect to anonymity. While there are different techniques and services providing that functionality, this specification will assume the usage of The Tor® Project's state-of-the-art anonymizing overlay network Tor.

Due to its deep dependency on Tor, the PELD defers the same possible goals as Tor:

  • The PELD does not try to conceal it is connected to the Tor network unless Tor bridge relays are used.

  • The PELD is not secure against end-to-end attacks, such as end-to-end timing or intersection attacks.

Moreover, the PELD is likely to be affected by any feasible attack against Tor; e.g. the PELD is not secure against some local attacks, such as confirmation attacks based on website fingerprinting: see Lexi and Dominik's Contemporary Profiling of Web Users conference at 27C3 for details.

2.1.2 Protection from data recovery after shutdown

The PELD aims at protecting the user from post-mortem analysis of the equipment (notably storage media and memory) he or she runs the PELD on. It is impossible for such a system to determine which information is sensitive and which is not. Thus, the PELD MUST be amnesic by default:

  • It is REQUIRED no trace is left on local storage devices unless the user explicitly asks for it: the PELD MUST take care not to use any filesystem or swap volume that might exist on the host machine hard drives.

  • The usage of encrypted removable storage devices (such as USB sticks) SHOULD be encouraged.

  • Volatile memory MUST be erased on shutdown to prevent memory recovery such as cold boot attack.

  • Secure erasure of files and free disk space SHOULD be made easy.

2.1.3 Working on sensitive documents

The PELD aims at providing a "safe" environment to produce and optionally publish sensitive documents. While the combination of anonymous access to the Internet and resistance against future equipment analysis does most of the job, some application-level attacks deserve special treatment: e.g. tools needed to inspect and cleanup metadata — such as EXIF data — in files SHOULD be available.

2.1.4 Portability

The PELD MUST be self-contained and portable (literally, not necessarily with respect to code portability), and thus possible to run in as many computing environments as possible from the same single distribution. In addition, while the PELD's main objective indeed is to act as a traditional Live Distribution (i.e. a Live CD or Live USB) it SHOULD also be compatible with popular virtual machine technologies for users who simply want a sandboxed environment within their usual operating system.

2.1.5 Target user

The PELD's target user is the average user in terms of computer literacy; he or she does not necessarily control fully the computer being used. Examples would be a public computer in a library, coffee shop, university or a residence. We assume that the target user does not want to do any configuration (at least with respect to security and anonymity) of the various applications and tools used themselves, either because of insufficient knowledge, lack of interest or other reasons. The PELD MUST provide strong anonymity with no need of advanced configuration whatsoever. It MUST be made as difficult as possible for the user to unknowingly compromise anonymity.

2.1.6 Filling some empty room in the Tor distributions landscape

XXX: update

The PELD is meant to be complementary to other Tor distributions. It has no such goal as replacing other existing tool that properly fulfills its use cases.

The PELD fills an empty place alongside of other Tor distributions. Using the PELD certainly requires the user to change more of his or her habits than installing e.g. the Tor Browser Bundle. On the other hand, the PELD currently is the only tool that makes it feasible to use the Internet, and more generally computers, for certain activities in contexts where the user cannot afford the risks involved by other Tor distributions.

On the online privacy side of things, the PELD is aimed at offering roughly the same protection level as e.g. the Tor Browser Bundle, but provides a full-blown Tor-ified operating system instead of a few selected and carefully configured applications; this e.g. allows to safely download and open files using external applications, as mentioned by the Torbutton warning popup when a user attempts such an operation outside the PELD.

About protection from post-shutdown data recovery, thanks to its amnesic-by-default behavior, the PELD can aim at providing a level of protection only a fine-tuned Live operating system can offer. On the contrary Tor distributions that rely on an untrusted underlying operating system could hardly guarantee anything in this area, regardless of the amount of resources and cleverness that is spent to leave less traces on local storage:

  • widespread operating systems and shipped Internet applications generally default to write all kinds of traces to local storage; Tor distributions that depend on such systems are therefore forced to adopt a blocklist approach to lessen the amount of traces left behind. Such an approach is known to be prone to human error, such as bug #7449 on Tor Project's Trac.

  • widespread operating systems typically offer very few control knobs to userspace applications over their not-amnesic-by-default behavior, especially when run without any kind of administrator credentials. Tor distributions that depend on such systems generally have no choice but hope no undesired trace will be left e.g. in the system's swap file or partition.

The PELD amnesic feature also allows the user to safely perform non-Internet activities, which is yet another distinctive trait compared to other Tor distributions.

To sum up, one can be a Tor expert and carefully configure a non-amnesic system to be as much Tor-ified as the PELD, but he or she won't get the same post-mortem analysis protection.

2.1.7 Summary

In short, the PELD aims at providing privacy on computers and on the Internet for anyone anywhere.

2.2 Threat model

The goal of staying anonymous and keeping sensitive information protected stands in direct conflict with the goals of several entities "present" on the Internet. The following threat model is meant to describe the intentions and capabilities of such attackers.

2.2.1 The goal of the attacker

The adversary may have one or more goals among the following ones.

  • Identify or locate the user, track his or her activities on the Internet: information such as the User-Agent HTTP header, locale and especially IP address can all be used in various degrees to identify or locate the user, and to track his or her activities on the Internet.

  • Eavesdrop on sensitive data: the Tor network only prevents the data from being traced (according to Tor's threat model) but does not protect it from eavesdropping.

  • Data recovery after system shutdown: "normal" operating systems keep a lot of traces about their users' Internet activities (notably browser cache, cookies and history) on local storage media; similary, working on a sensitive document with a "normal" operating system is very likely to leave traces of this document. User's data can remain on the equipment even after the machine is shut down; be it stored in the filesystem or in the memories, both RAM and swap, which might as well retain data (for example encryption keys or passwords). The adversary may want to recover such information by analyzing the equipment that has been used.

2.2.2 Capabilities, methods and other means of the attacker

The adversary may have capabilities needed to perform the following attacks.

  • Eavesdropping and content injection: it is assumed that the adversary is non-global and has full control over the network traffic of some portion of the Internet (e.g. some Tor exit nodes, upstream routers of exit nodes, or the ISP that provides the Internet connection the user is sitting behind). The adversary is thus able to eavesdrop, modify, delete or delay parts or all of the user's traffic on the Internet.

  • Bypass attacks: it is conceivable for attackers to mount attacks which bypass the proxy and DNS setup in the applications which could then be used to identify the user, either by injecting data or social engineering.

  • Exploit software vulnerabilities: the attacker might be able to run arbitrary code by exploiting vulnerabilities present in any of the software packages installed.

  • Application level attacks: the attacker can utilize certain applications' services and features to get identifying information. Examples are JavaScript and Java applets in web browsers, CTCP queries in IRC clients, etc.

