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Securing Debian Manual
Chapter 7 - Debian Security Infrastructure

7.1 The Debian Security Team

Debian has a Security Team, made up of five members and two secretaries who handle security in the stable distribution. Handling security means they keep track of vulnerabilities that arise in software (watching forums such as Bugtraq, or vuln-dev) and determine if the stable distribution is affected by it.

Also, the Debian Security Team is the contact point for problems that are coordinated by upstream developers or organizations such as CERT which might affect multiple vendors. That is, when problems are not Debian-specific. There are two contact points with the Security Team:

Sensitive information should be sent to the first address and, in some cases, should be encrypted with the Debian Security Contact key (key ID 363CCD95).

Once a probable problem is received by the Security Team it will investigate if the stable distribution is affected and if it is, a fix is made for the source code base. This fix will sometimes include backporting the patch made upstream (which usually is some versions ahead of the one distributed by Debian). After testing of the fix is done, new packages are prepared and published in the security-master.debian.org site so they can be retrieved through apt (see Execute a security update, Section 4.2). At the same time a Debian Security Advisory (DSA) is published on the web site and sent to public mailing lists including debian-security-announce and Bugtraq.

Some other frequently asked questions on the Debian Security Team can be found at Questions regarding the Debian security team, Section 12.3.

7.2 Debian Security Advisories

Debian Security Advisories (DSAs) are made whenever a security vulnerability is discovered that affects a Debian package. These advisories, signed by one of the Security Team members, include information of the versions affected as well as the location of the updates and their MD5 sums. This information is:

DSAs are published both in Debian's mainserver frontpage and in the Debian security pages. Usually this does not happen until the website is rebuilt (every four hours) so they might not be present immediately. The preferred channel is the debian-security-announce mailing list.

Interested users can, however (and this is done in some Debian-related portals) use the RDF channel to download automatically the DSAs to their desktop. Some applications, such as Evolution (an email client and personal information assistant) and Multiticker (a GNOME applet), can be used to retrieve the advisories automatically. The RDF channel is available at http://www.debian.org/security/dsa.rdf.

DSAs published on the website might be updated after being sent to the public-mailing lists. A common update is adding cross references to security vulnerability databases. Also, translations[49] of DSAs are not sent to the security mailing lists but are directly included in the website.

7.2.1 Vulnerability cross references

Debian provides a fully crossreferenced table including all the references available for all the advisories published since 1998. This table is provided to complement the reference map available at CVE.

You will notice that this table provides references to security databases such as Bugtraq, CERT/CC Advisories and US-CERT Vulnerability Notes Database as well as CVE names (see below). These references are provided for convenience use, but only CVE references are periodically reviewed and included. This feature was added to the website on June 2002.

One of the advantages of adding cross references to these vulnerability databases is that:

7.2.2 CVE compatibility

Debian Security Advisories were declared CVE-Compatible[50] in February 24, 2004.

Debian developers understand the need to provide accurate and up to date information of the security status of the Debian distribution, allowing users to manage the risk associated with new security vulnerabilities. CVE enables us to provide standardized references that allow users to develop a CVE-enabled security management process.

The Common Vulnerabilities and Exposures (CVE) project is maintained by the MITRE Corporation and provides a list of standardized names for vulnerabilities and security exposures.

Debian believes that providing users with additional information related to security issues that affect the Debian distribution is extremely important. The inclusion of CVE names in advisories help users associate generic vulnerabilities with specific Debian updates, which reduces the time spent handling vulnerabilities that affect our users. Also, it eases the management of security in an environment where CVE-enabled security tools -such as network or host intrusion detection systems, or vulnerability assessment tools- are already deployed regardless of whether or not they are based on the Debian distribution.

Debian started adding CVE names to DSAs in June 2002, and now provides CVE names for all DSAs released since September 1998 after a review process started on August 2002. All of the advisories can be retrieved on the Debian web site, and announcements related to new vulnerabilities include CVE names if available at the time of their release. Advisories associated with a given CVE name can be searched directly through the search engine.

Users who want to search for a particular CVE name can use the web search engine available in debian.org to retrieve advisories available (in English and translated to other languages) associated with CVE names. A search can be made for a specific name (like advisory CAN-2002-0001) or for partial names (like all the 2002 candidates included in advisories search for CAN-2002). Notice that you need to enter the word "advisory" together with the CVE name in order to retrieve only security advisories.

