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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 organisations 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.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 11.3.


7.2 Debian Security Advisories

Debian Security Advisories 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 MD5sums. 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 (once daily) so they might not be present inmediately, 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 [34] 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 provide 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[35] 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 standarised 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 since 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 yu might not find a given CVE name in published advisories either because:


7.3 Debian Security Build Infrastructure

Since Debian is currently supported in a large number of architectures, administrators sometimes wonder if a given arquitecture might take more time to receive security updates than another. As a matter of fact, except for rare circumstances, updates are available to all arquitectures 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 security.debian.org:/org/security.debian.org/queue/unchecked or ftp://security.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 arquitectures 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 publicising 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) arquitectures 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 behaviour 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.

Please note that the uptodate 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 11.3.7) or older distributions (see I use an older version of Debian, is it supported by the Debian Security Team?, Section 11.3.8).


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 can not 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 behaviour 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 sourcecode 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 import aspect: developers must always test your change. If their 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.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 can not 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 [36]

As of today (february 2004) Debian does not provide signed packages for the distribution and the woody release (3.0) does not integrate that feature. There is a solution for signed packages which will be, hopefully, provided in the next release (sarge).

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


7.4.1 The proposed 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).

Currently, this scheme is fully implemented in apt 0.6, which is available in the experimental distribution. This changes are based on the patch for apt (available in Bug #203741) which provides this implementation. Please check it out and provide information to the BTS in order to help get this included in the main release since it still needs extensive testing.

Notice that, when using this experimental apt version 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 0.5.17 and later, and apt 0.6.1 and later), the Release.gpg is just a dettached 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

apt-key is available for adding new keys to apt's keyring, which by default includes only the current Debian archive signing key.

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 Checking distribution releases

In case you want to add now the additional security checks and don't want to run the experimental apt version (although we would really appreciate testing of it) 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
     # This script is copyright (c) 2001, Anthony Towns
     #
     # 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` `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
             wget -q -O Release "${url}/dists/${dist}/Release"
     
             if ! grep -q '^' Release; then
                     echo "  * NO TOP-LEVEL Release FILE"
             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
     
                     wget -q -O Release.gpg "${url}/dists/${dist}/Release.gpg"
                     sigline="`gpgv --status-fd 3 Release.gpg Release 3>&1 >/dev/null 2>&1 | sed -n "s/^\[GNUPG:\] GOODSIG [0-9A-Fa-f]* //p"`"
                     if [ "$sigline" ]; then
                             echo "  o Signed by: $sigline"
                     else
                             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"

7.4.3 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-verifyand debsigs. These two packages can sign and verify embeded signatures in the .deb itself. Debian already has the capability to do this now, but implementing the policy and tools won't be started until after woody releases.

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

2.99 18 April 2004Wed, 3 Mar 2004 09:18:54 +0100

Javier Fernández-Sanguino Peña jfs@computer.org