  • Physical access, live monitoring, post-mortem equipment analysis: some users face adversaries with intermittent or constant physical access to the equipment they use. Users in Internet cafes, for example, face such a threat. This means the adversary might be physically monitoring the computer while the PELD is running on it. Moreover the adversary might raid the user at any moment and then confiscate and analyse the equipment, storage media and memory in particular.

2.3 Distribution

The PELD MUST be distributed in a common format that can easily be used to install the PELD on the selected medium. For instance, if distributed as an ISO 9660 compatible image file it can be burned to a DVD with almost any DVD recording software available.

Also, it is RECOMMENDED to make it possible for end-users to verify the downloaded PELD image's integrity using public-key cryptography.

2.4 Operational requirements

This section handles mostly the criteria that the PELD should be portable and able to run in as many environments as possible.

2.4.1 Platform

The binaries MUST all be executable on the most common computer hardware architecture(s). As of 2014, the x86 computer architecture seems to be the obvious choice as the vast majority of personal computers in use is compatible with it. The PELD SHOULD support UEFI. Supporting widespread hardware architectures used in mobile computers, such as phones, is also welcome.

2.4.2 Media

The PELD SHOULD be able to boot and run natively from either a DVD or a USB drive. While running the PELD in native mode it MUST be completely independent from the host operating system and all other storage media on the host computer unless the user explicitly tries to access any of them.

In all circumstances, binaries, dynamic libraries and other executable code susceptible to virus infections and similar MUST always be completely write-protected, even when running from a writeable USB medium. Such files SHOULD not even be modifiable temporarily, which could be the case even when running from DVD if the filesystem is loaded into memory (e.g. tmpfs).

Configuration files, temporary files, user home directories and similar files that most likely need to be modifiable during operation MUST only be saved temporarily in memory (e.g. by use of something like tmpfs or unionfs) unless the user explicitly enables some features of the Persistent Storage.

It is tempting to use the possibility to write back data when running from USB in order to allow user settings to be persistent. If this is considered, this feature MUST be optional and offer the possibility to use strong encryption for the Persistent Storage.

2.4.3 Virtual machines

As an alternative to running the PELD natively from a DVD or USB, it SHOULD also be possible to run it inside virtual machines.

Running the PELD in such a virtualized environment provides weakened security compared to running it natively. This drawback is due to the dependency on the host OS. When the PELD runs as a guest OS:

  • The PELD cannot defend against keyloggers, viruses and other malware that could be present in the host OS. The user activities in the PELD might thus be under surveillance by an attacker who would control the host OS enough.

  • The PELD cannot guarantee anything with respect to writing to local storage: the host OS does its own memory management and can write to swap any part of the memory being used by the PELD without the user being told.

On the other hand, running the PELD inside a virtual machine is useful in situations where the user is not in a position to run the PELD natively, which often is the case with public computers. Additionally, many users seem to prefer this mode of operation and would prefer to use their usual operating system instead of rebooting to run the PELD natively; that alone is a reason for making sure it works.

2.5 Other considerations

2.5.1 Maintainability

The procedure to update the PELD SHOULD NOT be prohibitive to provide timely software updates that address issues related to security or anonymity. A scripted, automatic build procedure is RECOMMENDED over manually setting up things.

2.5.2 Sustainability

PELD development SHOULD be a team work rather than a one person work, and the deep knowledge of this work SHOULD be shared among the team members. Thus the development infrastructure SHOULD be designed and deployed in order to share this knowledge.

2.5.3 Open-source transparency, easing peer review

For the sake of transparency the use of open-source software is RECOMMENDED. Binary blobs SHOULD only be used when no good alternative exists, which could be the case with certain hardware drivers or driver firmwares.

Having third-parties analyze the PELD security is necessary to ensure it is working as intended. It is thus REQUIRED for the PELD itself to be open-source. Moreover decisions with non-trivial implications SHOULD be clearly and publicly documented: such information about what a PELD implementation intends to achieve and how it does so SHOULD be made available to reviewers.

Third-parties SHALL be able to reproduce a PELD implementation by building it from the released source code and publicly available information. The process MUST yield consistent results.

2.5.4 Easy feedback

In order to collect bug reports and wanted features, the PELD project SHOULD offer easy ways for end-users to provide feedback to the developers (email, web forum, bug tracker, shipped-within application, ...). Efforts SHOULD be made to offer the most anonymous (or at least pseudonymous) possible way to send this feedback.

2.6 Implementation requirements

2.6.1 Kernel requirements

The role of the kernel is mainly to provide support for the features required elsewhere in this specification. This includes:

  • Good hardware support is REQUIRED: "good" is a sketchy word in a specification. The general idea is to include as much drivers for relevant hardware as possible, in particular for network cards (wired and wireless), video adapters and anything necessary for basic operation.

  • Support for a stateful firewall with packet filtering capabilities is REQUIRED: it must somehow be able to sort traffic out for transparent proxying (mentioned in the next section) to work. Similarly, it must be able to identify and drop traffic destined to the Internet that is not supported by the Tor network, such as transport layer protocols other than TCP.

  • Security features are RECOMMENDED: with the dangers of exploitable vulnerabilities in any code running, attempts to mitigate these on the kernel level is a good idea. Executable space protection with the NX bit, address space layout randomization and similar techniques are all interesting in this respect. Access control in the form of Mandatory Access Control, Role-Based Access Control and so on SHOULD also be considered.

2.6.2 Network requirements

2.6.2.1 Firewall

In order to prevent accidental leaks of information, proxy bypass attacks on Tor and similar, the access to the Internet MUST be heavily restricted by a firewall:

  • All non-TCP transport layer protocols SHOULD be dropped as they are not supported by the Tor network.

  • All TCP traffic not explicitly targeting Tor SHOULD be redirected to the transparent proxy (i.e. to the TransPort as set in torrc); alternatively this traffic SHOULD be dropped (then only applications explicitly configured to use Tor will reach the Internet).

  • All DNS lookups SHOULD be made through the Tor network (i.e. redirected to DNSPort as set in torrc).

Note that the above is not necessary (or desirable) for local network (RFC1918) addresses; it is RECOMMENDED to special-case DNS queries though as some home gateways and captive wifi portals reply the public IP provided by the ISP when one asks information about themselves to their DNS resolver (source: The State of the DNS and Tor Union (also: a DNS UDP - TCP shim) thread on the or-talk mailing list).

Any exception to these rules MUST be thoroughly thought through and properly documented. If an action that is excepted from the above rules is user initiated, that MUST be made obvious to the user, and user opt-out MUST be offered, if possible.

2.6.2.2 Fingerprinting

Efforts SHOULD be made so that it is harder to fingerprint PELD users as being using the PELD. Considering this goal can conflict with others, trying to reach perfection in this domain is not necessarily reasonable.