In some cases you might not find a given CVE name in published advisories, for example because:

7.3 Debian Security Build Infrastructure

Since Debian is currently supported in a large number of architectures, administrators sometimes wonder if a given architecture might take more time to receive security updates than another. As a matter of fact, except for rare circumstances, updates are available to all architectures at the same time.

While previously the task to build security updates was done by hand, it is currently not (as Anthony Towns describes in a mail sent to the debian-devel-announce mailing list dated 8th June 2002).

Packages uploaded by the security team (to ftp://security-master.debian.org:/org/security.debian.org/queue/unchecked or ftp://security-master.debian.org/pub/SecurityUploadQueue) with an appropriate patch are checked for signatures withing fifteen minutes of being uploaded. Once this is done they get added to the list of the autobuilders (which no longer do a daily archive run). Thus, packages can get automatically built for all architectures thirty minutes or an hour or so after they're uploaded. However, security updates are a little more different than normal uploads sent by package maintainers since, in some cases, before being published they need to wait until they can be tested further, an advisory written, or need to wait for a week or more to avoid publicizing the flaw until all vendors have had a reasonable chance to fix it.

Thus, the security upload archive works with the following procedure (called "Accepted-Autobuilding"):

This procedure, previously done by hand, was tested and put through during the freezing stage of Debian 3.0 woody (July 2002). Thanks to this infrastructure the Security Team was able to have updated packages ready for the apache and OpenSSH issues for all the supported (almost twenty) architectures in less than a day.

7.3.1 Developer's guide to security updates

This mail was sent by Wichert Akkerman to the Debian-devel-announce mailing list in order to describe Debian developer's behavior for handling security problems in their packages. It is published here both for the benefit of developers as well as for users to understand better how security is handled in Debian.

FIXME: Please note that the up to date reference for this information is the Debian Developer's Reference, this section will be removed in the near future.

7.3.1.1 Coordinating with the security team

If a developer learns of a security problem, either in his package or someone else's he should always contact the security team (at team@security.debian.org). They keep track of outstanding security problems, can help maintainers with security problems or fix them themselves, are responsible for sending security advisories and maintaining security.debian.org.

Please note that security advisories are only done for release distributions, not for testing, unstable (see How is security handled for testing and unstable?, Section 12.3.8) or older distributions (see I use an older version of Debian, is it supported by the Debian Security Team?, Section 12.3.9).

7.3.1.2 Learning of security problems

There are a few ways a developer can learn of a security problem:

In the first two cases the information is public and it is important to have a fix as soon as possible. In the last case however it might not be public information. In that case there are a few possible options for dealing with the problem:

In all cases if the person who reports the problem asks to not disclose the information that should be respected, with the obvious exception of informing the security team (the developer should make sure he tells the security team that the information cannot be disclosed).

Please note that if secrecy is needed the developer can also not upload a fix to unstable (or anywhere else), since the changelog information for unstable is public information.

There are two reasons for releasing information even though secrecy is requested/required: the problem has been known for too long, or the information becomes public.

7.3.1.3 Building a package

The most important guideline when making a new package that fixes a security problem is to make as few changes as possible. People are relying on the exact behavior of a release once it is made, so any change made to it can possibly break someone's system. This is especially true of libraries: the developer must make sure he never changes the API or ABI, no matter how small the change.

This means that moving to a new upstream version is not a good solution, instead the relevant changes should be backported. Generally upstream maintainers are willing to help if needed, if not the Debian Security Team might be able to help.

In some cases it is not possible to backport a security fix, for example when large amounts of source code need to be modified or rewritten. If that happens it might be necessary to move to a new upstream version, but it should always be coordinated with the security team beforehand.

Related to this is another important aspect: developers must always test your change. If there is an exploit the developer should try if it indeed succeeds on the unpatched package and fails on the fixed package. The developer should try normal usage as well, sometimes a security fix can break normal use subtly.

Finally a few technical things for developers to keep in mind:

7.3.1.4 Uploading security fixes

After the developer has created and tested the new package it needs to be uploaded so it can be installed in the archives. For security uploads the place to upload to is ftp://security-master.debian.org/pub/SecurityUploadQueue/ .

Once an upload to the security queue has been accepted the package will automatically be rebuilt for all architectures and stored for verification by the security team.

Uploads waiting for acceptance or verification are only accessible by the security team. This is necessary since there might be fixes for security problems that cannot be disclosed yet.

