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:
team@security.debian.org
which only the members of the security team read.
security@debian.org which
is read by all Debian developers (including the security team). Mails sent to
this list are not published in the Internet (it's not a public mailing list).
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, if it is, a fix is worked 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
4.8). 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.
Debian Security Advisories are made whenever a security vulnerability is discovered that affects a Debian package. This 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 such as CVE, CERT/CC vulnerability notes or Bugtraq. This feature was
added to the website on june 2002.
One of the advantages of adding cross references to these vulnerability databases is that:
Since Debian is currently supported in a large number of arquitectures, 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 Thowns describes in a mail sent to the debian-devel-announce mailing list dated 8th june 2002. FIXME: add pointer).
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"):
dpkg-scanpackages, dpkg-scansources...)
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.
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.
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 11.3.5) or older distributions (see I use an older version of Debian, is it supported by the Debian Security Team?, Section 11.3.6).
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.
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:
pbuilder
and debootstrap packages can be helpful in that case).)
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.
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.
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 [18]
As of today (december 2001) 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.
The current (unimplemented) 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).
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/.
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.
In case you want to add now the additional security checks you can use the script below, provided by Anthony Thown. 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:
http://ftp-master.debian.org/ziyi_key_2002.asc
and add them to ~/.gnupg/trustedkeys.gpg (which is what
gpgv uses by default).
gpg --no-default-keyring --keyring trustedkeys.gpg --import ziyi_key_2002.asc
/etc/apt/sources.list lines that don't use the normal
"dists" structure, or change the script so that it works with them.
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"
Securing Debian Manual
2.6 10 October 2002Wed, 18 Sep 2002 14:09:35 +0200jfs@computer.org