2.6.3 User interface and applications

2.6.3.1 General user interface

The user SHOULD be able to do all relevant things with easy-to-use graphical interfaces. The PELD SHOULD present a solid, user-friendly desktop environment with all the expected features after booting: file management, system settings configuration, support applications etc.

2.6.3.2 Internet applications

At least clients for the following Internet activities MUST be supported:

  • Web browsing: since the web browser is arguably the most used end-user Internet application as well as the one that offers the largest attack surface, the Tor Project has spent significant resources on analyzing and mitigating the many pitfalls of modern web browsers with respect to anonymity: media plugins, browser-specific bugs, web technologies such as JavaScript, CSS and cookies, etc. Benefiting from this work is essential for maintaining anonymity, so it is REQUIRED to include only web browsers that are endorsed by the Tor Project, accompanied with any configurations, extensions/plugins, etc. that they recommend (for instance, Tor Browser Bundle). The PELD browser fingerprint SHOULD make the PELD users appear uniformly among Tor users with generally recommended configuration, such as Tor Browser Bundle users.

  • Email: support for PGP or S/MIME is highly RECOMMENDED. Also, beware that the EHLO/HELO sent to the SMTP-server will contain the host's IP address in many email clients. The Message-ID headers and HTML/JavaScript email support are other usual leaking spots that MUST be taken care of.

  • Instant messaging, including IRC and XMPP.

  • Secure Shell client such as OpenSSH.

Other RECOMMENDED clients for Internet activities include:

  • P2P file-sharing such as BitTorrent: note, however, that large scale file-sharing activity in general is frowned upon in the Tor community as it consumes extreme amounts of network resources compared to other kinds of services.

  • Collaborative text editing such as Gobby.

  • Remote desktop such as VNC or RDP.

  • Feed aggregator for RSS/RDF, Atom and other widely spread feed formats.

Given that these applications will be the user's interface to the Internet, they MUST be chosen and configured cautiously, and with security in mind. In general, as little information as possible SHOULD leak about the user, the applications used and the system settings.

2.6.3.3 Document production applications

A great deal of communication over the Internet is done via the distribution of different types of commonly used media and document formats, so the PELD MUST contain the basic facilities for creating and editing such formats. More specifically, at least the following media and text production tasks MUST be possible using software shipped by the PELD:

  • Word processing
  • Bitmap and vector graphics
  • Sound recording and editing
  • Desktop publishing
  • Printing and scanning

Other RECOMMENDED media and text manipulation tools include:

  • Video editor
  • Spreadsheet
  • Presentation software
  • Gettext catalogs (.po files) editor

These applications SHOULD be compatible with widely spread file formats in their domain.

2.6.3.4 Cryptographic tools

Tools for securely signing, verifying, encrypting and decrypting files and messages SHOULD be available. In particular, some implementation of OpenPGP SHOULD be included as it is the de-facto standard for these tasks in the Free Software world. GUIs for managing keys and performing the relevant cryptographic tasks SHOULD be available. The OpenPGP implementation SHOULD be pre-configured to use an encrypted tunnel when connecting to a keyserver (read: hkps://).

Tools for creating and using encrypted storage containers are also RECOMMENDED.

A password manager SHOULD be included and allow one to store many passwords in an encrypted database which is protected by a single key. This is meant to encourage users to use strong passwords, and to discourage password reuse.

Any cryptographic tool shipped in the PELD SHOULD by default use up-to-date parameters with respect to the current commonly agreed best practices: digests, ciphers and key sizes.

2.6.3.5 Tor

Only stable releases SHOULD be considered since Tor really is at the core of the PELD.

Tor SHOULD be set up to enable its DNS server (DNSPort) to allow DNS lookups through the Tor network; alternatively a local DNS server can be configured to use Tor.

If transparent proxying (as opposed to dropping non-Tor traffic) was chosen in the network section, then Tor MUST be set up to enable its transparent proxy (TransPort, TransListen); alternatively any transparent proxy configured to use Tor as the parent proxy can be used.

While there are many other interesting configuration possibilities described in the Tor manual, care MUST be taken to avoid those that may impair anonymity or security.

A GUI Tor controller application such as Vidalia or TorK is highly RECOMMENDED. However, this requires opening the control port in Tor, and thus some means of authentication will be REQUIRED (CookieAuthentication preferably) to hinder attacks on the Tor software. XXX: this paragraph is mostly obsolete.

2.6.3.6 Hardened tool chain and compiling

As an addition to the security against exploitable vulnerabilities provided by the kernel, compiling software with stack smashing protection, Position Independant Executable (PIE) and similar compiler security enhancements is RECOMMENDED. Note that some compiler-level options may be necessary to take advantage of in-kernel security features. Thus the use of a hardened tool chain might depend on the vendor distribution used to build the PELD upon. If such techniques are not widely deployed at this level, using them to build the PELD can require a lot of time from its developers and impact its ease of maintenance, which would make it harder for new contributors to get involved in the project. For this reason, compile-time hardening features should be carefully selected to balance the costs they impose against the extra security they bring. If using a hardened tool chain to build the PELD is deemed to require too much energy, resources could be better spent pushing usage of such hardening features in the base operating system.

2.6.3.7 "Virtual" input system

Since one goal of the PELD is to permit usage of untrusted computers while preserving anonymity, measures MUST be taken against hardware that might deanonymize or facilitate recording of a user, such as hardware keyloggers. Thus a virtual keyboard (usable with the mouse) MUST be available.

2.6.3.8 Entropy

Some crucial applications of the PELD, such as the Tor client, make heavy use of cryptographic techniques and therefore rely on a high quality pseudo-random number generator (PRNG). Initializing (seeding) a PRNG is tricky in a Live system context as the system state after boot, if not fully deterministic, is parameterized by far fewer variables than in the case of a non-Live system.

Using an auxiliary entropy source such as haveged is thus REQUIRED.

2.6.4 Usability

Security is usually hard to get. Therefore steps SHOULD be taken in order to make users more comfortable with the PELD, and also to educate users about specific risks and non-intuitive situations that may affect their anonymity on the Internet.

2.6.4.1 Internationalization and localization

The user MUST be able to easily select his or her language of preference among a great number of widespread ones. User applications SHOULD be localized to fit this preference, as SHOULD system settings such as keyboard layout.

The PELD documentation, either online or shipped within, SHOULD be easily translatable. Software written specifically for the PELD SHOULD be developed with i18n/l10n-awareness in mind.

2.6.4.2 Education and user help

The PELD SHOULD include an easy-to-read document explaining:

  1. what the PELD goals (and non-goals) are. The PELD is no magic wand.

  2. how to securely use the PELD and the bundled software.

As the user is assumed to only have the knowledge of an average computer user, it will be required to explain some general security concepts.