If a member of the security team accepts a package it will be installed on security.debian.org as well as the proper <codename>-proposed-updates in ftp-master or non-US archive.

7.3.1.5 The security advisory

Security advisories are written and posted by the security team. However they certainly do not mind if a maintainer can supply (part of) the text for them. Information that should be in an advisory is described in Debian Security Advisories, Section 7.2.

7.4 Package signing in Debian

This section could also be titled "how to upgrade/update safely your Debian GNU/Linux system" and it deserves its own section basically because it is an important part of the Security Infrastructure. Package signing is an important issue since it avoids tampering of packages distributed in mirrors and of downloads with man-in-the-middle attacks. Automatic software update is an important feature but it's also important to remove security threats that could help the distribution of trojans and the compromise of systems during updates[51].

Debian does not provide signed packages but provides a mechanism available since Debian 4.0 (codename etch) to check for downloaded package's integrity[52]. For more information, see Secure apt, Section 7.4.2.

This issue is better described in the Strong Distribution HOWTO by V. Alex Brennen.

7.4.1 The current scheme for package signature checks

The current scheme for package signature checking using apt is:

By following the chain of MD5 sums apt is capable of verifying that a package originates from a a specific release. This is less flexible than signing each package one by one, but can be combined with that scheme too (see below).

This scheme is fully implemented in apt 0.6 and is available since the Debian 4.0 release. For more information see Secure apt, Section 7.4.2. Packages that provide a front-end to apt need to be modified to adapt to this new feature; this is the case of aptitude which was modified to adapt to this scheme. Front-ends currently known to work properly with this feature include aptitude and synaptic.

Package signing has been discussed in Debian for quite some time, for more information you can read: http://www.debian.org/News/weekly/2001/8/ and http://www.debian.org/News/weekly/2000/11/.

7.4.2 Secure apt

The apt 0.6 release, available since Debian 4.0 etch and later releases, includes apt-secure (also known as secure apt) which is a tool that will allow a system administrator to test the integrity of the packages downloaded through the above scheme. This release includes the tool apt-key for adding new keys to apt's keyring, which by default includes only the current Debian archive signing key.

These changes are based on the patch for apt (available in Bug #203741) which provides this implementation.

Secure apt works by checking the distribution through the Release file, as discussed in Per distribution release check, Section 7.4.3. Typically, this process will be transparent to the administrator although you will need to intervene every year[53] to add the new archive key when it is rotated, for more information on the steps an administrator needs to take a look at Safely adding a key, Section 7.4.3.7.

This feature is still under development, if you believe you find bugs in it, please, make first sure you are using the latest version (as this package might change quite a bit before it is finally released) and, if running the latest version, submit a bug against the apt package.

You can find more information at the wiki pages and the official documentation: Migration to APT 0.6 and APT Signature Checking.

7.4.3 Per distribution release check

This section describes how the distribution release check mechanism works, it was written by Joey Hess and is also available at the Debian Wiki.

7.4.3.1 Basic concepts

Here are a few basic concepts that you'll need to understand for the rest of this section.

A checksum is a method of taking a file and boiling it down to a reasonably short number that uniquely identifies the content of the file. This is a lot harder to do well than it might seem, and the most commonly used type of checksum, the MD5 sum, is in the process of being broken.

Public key cryptography is based on pairs of keys, a public key and a private key. The public key is given out to the world; the private key must be kept a secret. Anyone possessing the public key can encrypt a message so that it can only be read by someone possessing the private key. It's also possible to use a private key to sign a file, not encrypt it. If a private key is used to sign a file, then anyone who has the public key can check that the file was signed by that key. No one who doesn't have the private key can forge such a signature.

These keys are quite long numbers (1024 to 2048 digits or longer), and to make them easier to work with they have a key id, which is a shorter, 8 or 16 digit number that can be used to refer to them.

gpg is the tool used in secure apt to sign files and check their signatures.

apt-key is a program that is used to manage a keyring of gpg keys for secure apt. The keyring is kept in the file /etc/apt/trusted.gpg (not to be confused with the related but not very interesting /etc/apt/trustdb.gpg). apt-key can be used to show the keys in the keyring, and to add or remove a key.

7.4.3.2 Release checksums

A Debian archive contains a Release file, which is updated each time any of the packages in the archive change. Among other things, the Release file contains some MD5 sums of other files in the archive. An excerpt of an example Release file:

MD5Sum: 6b05b392f792ba5a436d590c129de21f 3453 Packages 1356479a23edda7a69f24eb8d6f4a14b 1131 Packages.gz 2a5167881adc9ad1a8864f281b1eb959 1715 Sources 88de3533bf6e054d1799f8e49b6aed8b 658 Sources.gz

The Release files also include SHA-1 checksums, which will be useful once MD5 sums become fully broken, however apt doesn't use them yet.

Now if we look inside a Packages file, we'll find more MD5 sums, one for each package listed in it. For example:

Package: uqm Priority: optional ... Filename: unstable/uqm_0.4.0-1_i386.deb Size: 580558 MD5sum: 864ec6157c1eea88acfef44d0f34d219

These two checksums can be used to verify that you have downloaded a correct copy of the Packages file, with a md5sum that matches the one in the Release file. And when it downloads an individual package, it can also check its md5sum against the content of the Packages file. If apt fails at either of these steps, it will abort.

None of this is new in secure apt, but it does provide the foundation. Notice that so far there is one file that apt doesn't have a way to check: The Release file. Secure apt is all about making apt verify the Release file before it does anything else with it, and plugging this hole, so that there is a chain of verification from the package that you are going to install all the way back to the provider of the package.

7.4.3.3 Verification of the Release file

To verify the Release file, a gpg signature is added for the Release file. This is put in a file named Release.gpg that is shipped alongside the Release file. It looks something like this [54] , although only gpg actually looks at its contents normally:

-----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.1 (GNU/Linux) iD8DBQBCqKO1nukh8wJbxY8RAsfHAJ9hu8oGNRAl2MSmP5+z2RZb6FJ8kACfWvEx UBGPVc7jbHHsg78EhMBlV/U= =x6og -----END PGP SIGNATURE-----

7.4.3.4 Check of Release.gpg by apt

Secure apt always downloads Release.gpg files when it's downloading Release files, and if it cannot download the Release.gpg, or if the signature is bad, it will complain, and will make note that the Packages files that the Release file points to, and all the packages listed therein, are from an untrusted source. Here's how it looks during an apt-get update:

W: GPG error: http://ftp.us.debian.org testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 010908312D230C5F

Note that the second half of the long number is the key id of the key that apt doesn't know about, in this case that's 2D230C5F.

If you ignore that warning and try to install a package later, apt will warn again:

WARNING: The following packages cannot be authenticated! libglib-perl libgtk2-perl Install these packages without verification [y/N]?

If you say Y here you have no way to know if the file you're getting is the package you're supposed to install, or if it's something else entirely that somebody that can intercept the communication against the server[55] has arranged for you, containing a nasty suprise.

Note that you can disable these checks by running apt with --allow-unauthenticated.

It's also worth noting that newer versions of the Debian installer use the same signed Release file mechanism during their debootstrap of the Debian base system, before apt is available, and that the installer even uses this system to verify pieces of itself that it downloads from the net. Also, Debian does not currently sign the Release files on its CDs; apt can be configured to always trust packages from CDs so this is not a large problem.

7.4.3.5 How to tell apt what to trust

So the security of the whole system depends on there being a Release.gpg file, which signs a Release file, and of apt checking that signature using gpg. To check the signature, it has to know the public key of the person who signed the file. These keys are kept in apt's own keyring (/etc/apt/trusted.gpg), and managing the keys is where secure apt comes in.

By default, Debian systems come preconfigured with the Debian archive key in the keyring.

# apt-key list /etc/apt/trusted.gpg -------------------- pub 1024D/4F368D5D 2005-01-31 [expires: 2006-01-31] uid Debian Archive Automatic Signing Key (2005) <ftpmaster@debian.org>

Here 4F368D5D is the key id, and notice that this key was only valid for a one year period. Debian rotates these keys as a last line of defense against some sort of security breach breaking a key.

That will make apt trust the official Debian archive, but if you add some other apt repository to /etc/apt/sources.list, you'll also have to give apt its key if you want apt to trust it. Once you have the key and have verified it, it's a simple matter of running apt-key add file to add it. Getting the key and verifying it are the trickier parts.

7.4.3.6 Finding the key for a repository

The debian-archive-keyring package is used to distribute keys to apt. Upgrades to this package can add (or remove) gpg keys for the main Debian archive.