Real-world experience demonstrates that the average user quite rarely thinks about upgrading his or her PELD copy, and is often pretty happy unknowingly running an obsolete version affected by numerous well-known security issues. A PELD running copy SHOULD therefore notice it is affected by known security issues and notify the user if a newer release fixes some.

2.7 The future

A few more or less well known issues shall be thought through so that this specification can provide reasonable guidance about them.

2.7.1 Memory recovery attacks

Research aims at recovering a Live system's in-memory filesystem and partial recovery of its previously deleted contents. Most current Live systems do not protect against that kind of attacks: at best, they erase free memory on shutdown, leaving intact in memory any data saved in the unionfs (aufs, overlayfs, etc.) ramdisk branch.

This was discussed on the or-talk mailing list, and a new system that mitigates such attacks is part of Tails 0.7 and newer.

This specification must warn about such matters.

2.7.2 HTTP keepalive

Quoting the Live CD Best Practices document on the Tor wiki:

OPTIONAL? To prevent the browser from keeping HTTP sessions open over existing circuits the following network settings should be applied. This will ensure that new circuits, such as requested via NEWNYM, will service subsequent HTTP requests.

The impact of HTTP keepalive and possible solutions are being discussed on the tails-dev@boum.org mailing list.

2.7.3 Mounting of filesystems stored on removable devices

Some attacks recently put under the spotlights exploit vulnerabilities in the desktop software stack that triggers automatic mounting, display and files preview of filesystems stored on removable devices.

This specification must warn about such matters.

2.7.4 Miscellaneous

  • FireWire is known to allow read access to the system memory, at least when such devices are allowed to use DMA (Linux: options ohci1394 phys_dma=0 helps mitigating that).

  • IrDA and other network links might allow attacks.

3 Implementation

Tails is an implementation of the PELD specification above. It is licensed under the GNU GPL version 3 or (at your option) any later version.

Critical parts of the configuration are based on the ones from well-known and trusted sources, namely Tails ancestor Incognito and the Tor BrowserBundle. This is for example the case for the firewall and Tor configurations.

NOTICE: this distribution is provided as-is with no warranty of fitness for a particular purpose, including total anonymity. Anonymity depends not only on the software but also on the user understanding the risks involved and how to manage those risks.

3.0 Other Tails design documents

3.1 Download

See the installation section on Tails's website for download information. Published Tails images integrity can be checked using OpenPGP detached signatures made with a key that is well linked to the Web of Trust.

The sources are stored in a bunch of Git repositories. The git page on the Tails website provides all needed information to access these repositories.

The latest version of this document can be found on the Tails website: contribute/design.

3.2 Software

Tails ships the following software. This list is not complete, but only contains packages deemed as important for whatever reason. The full packages list can be found in the BitTorrent files download directory (look for files with the .packages extension).

3.2.1 Core software

Being based in Debian, Tails benefits from its great package management tools, facilitating its build and the inclusion of new software. Sadly compile time options that would enhance Tails security (-fPIE, -fPIC, -fstack-protector etc.) are not widely used in Debian yet. Thus having them included in Tails would require to recompile every package with the right compile-time options. This would badly impact the development and build efforts required to release Tails. As an alternative, efforts have been started to push usage of such hardening features in Debian.

3.2.2 Other applications

See features.

3.3 Localization

3.3.1 Best supported (Tier-1) languages

See translate.

3.3.2 Locales and spell checking

Tails ships, as is, localization files provided by the installed Debian packages.

Additionally, Tails installs by default:

  • All available Tor Browser localization packages
  • For each Tier-1 language, when available in Debian:
    • Spell checking dictionaries (usable at least in Tor Browser, LibreOffice and GNOME Text Editor)
    • Localization packages for LibreOffice and Thunderbird

3.3.3 Input methods

Tails ships with IBus and a few engines (Mozc for Japanese, Pinyin and Bopomofo for Chinese, and Hangul for Korean).

A login script prepares and configures IBus. When a Japanese, Chinese or Korean locale is selected, this login script selects the right default input method. GNOME starts the IBus daemon itself.

Still, since one may want to work on documents written in Chinese, Japanese or Korean even when selecting English as their preferred language, IBus is also started in other locales, with all supported input engines pre-loaded.

IBus' environment variables are always exported on login to make this work.

3.4 Notification of security issues and new Tails releases

Tails ships a script that checks if there are security issues that are not fixed by any Tails release (and thus, affect the currently running Tails regardless of its version). A desktop notification is displayed for every such issue.

The connections are made to the Tails website through Tor, over HTTPS with a pinned CA. The only piece of information leaked to the Tails web server (apart of LWP::UserAgent's specific User-Agent and HTTP behavior) is the first two chars of the LANG environment variable.

This script is run after the user has logged-in and Tor is in known-working state.

Security issues that were fixed in a newer version of Tails are taken care of by Tails Upgrader that tells the user whenever they should upgrade and leads them through the necessary steps.

3.5 Feedback

Users can send feedback in several ways to Tails developers. A task tracker is available. Users can also send email to the private developers mailing list or to the private support mailing list.

A dedicated application called WhisperBack is also available in every running Tails copy. WhisperBack allows users to send anonymous or pseudonymous feedback via OpenPGP-encrypted email. It is configured in Tails to use a Tor onion service run by Tails developers as the outgoing SMTP server. WhisperBack only sends email over a STARTTLS session after carefully verifying the remote SMTP SSL certificate. Some minimal information about the system is gathered and sent along with the report; the reporting user can opt-out of this though. Users can also provide an email address and an OpenPGP public key in case further discussion is needed to address the reported issue. WhisperBack's graphical interface fully supports internationalization and is ready to be translated.

3.6 Configuration

In this section we briefly present the setup of several key software packages and system settings of Tails with respect to security and anonymity. There are of course other minor tweaks here and there, but those are mainly for usability issues and similar.

3.6.1 The Tor® software

The Tor software is currently configured as a client only (onion proxy). The client listens for control connections on port 9052 (using cookie authentication) which is non-standard (see about the "control port filter" below), as a transparent proxy on port 9040 (only used for remapped onion services) and as a DNS server on port 8853.

The client listens on a few SOCKS address/ports. For details, see the Tor stream isolation design page).

Only connections from localhost or from specific network namespaces are accepted. It can be argued that running a Tor server (onion router) would increase one's anonymity for a number for reasons but we still feel that most users probably would not want this due to the added consumption of bandwidth.

If a compromised software had access to the Tor control port, an attacker who controls it could simply ask Tor the public IP through the GETINFO address command. To prevent this, access to the Tor control port is only granted to the root user, as well as to the members of the debian-tor group (via the control socket). A filtering proxy to the control port runs on port 9051 (i.e. the default Tor ControlPort), so for instance Torbutton still can perform safe commands like SIGNAL NEWNYM. It allows defining fine-grained lists of allowed commands (and their arguments) and events on a per-application basis, which can enforce rules like "this $user (e.g. amnesia) when running this $application (e.g. /usr/bin/onionshare) from this network namespace, can only run these commands (ADD_ONION etc.) and listen to these events (e.g. HS_DESC, which is expected after a successfull use of ADD_ONION)".