For other archives, there is not yet a standard location where you can find the key for a given apt repository. There's a rough standard of putting the key up on the web page for the repository or as a file in the repository itself, but no real standard, so you might have to hunt for it.

The Debian archive signing key is available at http://ftp-master.debian.org/ziyi_key_2006.asc (replace 2006 with current year).[56]

gpg itself has a standard way to distribute keys, using a keyserver that gpg can download a key from and add it to its keyring. For example:

$ gpg --keyserver pgpkeys.mit.edu --recv-key 2D230C5F gpg: requesting key 2D230C5F from hkp server pgpkeys.mit.edu gpg: key 2D230C5F: public key "Debian Archive Automatic Signing Key (2006) <ftpm aster@debian.org>" imported gpg: Total number processed: 1 gpg: imported: 1

You can then export that key from your own keyring and feed it to apt-key:

$ gpg -a --export 2D230C5F | sudo apt-key add - gpg: no ultimately trusted keys found OK

The "gpg: no ultimately trusted keys found" warning means that gpg was not configured to ultimately trust a specific key. Trust settings are part of OpenPGPs Web-of-Trust which does not apply here. So there is no problem with this warning. In typical setups the user's own key is ultimately trusted.

7.4.3.7 Safely adding a key

By adding a key to apt's keyring, you're telling apt to trust everything signed by the key, and this lets you know for sure that apt won't install anything not signed by the person who possesses the private key. But if you're sufficiently paranoid, you can see that this just pushes things up a level, now instead of having to worry if a package, or a Release file is valid, you can worry about whether you've actually gotten the right key. Is the http://ftp-master.debian.org/ziyi_key_2006.asc file mentioned above really Debian's archive signing key, or has it been modified (or this document lies).

It's good to be paranoid in security, but verifying things from here is harder. gpg has the concept of a chain of trust, which can start at someone you're sure of, who signs someone's key, who signs some other key, etc., until you get to the archive key. If you're sufficiently paranoid you'll want to check that your archive key is signed by a key that you can trust, with a trust chain that goes back to someone you know personally. If you want to do this, visit a Debian conference or perhaps a local LUG for a key signing [57].

If you can't afford this level of paranoia, do whatever feels appropriate to you when adding a new apt source and a new key. Maybe you'll want to mail the person providing the key and verify it, or maybe you're willing to take your chances with downloading it and assuming you got the real thing. The important thing is that by reducing the problem to what archive keys to trust, secure apt lets you be as careful and secure as it suits you to be.

7.4.3.8 Verifying key integrity

You can verify the fingerprint as well as the signatures on the key. Retrieving the fingerprint can be done for multiple sources, you can check The Debian System Book, talk to Debian Developers on IRC, read the mailing list where the key change will be announced or any other additional means to verify the fingerprint. For example you can do this:

$ GET http://ftp-master.debian.org/ziyi_key_2006.asc | gpg --import gpg: key 2D230C5F: public key "Debian Archive Automatic Signing Key (2006) <ftpmaster&debian.org>" imported gpg: Total number processed: 1 gpg: imported: 1 $ gpg --check-sigs --fingerprint 2D230C5F pub 1024D/2D230C5F 2006-01-03 [expires: 2007-02-07] Key fingerprint = 0847 50FC 01A6 D388 A643 D869 0109 0831 2D23 0C5F uid Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.org> sig!3 2D230C5F 2006-01-03 Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.org> sig! 2A4E3EAA 2006-01-03 Anthony Towns <aj@azure.humbug.org.au> sig! 4F368D5D 2006-01-03 Debian Archive Automatic Signing Key (2005) <ftpmaster@debian.org> sig! 29982E5A 2006-01-04 Steve Langasek <vorlon@dodds.net> sig! FD6645AB 2006-01-04 Ryan Murray <rmurray@cyberhqz.com> sig! AB2A91F5 2006-01-04 James Troup <james@nocrew.org>

and then check the trust path from your key (or a key you trust) to at least one of the keys used to sign the archive key. If you are sufficiently paranoid you will tell apt to trust the key only if you find an acceptable path:

$ gpg --export -a 2D230C5F | sudo apt-key add - Ok

Note that the key is signed with the previous archive key, so theoretically you can just build on your previous trust.

7.4.3.9 Debian archive key yearly rotation

As mentioned above, the Debian archive signing key is changed each year, in January. Since secure apt is young, we don't have a great deal of experience with changing the key and there are still rough spots.