We disabled the default warning messages of Tor (WarnPlaintextPorts) when connecting to ports 110 (POP3) and 143 (IMAP). These ports are used for both plaintext and StartTLS connections. As more and more email providers recommend and even enforce StartTLS on these ports, the effect of these warnings were most of the time counterproductive as people had to click through needlessly scary security warnings.

Onion Circuits allows the user to view the state of their Tor circuits and streams. The Tor Status extension provides a permanent visual indication of whether Tor has bootstrapped already.

3.6.2 IPv6

See the dedicated design document.

3.6.3 DNS

Tor does not support UDP so we cannot simply redirect DNS queries to the Tor transparent proxy.

Most DNS leaks are avoided by having the system resolver query the Tor network using the DNSPort configured in torrc.

There is a concern that any application could attempt to do its own DNS resolution without using the system resolver; UDP datagrams are therefore blocked in order to prevent leaks. Another solution may be to use the Linux network filter to forward outgoing UDP datagrams to the local DNS proxy.

Tails also forbids DNS queries to RFC1918 addresses; those might indeed allow the system to learn the local network's public IP address.

resolv.conf is configured to point to the Tor DNS resolver, and we configure NetworkManager not to manage resolv.conf at all:

Some applications need to be able to do clearnet DNS resolutions, so we save the DNS configuration obtained by NetworkManager:

The following is the complete list of the applications allowed to use the clearnet DNS configuration:

3.6.4 HTTP Proxy

Tails does not include any HTTP proxy anymore.

3.6.5 SOCKS libraries

torsocks and torify are installed. Since Tor-ification is done at a lower level (in-kernel network filter, Tor-ified DNS), these tools are actually unnecessary. They are solely included due to dependencies and configured for completeness.

3.6.6 Network Filter

One serious security issue is that we don't know what software will attempt to contact the network and whether their proxy settings are set up to use the Tor SOCKS proxy correctly. This is solved by blocking all outbound Internet traffic except Tor, and explicitly configuring all applications to use one of these.

The default case is to block all outbound network traffic; let us now document all exceptions and some clarifications to this rule.

Tor user

Tor itself obviously has to connect to the Internet without going through the Tor network. This is achieved by special-casing connections originating from the debian-tor Unix user.

clearnet user

The clearnet user is used to run tails-get-network-time.

It is granted full network access (but no loopback access). It most specifically needs to resolve hosts and to make an HTTP connection.

Unsafe Browser

The Unsafe Browser is run by the amnesia user, in the clearnet network namespace. This is not to be confused with the clearnet user.

The clearnet network namespace has full network access (UDP and TCP) but no loopback access.

Local Area Network (LAN)

Tails short description talks of sending through Tor outgoing connections to the Internet. Indeed: traffic to the local LAN (RFC1918 addresses) is wide open as well as the loopback traffic obviously.

LAN DNS queries are forbidden to protect against some attacks.

Local services allowlist

The Tails firewall uses a allowlist which only grants access to each local service to the users that actually need it. This blocks potential leaks due to misconfigurations or bugs, and deanonymization attacks by compromised processes.

One exception is for the amnesia user, which is allowed to connect to any local TCP port apart for a blocklist, because some applications, such as Audacity or OnionShare, rely on the ability to connect to random local ports.

For specifics, see the firewall configuration where this is well commented: config/chroot local-includes/etc/ferm/ferm.conf

Automapped addresses

AutomapHostsOnResolve is enabled in Tor configuration, and a firewall rule transparently redirects to the Tor transparent proxy port the connections targeted at the 127.192.0.0/10 virtual mapped address space.

Only the amnesia user is granted access to the Tor transparent proxy port, so in practice only this user can use this hostname-to-address mapping facility.

IPv6

Tails does not support connecting to the Tor network via IPv6 yet: #20490.

Also see the Firewall section of the IPv6 design.

UDP, ICMP and other non-TCP protocols

Tor only supports TCP. Non-TCP traffic to the Internet, such as UDP datagrams and ICMP packets, is dropped.

An unfortunate consequence of fully blocking ICMP is that Path MTU Discovery is broken. We workaround this problem by enabling Packetization Layer Path MTU Discovery. For details, see:

RELATED packets

As a general rule, the Tails' firewall does not accept RELATED packets: accepting them enables quite a lot of code in the kernel that we don't need, so we prefer reducing the attack surface a bit by blocking them. See the "[Tails-dev] Reducing attack surface of kernel and tightening firewall/sysctls" thread for details.

However, RELATED ICMP packets to the loopback interface are let through, in order to smooth user experience whenever a program's local network connection is blocked, and the TCP stack generates ICMP packets (e.g. with TYPE=3 CODE=3, i.e. the destination port is unreachable) to let the program know what is going on early, instead of letting it wait for a timeout.

3.6.7 MAC address spoofing

See the dedicated design document.

3.6.8 Host system swap

Tails takes care not to use any swap filesystem that might exist on the host machine hard drive. Most of this is done at build time: the /sbin/swapon binary is replaced by a fake no-op script, and live-boot's swapon option is not set.

3.6.9 Host system RAM

In order to protect against memory recovery such as cold boot attack, most of the system RAM is overwritten when Tails is being shutdown or when the boot medium is physically removed. Also, memory allocated to processes is erased upon process termination.

The big picture

Tails now relies on the Linux kernel's freed memory poisoning feature.

But memory poisoning only works when memory is actually freed, and a regular shutdown would not free the memory used by the overlayfs read-write branch. So we use the systemd-shutdown ability to return to the initramfs, to ensure the root filesystem is unmounted.

The initramfs is unpacked in /run/initramfs at boot time:

… so that the copy of systemd-shutdown that runs in the real, non-initramfs system can switch root into /run/initramfs, and run another copy of systemd-shutdown that's included in the initramfs. That one will unmount all filesystems, run a custom hook that helps us automatically test this behavior, and finally perform the requested poweroff/reboot action.

To make this work, a dedicated tmpfs filesystem is mounted on /run/initramfs: /run is mounted with the noexec option and while our attempts to remount it with exec worked for clean shutdown, they failed for emergency shutdown, i.e. when the boot medium is physically removed.

For details about the underlying systemd mechanisms, see:

In our experience, jumping back to the initramfs to unmount the remaining filesystems, as described above, was necessary but not sufficient to free the memory used by the overlayfs read-write branch. That's why additionally, we manually delete the content of that branch via a systemd service late in the shutdown process, before we jump back to the initramfs. It's unclear to us why this works when the boot medium is physically removed: in that case, systemctl --force poweroff is not supposed to stop tails-remove-overlayfs-dirs.service, and thus this additional clean up step should be skipped; it could be that in this emergency shutdown situation, systemd-shutdown somehow manages to clean things up by itself and there's no need for tails-remove-overlayfs-dirs.service.

Triggers

Different kinds of events trigger the memory erasure process. All lead to run the shutdown process that erases memory.

First, most memory is erased at the end of a normal shutdown/reboot sequence. This is implemented by the Linux kernel's freed memory poisoning feature, more specifically init_on_free=1.

Automated tests ensure that the most important parts of memory are erased this way.

Second, the memory erasure process is triggered when the boot medium is physically removed during runtime (USB boot medium is unplugged or boot DVD is ejected). This is implemented by a custom udev-watchdog program monitors the boot medium; it's run by a wrapper, started at boot time, that brutally invokes the memory erasure process, bypassing other system shutdown scripts, when this medium happens to be physically removed.

Note that the udev-watchdog is disabled while the system is suspended to RAM, in order to avoid a race condition when resuming from suspend, which used to occasionally trigger the emergency shutdown (see #11729). This means that the memory erasure process is not triggered if the boot medium is removed while the system is suspended.

Making sure needed files are available

The memlockd daemon, appropriately configured, ensures every file needed by the memory erasure process is locked into memory from boot to memory erasure time.

Limitations

As discussed in an email thread with the authors of PAX_MEMORY_SANITIZE, kernel memory poisoning does not clear all kinds of memory once it's freed:

  • we enable free poisoning for the buddy allocator, the slub/slab ones, and heap memory, but there may be other ways the Linux kernel allocates memory, that are not subject to poisoning;

  • on shutdown all process memory is freed (and thus erased), but some kernel memory is not erased on shutdown, and is currently not erased.

It's not obvious that our previous approaches (see below) did any better, and this one is much more reliable, so we think this trade-off is the most sensible one with the resources and skills currently available for Tails.

Obsolete approaches

The initial implementation of the Tails memory erasure feature suffered from flaws that were demonstrated by external audit. In short, it only erased free memory and let data in the union filesystem read-write branch in recoverable state.

Then, in order to erase the biggest possible part of the system memory, a new implementation, shipped from Tails 0.7 to 2.12, runs in a fresh environment provided by a newly started Linux kernel. This way, a given part of the memory either is used by the memory erasure process itself or it is considered as free and thus erased by this process; in any case, it is at least overwritten once.

Sadly, this approach suffered from severe usability and reliability problems (e.g. #12354, #11786). So it was removed in Tails 3.0.

3.6.10 Host system disks and partitions

Tails takes care not to use any filesystem that might exist on the host machine hard drive, unless explicitly told to do so by the user. The Debian Live persistence feature is disabled by passing nopersistence over the kernel command line to live-boot.

3.6.11 Filesystems stored on removable devices

Removable drives auto-mounting is disabled in Tails 0.7 and newer.

3.6.12 Passwords

Two users are intended to be used for logins: amnesia and root. None have a password by default; the amnesia user is allowed to gain super user privileges, using sudo, if an administrator password is set in tails-greeter.

The PELD specification recommends to prevent executable code to be modifiable, even temporarily; Tails does not implement this recommendation. Instead, thanks to the super user privileges being available to the end-user, Tails makes it possible to modify or add executable code by:

  • upgrading bundled software: this allows (technical) users to protect themselves from serious security issues until an updated Tails is released

  • installing additional software: this helps achieving the PELD "Working on sensitive documents" goal by enabling users to perform tasks that need software not shipped in Tails.

Such modifications happen only in RAM, the user will remove the DVD/USB when done and there are no services allowing logins from the network.

As a first step Tails has stopped granting sudo privileges to the amnesia user by default. Unless an administrator password is set in tails-greeter, no root access is possible afterwards.

3.6.13 Tor Browser

Tails ships with the Tor Browser, which is based on Mozilla Firefox and patched by the Tor Project for improved anonymity by reducing information leaks, decreasing attack surface and similar. The actual binaries etc. used in Tails are those distributed by the Tor Project, but the configuration differs slightly, which is described below.

In Tails we diverge from the Tor Browser's one-profile-only design, and install the Tor Browser in a globally accessible directory used by all browser profiles (and other XUL applications). In particular, the global Tor Browser installation is also used for the Unsafe Browser, although it is user-isolated and uses a separate profile with very different configuration (most importantly, it does not use tor for outgoing connections).

The default profile is split from the binaries and application data:

We only modify this Tor Browser installation slightly:

  • Tails installs the uBlock Origin extension.

  • We use the myspell/hunspell dictionaries provided by Debian.

  • We ship all languages supported by Tor Browser, which will select the UI language depending on the locale chosen by the user in our Welcome Screen.

  • Tails does not install the Tor Launcher extension as part of the browser. Rationale: Tor Browser in Tails is not allowed to configure tor.

  • We modify preferences in browser/omni.ja to slightly adjust the UI.

  • We append Tails-specific preferences to browser/omni.ja.

In Tails we do not use the start-browser script, since it does a lot of stuff not needed in Tails (error checking mainly) and isn't flexible since it looks for the browser profile in a specific place. Our custom script makes use of the global installation and also makes sure the default profile is used as a basis. Any shared libraries shipped inside the Tor Browser are also used (via LD_LIBRARY_PATH) since Debian stable often has too old versions to start the browser.

Whenever the user tries to start the Tor Browser before Tor is ready, they are informed it won't work, and asked whether to start the browser anyway:

The remaining configuration differences can be found in:

Finally, here are some useful resources when working on the integration of Tor Browser in Tails:

3.6.14 Thunderbird

Thunderbird sends email through Tor.

Thunderbird leaks a lot of information by default. This is mitigated in Tails by a set of preferences, that were initially inherited from the now defunct TorBirdy add-on. For example:

  • Thunderbird is configured to say EHLO[127.0.0.1] to the SMTP server instead of disclosing the real IP address and hostname.

  • We disable HTML email and inline attachments in order to get rid of a whole class of privacy concerns.

  • When setting up an IMAP account, the drafts folder, instead of being remote, is set to ~$HOME/.thunderbird/Local Folders.

Implementation:

OpenPGP support is integrated in Thunderbird since version 78.

Thunderbird's email configuration wizard has security issues in its default configuration. For example, it trusts the result of DNS requests and may retrieve mail server configurations from a remote server over an insecure channel. So in Tails, we patch Thunderbird to solve these security problems. We are in the process of (upstreaming these patches). Until they are all upstreamed, we apply these patches, on top of Debian's Thunderbird, while building Tails images:

These account configuration wizard patches allow using only secure protocols for the domain and ISPDB lookups as well as for the actual mail account configuration. In detail this means:

  1. we prevent all testing of plaintext protocols when guessing configurations.