In January 2006, a new key for 2006 was made and the Release file began to be signed by it, but to try to avoid breaking systems that had the old 2005 key, the Release file was signed by that as well. The intent was that apt would accept one signature or the other depending on the key it had, but apt turned out to be buggy and refused to trust the file unless it had both keys and was able to check both signatures. This was fixed in apt version 0.6.43.1. There was also confusion about how the key was distributed to users who already had systems using secure apt; initially it was uploaded to the web site with no announcement and no real way to verify it and users were forced to download it by hand.

In January 2006, a new key for 2006 was made and the Release file began to be signed by it, but to try to avoid breaking systems that had the old 2005 key, the Release file was signed by that as well. In order to prevent confusion on the best distribution mechanism for users who already have systems using secure apt, the debian-archive-keyring package was introduced, which manages apt keyring updates.

7.4.3.10 Known release checking problems

One not so obvious problem is that if your clock is very far off, secure apt will not work. If it's set to a date in the past, such as 1999, apt will fail with an unhelpful message such as this:

W: GPG error: http://archive.progeny.com sid Release: Unknown error executing gpg

Although apt-key list will make the problem plain:

gpg: key 2D230C5F was created 192324901 seconds in the future (time warp or clock problem) gpg: key 2D230C5F was created 192324901 seconds in the future (time warp or clock problem) pub 1024D/2D230C5F 2006-01-03 uid Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.org>

If it's set to a date too far in the future, apt will treat the keys as expired.

Another problem you may encouter if using testing or unstable is that if you have not run apt-get update lately and apt-get install a package, apt might complain that it cannot be authenticated (why does it do this?). apt-get update will fix this.

7.4.3.11 Manual per distribution release check

In case you want to add now the additional security checks and don't want or cannot run the latest apt version[58] you can use the script below, provided by Anthony Towns. This script can automatically do some new security checks to allow the user to be sure that the software s/he's downloading matches the software Debian's distributing. This stops Debian developers from hacking into someone's system without the accountability provided by uploading to the main archive, or mirrors mirroring something almost, but not quite like Debian, or mirrors providing out of date copies of unstable with known security problems.

This sample code, renamed as apt-check-sigs, should be used in the following way:

# apt-get update # apt-check-sigs (...results...) # apt-get dist-upgrade

First you need to:

This is the example code for apt-check-sigs, the latest version can be retrieved from http://people.debian.org/~ajt/apt-check-sigs. This code is currently in beta, for more information read http://lists.debian.org/debian-devel/2002/debian-devel-200207/msg00421.html.