  2. we make ISP autoconfiguration lookups first try https, then http, but only if we allow insecure protocols.

  3. we discard any configurations using plaintext protocols.

In addition, even when plaintext protocols are allowed, the patches make us prefer secure options if available. This setting can be disabled (opt-out by the user). This also applies to manual configuration.

3.6.15 Pidgin

Pidgin is configured in Tails to not log anything as well as not to reveal too much of user activity by disabling reporting of online/away/typing status. Only IRC and Jabber/XMPP protocols are left available, to avoid the usage of less well audited plugins. The Off-the-record plugin is enabled to help one-to-one conversations being as private and unrecordable as possible.

3.6.16 GnuPG

GnuPG and Kleopatra are configured to use https://keys.openpgp.org via its Onion service, since it's reliable.

GnuPG is configured accordingly to the OpenPGP Best Practices, e.g. to prefer non-outdated digest algorithms from the SHA-2 family, to force exclusion of the version string in ASCII armored output, to avoid automatically locating and retrieving keys, and to disregard the preferred keyserver assigned to specific keys.

3.6.17 Persistent Storage

An opt-in data persistence feature is available in Tails 0.11 and newer. See persistence for details.

3.6.18 Installation on USB sticks and SD cards

An easy (read: not command-line based) way to install and upgrade Tails on USB sticks and SD cards is available in Tails 0.11 and newer. SD cards readers wired via SDIO are supported since Tails 0.21. See installation for details.

3.6.19 Wireless devices handling

Tails puts the wireless devices in a sensible state at boot time.

At boot time, Tails unblocks Wi-Fi, WWAN and WiMAX radios and soft-blocks all other kinds of wireless devices (e.g. Bluetooth, UWB, GPS, FM).

3.6.20 OpenSSH

The OpenSSH client is configured to use the Tor SOCKS proxy.

3.6.21 Automatic upgrades

When a Tails release is out, Tails users are proposed to download and apply a partial upgrade (that is, only what has changed between two releases). See upgrades for details.

3.6.22 Panel applets and GNOME Shell extensions

Tails ships a few custom GNOME panel applets and GNOME Shell extensions.

The status-menu-helper GNOME Shell extension provides two-clicks shutdown and restart actions, as well as screen locking.

3.6.23 DHCP hostname leaks

We patch NetworkManager so its DHCP client does not send the hostname in DHCP requests, until GNOME bug #768076 is fixed.

3.6.25 Application isolation

Tails has some minimal application isolation to mitigate a bit the consequences of security issues in individual applications being exploited by attackers.

3.6.26 wget

We wrap wget with torsocks, after unsetting the http_proxy environment variable and friends, so that it talks directly to the Tor SOCKS port.

3.6.27 APT

During most of the ISO build process, APT uses the proxy configured through live-build (that is, usually a local apt-cacher-ng).

However, at boot time, a hook does (a more elaborate version of) s,http://,tor+http:// in APT sources. Then, APT will use the tor+http method, that is provided by apt-transport-tor.

3.6.28 Electrum

We install the Electrum Bitcoin client and the default configuration tells it to use the default of Tor's SOCKSPort:s, and sync the necessary parts of the Bitcoin blockchain (as a lightweight client) from the default server pool using SSL.

There is also a feature of the Persistent Storage for the live user's .electrum configuration folder, which stores the Bitcoin wallet, application preferences and the cached Bitcoin blockchain.

3.6.29 Kernel hardening

A lot of security features have been implemented upstream at the kernel level (ASLR, removal of /dev/kmem, /dev/mem protections, various stack and heap hardening features, /proc or /sys not leaking sensitive information, etc.), most of them being slowly integrated into Debian. This is the reason why the Tails kernel has only a few more security features enabled than the stock Debian kernel.

We pass a few kernel parameters on the boot command line and /proc/sys to increase security at little to no cost.

See #11143 and #10951 for the discussion that lead to this choice of parameters. Most of what follows come straight from what "cypherpunks" wrote there.

init_on_free=1

Fill freed pages and heap objects with zeroes.

Also enables poisoning for some freed memory. Poisoning writes an arbitrary value to freed objects, so any modification or reference to that object after being freed or before being initialized will be detected and prevented. This prevents many types of use-after-free vulnerabilities at little performance cost.

slab_nomerge

Disables the merging of slabs of similar sizes. Many times some obscure slab will be used in a vulnerable way, allowing an attacker to mess with it more or less arbitrarily. Most slabs are not usable even when exploited, so this isn't too big of a deal. Unfortunately the kernel will merge similar slabs to save a tiny bit of space, and if a vulnerable and useless slab is merged with a safe but useful slab, an attacker can leverage that aliasing to do far more harm than they could have otherwise. In effect, this reduces kernel attack surface area by isolating slabs from each other. The trade-off is a very slight increase in kernel memory utilization. slabinfo -a can be used to tell what the memory footprint increase would be on a given system.

slub_debug=FZ

Enables sanity checks (F) and redzoning (Z). Sanity checks are self-evident and come with a modest performance impact, but this is unlikely to be significant on an average Tails system. The checks are basic but are still useful both for security and as a debugging measure. Redzoning adds extra areas around slabs that detect when a slab is overwritten past its real size, which can help detect overflows. Its performance impact is negligible.

An additional note: any time slub_debug= is put in the kernel command line, slab_nomerge is implied. But having slab_nomerge explicitely declared can help prevent regressions where disabling of debugging features is desired but re-enabling of merging is not.

vsyscall=none

Virtual syscalls are the obsolete predecessor of vDSO calls. Unfortunately, both vsyscall=native and vsyscall=emulate (the default) have a negative security impact, with the latter a little less so. Namely, they provide a target for any attacker who has control of the return instruction pointer, which is increasingly common these days now that attackers need to resort to ROP and similar attacks which target a process' control flow. The impact of this is with reduced compatibility, however only legacy statically compiled binaries and old versions of glibc used vsyscalls. All software on modern Tails uses vDSO instead. If for some reason a program does try to use a vsyscall, the process will crash with a memory access violation, and won't bring the whole system down.

mce=0

Mostly useful for systems with ECC memory, setting mce to 0 will cause the kernel to panic on any uncorrectable errors detected by the machine check exception system. Corrected errors will just be logged. The default is mce=1, which will SIGBUS on many uncorrected errors. Unfortunately this means malicious processes which try to exploit hardware bugginess (such as rowhammer) will be able to try over and over, suffering only a SIGBUS at failure. Setting mce=0 should have no impact. Any hardware which regularly triggers a memory-based MCE is unlikely to even boot, and the default is 1 only for long-lived servers.

page_alloc.shuffle=1

Enables page allocator freelist randomization.

kASLR

Linux kASLR is known as not being particularly strong, but one has to start somewhere. See self-protection.txt for details.

kASLR is enabled by default in the Debian kernel since 4.7~rc7-1~exp1 (CONFIG_RANDOMIZE_BASE and CONFIG_RANDOMIZE_MEMORY) so there is no need to enable it with a specific kernel parameter.

kernel.kptr_restrict=2

Some off-the-shelf malware exploit kernel addresses exposed via /proc/kallsyms so by not making these addresses easily available we increase the cost of such attack some what; now such malware has to check which kernel Tails is running and then fetch the corresponding kernel address map from some external source. This is not hard, but certainly not all malware has such functionality.