#!/bin/bash # Copyright (c) 2001 Anthony Towns <ajt@debian.org> # # This program is free software; you can redistribute it and/or modify # it under the terms of the GNU General Public License as published by # the Free Software Foundation; either version 2 of the License, or # (at your option) any later version. # # This program is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the # GNU General Public License for more details. rm -rf /tmp/apt-release-check mkdir /tmp/apt-release-check || exit 1 cd /tmp/apt-release-check >OK >MISSING >NOCHECK >BAD arch=`dpkg --print-installation-architecture` am_root () { [ `id -u` -eq 0 ] } get_md5sumsize () { cat "$1" | awk '/^MD5Sum:/,/^SHA1:/' | MYARG="$2" perl -ne '@f = split /\s+/; if ($f[3] eq $ENV{"MYARG"}) { print "$f[1] $f[2]\n"; exit(0); }' } checkit () { local FILE="$1" local LOOKUP="$2" Y="`get_md5sumsize Release "$LOOKUP"`" Y="`echo "$Y" | sed 's/^ *//;s/ */ /g'`" if [ ! -e "/var/lib/apt/lists/$FILE" ]; then if [ "$Y" = "" ]; then # No file, but not needed anyway echo "OK" return fi echo "$FILE" >>MISSING echo "MISSING $Y" return fi if [ "$Y" = "" ]; then echo "$FILE" >>NOCHECK echo "NOCHECK" return fi X="`md5sum < /var/lib/apt/lists/$FILE | cut -d\ -f1` `wc -c < /var/lib /apt/lists/$FILE`" X="`echo "$X" | sed 's/^ *//;s/ */ /g'`" if [ "$X" != "$Y" ]; then echo "$FILE" >>BAD echo "BAD" return fi echo "$FILE" >>OK echo "OK" } echo echo "Checking sources in /etc/apt/sources.list:" echo "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~" echo (echo "You should take care to ensure that the distributions you're downloading " echo "are the ones you think you are downloading, and that they are as up to" echo "date as you would expect (testing and unstable should be no more than" echo "two or three days out of date, stable-updates no more than a few weeks" echo "or a month)." ) | fmt echo cat /etc/apt/sources.list | sed 's/^ *//' | grep '^[^#]' | while read ty url dist comps; do if [ "${url%%:*}" = "http" -o "${url%%:*}" = "ftp" ]; then baseurl="${url#*://}" else continue fi echo "Source: ${ty} ${url} ${dist} ${comps}" rm -f Release Release.gpg lynx -reload -dump "${url}/dists/${dist}/Release" >/dev/null 2>&1 wget -q -O Release "${url}/dists/${dist}/Release" if ! grep -q '^' Release; then echo " * NO TOP-LEVEL Release FILE" >Release else origline=`sed -n 's/^Origin: *//p' Release | head -1` lablline=`sed -n 's/^Label: *//p' Release | head -1` suitline=`sed -n 's/^Suite: *//p' Release | head -1` codeline=`sed -n 's/^Codename: *//p' Release | head -1` dateline=`grep "^Date:" Release | head -1` dscrline=`grep "^Description:" Release | head -1` echo " o Origin: $origline/$lablline" echo " o Suite: $suitline/$codeline" echo " o $dateline" echo " o $dscrline" if [ "${dist%%/*}" != "$suitline" -a "${dist%%/*}" != "$codeline" ]; then echo " * WARNING: asked for $dist, got $suitline/$codeline" fi lynx -reload -dump "${url}/dists/${dist}/Release.gpg" >/dev/null 2>&1 wget -q -O Release.gpg "${url}/dists/${dist}/Release.gpg" gpgv --status-fd 3 Release.gpg Release 3>&1 >/dev/null 2>&1 | sed -n "s/^\[GNUPG:\] //p" | (okay=0; err=""; while read gpgcode rest; do if [ "$gpgcode" = "GOODSIG" ]; then if [ "$err" != "" ]; then echo " * Signed by ${err# } key: ${rest#* }" else echo " o Signed by: ${rest#* }" okay=1 fi err="" elif [ "$gpgcode" = "BADSIG" ]; then echo " * BAD SIGNATURE BY: ${rest#* }" err="" elif [ "$gpgcode" = "ERRSIG" ]; then echo " * COULDN'T CHECK SIGNATURE BY KEYID: ${rest %% *}" err="" elif [ "$gpgcode" = "SIGREVOKED" ]; then err="$err REVOKED" elif [ "$gpgcode" = "SIGEXPIRED" ]; then err="$err EXPIRED" fi done if [ "$okay" != 1 ]; then echo " * NO VALID SIGNATURE" >Release fi) fi okaycomps="" for comp in $comps; do if [ "$ty" = "deb" ]; then X=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/binary-${arch}/Release" | sed 's,//*,_,g'`" "${comp}/binary-${arch}/Release") Y=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/binary-${arch}/Packages" | sed 's,//*,_,g'`" "${comp}/binary-${arch}/Packages") if [ "$X $Y" = "OK OK" ]; then okaycomps="$okaycomps $comp" else echo " * PROBLEMS WITH $comp ($X, $Y)" fi elif [ "$ty" = "deb-src" ]; then X=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/source/Release" | sed 's,//*,_,g'`" "${comp}/source/Release") Y=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/source/Sources" | sed 's,//*,_,g'`" "${comp}/source/Sources") if [ "$X $Y" = "OK OK" ]; then okaycomps="$okaycomps $comp" else echo " * PROBLEMS WITH component $comp ($X, $Y)" fi fi done [ "$okaycomps" = "" ] || echo " o Okay:$okaycomps" echo done echo "Results" echo "~~~~~~~" echo allokay=true cd /tmp/apt-release-check diff <(cat BAD MISSING NOCHECK OK | sort) <(cd /var/lib/apt/lists && find . -type f -maxdepth 1 | sed 's,^\./,,g' | grep '_' | sort) | sed -n 's/^> //p' >UNVALIDATED cd /tmp/apt-release-check if grep -q ^ UNVALIDATED; then allokay=false (echo "The following files in /var/lib/apt/lists have not been validated." echo "This could turn out to be a harmless indication that this script" echo "is buggy or out of date, or it could let trojaned packages get onto" echo "your system." ) | fmt echo sed 's/^/ /' < UNVALIDATED echo fi if grep -q ^ BAD; then allokay=false (echo "The contents of the following files in /var/lib/apt/lists does not" echo "match what was expected. This may mean these sources are out of date," echo "that the archive is having problems, or that someone is actively" echo "using your mirror to distribute trojans." if am_root; then echo "The files have been renamed to have the extension .FAILED and" echo "will be ignored by apt." cat BAD | while read a; do mv /var/lib/apt/lists/$a /var/lib/apt/lists/${a}.FAILED done fi) | fmt echo sed 's/^/ /' < BAD echo fi if grep -q ^ MISSING; then allokay=false (echo "The following files from /var/lib/apt/lists were missing. This" echo "may cause you to miss out on updates to some vulnerable packages." ) | fmt echo sed 's/^/ /' < MISSING echo fi if grep -q ^ NOCHECK; then allokay=false (echo "The contents of the following files in /var/lib/apt/lists could not" echo "be validated due to the lack of a signed Release file, or the lack" echo "of an appropriate entry in a signed Release file. This probably" echo "means that the maintainers of these sources are slack, but may mean" echo "these sources are being actively used to distribute trojans." if am_root; then echo "The files have been renamed to have the extension .FAILED and" echo "will be ignored by apt." cat NOCHECK | while read a; do mv /var/lib/apt/lists/$a /var/lib/apt/lists/${a}.FAILED done fi) | fmt echo sed 's/^/ /' < NOCHECK echo fi if $allokay; then echo 'Everything seems okay!' echo fi rm -rf /tmp/apt-release-check

You might need to apply the following patch for sid since md5sum adds an '-' after the sum when the input is stdin:

@@ -37,7 +37,7 @@ local LOOKUP="$2" Y="`get_md5sumsize Release "$LOOKUP"`" - Y="`echo "$Y" | sed 's/^ *//;s/ */ /g'`" + Y="`echo "$Y" | sed 's/-//;s/^ *//;s/ */ /g'`" if [ ! -e "/var/lib/apt/lists/$FILE" ]; then if [ "$Y" = "" ]; then @@ -55,7 +55,7 @@ return fi X="`md5sum < /var/lib/apt/lists/$FILE` `wc -c < /var/lib/apt/lists/$FILE`" - X="`echo "$X" | sed 's/^ *//;s/ */ /g'`" + X="`echo "$X" | sed 's/-//;s/^ *//;s/ */ /g'`" if [ "$X" != "$Y" ]; then echo "$FILE" >>BAD echo "BAD"

7.4.4 Release check of non Debian sources

Notice that, when using the latest apt version (with secure apt) no extra effort should be required on your part unless you use non-Debian sources, in which case an extra confirmation step will be required by apt-get. This is avoided by providing Release and Release.gpg files in the non-Debian sources. The Release file can be generated with apt-ftparchive (available in apt-utils 0.5.0 and later), the Release.gpg is just a detached signature. To generate both follow this simple procedure:

$ rm -f dists/unstable/Release $ apt-ftparchive release dists/unstable > dists/unstable/Release $ gpg --sign -ba -o dists/unstable/Release.gpg dists/unstable/Release

7.4.5 Alternative per-package signing scheme

The additional scheme of signing each and every packages allows packages to be checked when they are no longer referenced by an existing Packages file, and also third-party packages where no Packages ever existed for them can be also used in Debian but will not be default scheme.

This package signing scheme can be implemented using debsig-verify and debsigs. These two packages can sign and verify embedded signatures in the .deb itself. Debian already has the capability to do this now, but there is no feature plan to implement the policy or other tools since the archive signing scheme is prefered. These tools are available for users and archive administrators that would rather use this scheme instead.

Latest dpkg versions (since 1.9.21) incorporate a patch that provides this functionality as soon as debsig-verify is installed.

NOTE: Currently /etc/dpkg/dpkg.cfg ships with "no-debsig" as per default.

NOTE2: Signatures from developers are currently stripped when they enter off the package archive since the currently preferred method is release checks as described previously.

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Securing Debian Manual

Version: 3.13, Mon, 10 Nov 2008 23:32:30 +0000

Javier Fernández-Sanguino Peña jfs@debian.org
Authors, Section 1.1