For this reason, we also make sure to purge /boot/System.map.

vm.mmap_rnd_bits, vm.mmap_rnd_compat_bits

These settings are set to the maximum supported value in order to improve ASLR effectiveness for mmap, at the cost of increased address-space fragmentation.

kernel.kexec_load_disabled = 1

kexec is dangerous: it enables replacement of the running kernel.

mds=full,nosmt

As per https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html, if the CPU is vulnerable, this:

  1. enables "all available mitigations for the MDS vulnerability, CPU buffer clearing on exit to userspace";
  2. disables SMT which is another avenue for exploiting this class of attacks.

randomize_kstack_offset=on

Randomize kernel stack offset on syscall entry.

This is recommended by https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Recommended_Settings and enabled by default in more recent Debian kernels Debian bug #1016056.

net.core.bpf_jit_harden = 2

Turn on BPF JIT hardening, if the JIT is enabled.

This is recommended by https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Recommended_Settings.

spec_store_bypass_disable=on

Unconditionally disable the Speculative Store Bypass (SSB) on affected CPUs (see for example https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SpectreAndMeltdown/MitigationControls#CVE-2018-3639).

The default is prctl which means that SSB is enabled by default and can be disabled by a process via prctl. That's the default because disabling SSB can have a performance impact. The performance impact is not expected to be significant in Tails and we prefer the security benefits.

3.6.30 Time zone

Tails sets the system time zone to UTC to make it harder to distinguish Tails users from each other.

This confuses users, so we aim to provide ways to display the date and time in the user's own time zone, without changing the actual system time that applications would leak:

An adversary who can run arbitrary code as the amnesia user can use sudo tails-get-date to discover the timezone that the user chose. This is mitigated by the fact the most security-sensitive applications running as the amnesia user (e.g. Tor Browser, Thunderbird, and Pidgin) are not allowed to run sudo, thanks to AppArmor confinement and/or bubblewrap.

3.7 Running Tails in virtual machines

3.7.1 Current support

Tails may of course be run in virtual machines. Due to the popularity of VMWare we include open-vm-tools (an open-source alternative to VMware tools) as well as special video drivers for an improved user experience in that environment. Due to the closed-source nature of VMWare we try to encourage users of open VMs, like VirtualBox and QEMU, by making sure that these also work. In the case of VirtualBox both video and input drivers are included, as well as the guest utilities.

3.7.2 Security concerns

Security concerns for all VMs are numerous. Tails therefore tries to detect whether it is run inside a VM and warns the user if it is.

3.7.3 Running Tails inside a Windows session

Potential work may make it easier to run the DVD/USB in a virtual machine inside a Windows session whenever native boot is impossible or not desirable. This probably will be implemented by shipping a QEMU or VirtualBox binary for Microsoft Windows.

3.8 Build process and maintenance

3.8.1 Build tools

The Debian Live is a toolkit to build Live systems based on Debian, such as Tails. Debian Live is designed to automate the build process of the target distribution, which eases Tails development and maintenance. As an added bonus, Debian Live makes it possible for other people to build custom systems based on Tails, e.g. to include additional software.

For detailed instructions on how to build and modify Tails, see the build page and the contribute section on the wiki.

3.8.2 Testing process

An automated build and test environment is useful to avoid regressions in Tails, especially anonymity and security related ones. It also makes it easier for developers to work on Tails with more confidence, and at release time to cut down the time needed for quality assurance work.

Tails' manual test suite is "run" against Tails release candidates images before they are officially published. Automating this test suite is partly done, and a work in progress.

3.8.3 Upgrades

Keeping Tor (stable releases only, unless the Tor core developers recommend otherwise) and the Tor Browser up-to-date is a priority.

Remaining applications, including the base system, will be upgraded using Debian standard upgrade process, and generally based on the latest Debian stable release so there are not many problems.

We intend to build and publish bugfix releases (e.g. 0.6.1) in a timely manner when security issues affect Tails. Such releases are based on the stable Git branch and can thus also fix important — although not security-related — bugs.

3.9 Hardware support

Tails generally ships the latest Linux kernel from Debian unstable or Debian Backports as a compromise between stability and recent hardware support. Recent Intel and AMD microcode are included as well.

Tails supports only the x86-64 hardware architecture.

A 64-bit Linux kernel (amd64 flavour) and userspace are included.

Binary firmware blobs from the Debian non-free archive are installed when no good Free Software alternative exists. Our reasoning behind this choice is:

  • If Tails does not work out-of-the-box on the computer(s) they have available, most potential users will simply use something else, that will:

    1. just work ("thanks" to the inclusion of binary firmware);
    2. be less safe in the vast majority of real-world cases.

  • Much hardware that does not require proprietary firmware to be injected into it at runtime is simply embedding such proprietary blobs, often on read-only memory. Not only this does not provide much more security than injecting proprietary firmware at runtime, but it prevents hardware vendors from fixing (potentially security-relevant) bugs in the firmware once it's been shipped to users.

3.10 Caveats

Tails currently offers almost no protection against live physical monitoring, except for hardware keyloggers.

UDP is a problem. The Tor network does not support it. Outgoing UDP packets are dropped altogether by netfilter for this reason.

Tails does not support arbitrary DNS queries.

Some tools currently available to command-line users lack the integration into Tails and/or graphical user interface that would be needed to make them useful to anyone.

Other caveats are listed on the known issues page.

3.11 Fingerprint

Tails tries to make it as difficult as possible to distinguish Tails users from other Tor users.

The Tor Browser used in Tails is configured to match the fingerprint of the Tor Browser Bundle and the known differences, if any, are listed in the known issues page.

However the fact that different browser extensions are installed in Tails and in the Tor Browser surely allows more sophisticated attacks that usual fingerprint as returned by tools such as https://coveryourtracks.eff.org/ and http://ip-check.info/. For example, the fact that uBlock Origin is removing ads could be analysed.

From the point of view of the local network administrator:

  • When the user chooses in Tor Connection to hide the fact they are using Tor, Tails is almost exclusively generating Tor activity and that is probably quite different from other Tor Browser users. We believe this would be hard to avoid.

  • When the user chooses to configure Tor automatically, and not hide the fact they are using Tor, they consent to Tails initiating Internet activity without going through Tor, in order to help them connect to Tor. For details, see our design documentation about non-Tor traffic.

5 Bibliography