[ previous ] [ Contents ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ] [ 10 ] [ 11 ] [ 12 ] [ A ] [ B ] [ C ] [ D ] [ E ] [ F ] [ G ] [ H ] [ next ]
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:
[email protected]
which only the members of the security team read.
[email protected] 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 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.
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:
version number for the fix.
problem type.
whether it is remote or locally exploitable.
short description of the package.
description of the problem.
description of the exploit.
description of the fix.
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[48] of DSAs are not sent to the security mailing lists but are directly included in the website.
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:
it makes it easier for Debian users to see and track which general (published) advisories have already been covered by Debian.
system administrators can learn more about the vulnerability and its impact by following the cross references.
this information can be used to cross-check output from vulnerability scanners that include references to CVE to remove false positives (see Vulnerability assessment scanner X says my Debian system is vulnerable!, Section 12.2.1).
Debian Security Advisories were declared
CVE-Compatible[49]
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:
No Debian products are affected by that vulnerability.
There is not yet an advisory covering that vulnerability (the security issue
might have been reported as a security
bug but a fix has not been tested and uploaded).
An advisory was published before a CVE name was assigned to a given vulnerability (look for an update at the web site).
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"):
Someone finds a security problem.
Someone fixes the problem, and makes an upload to security-master.debian.org's incoming (this someone is usually a Security Team member but can be also a package maintainer with an appropriate fix that has contacted the Security Team previously). The Changelog includes a testing-security or stable-security as target distribution.
The upload gets checked and processed by a Debian system and moved into queue/accepted, and the buildds are notified. Files in here can be accessed by the security team and (somewhat indirectly) by the buildds.
Security-enabled buildds pick up the source package (prioritized over normal builds), build it, and send the logs to the security team.
The security team reply to the logs, and the newly built packages are uploaded to queue/unchecked, where they're processed by a Debian system, and moved into queue/accepted.
When the security team find the source package acceptable (i.e., that it's been correctly built for all applicable architectures and that it fixes the security hole and doesn't introduce new problems of its own) they run a script which:
installs the package into the security archive.
updates the Packages, Sources and
Release files of security.debian.org in the usual way
(dpkg-scanpackages, dpkg-scansources, ...).
sets up a template advisory that the security team can finish off.
(optionally) forwards the packages to the appropriate proposed-updates so that it can be included in the real archive as soon as possible.
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.
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.
If a developer learns of a security problem, either in his package or someone else's he should always contact the security team (at [email protected]). 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).
There are a few ways a developer can learn of a security problem:
he notices it on a public forum (mailing list, website, etc.).
someone files a bugreport (the Security tag should be used, or added by the developer).
someone informs him via private email.
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:
if it is a trivial problem (like insecure temporary files) there is no need to keep the problem a secret and a fix should be made and released.
if the problem is severe (remote exploitable, possibility to gain root privileges) it is preferable to share the information with other vendors and coordinate a release. The security team keeps contacts with the various organizations and individuals and can take care of that.
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.
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:
Make sure you target the right distribution in your debian/changelog. For stable this is stable-security and for testing this is testing-security. Do not target <codename>-proposed-updates.
Make sure the version number is proper. It has to be higher than the current package, but lower than package versions in later distributions. For testing this means there has to be a higher version in unstable. If there is none yet (testing and unstable have the same version for example) upload a new version to unstable first.
Do not make source-only uploads if your package has any binary-all packages. The buildd infrastructure will not build those.
Make sure when compiling a package you compile on a clean system which only has
packages installed from the distribution you are building for. If you do not
have such a system yourself you can try a debian.org machine (see
http://db.debian.org/machines.cgi) or set up a chroot (the
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-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.
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[50].
Debian does not provide signed packages but provides a mechanism available since Debian 4.0 (codename etch) to check for downloaded package's integrity[51]. For more information, see Secure apt, Section 7.4.2.
This issue is better described in the Strong
Distribution HOWTO by V. Alex Brennen.
The current scheme for package signature checking using apt is:
the Release file includes the MD5 sum of Packages.gz
(which contains the MD5 sums of packages) and will be signed. The signature is
one of a trusted source.
This signed Release file is downloaded by 'apt-get update' and
stored along with Packages.gz.
When a package is going to be installed, it is first downloaded, then the MD5 sum is generated.
The signed Release file is checked (signature ok) and it extracts
from it the MD5 sum for the Packages.gz file, the
Packages.gz checksum is generated and (if ok) the MD5 sum of the
downloaded package is extracted from it.
If the MD5 sum from the downloaded package is the same as the one in the
Packages.gz file the package will be installed, otherwise the
administrator will be alerted and the package will be left in the cache (so the
administrator can decide whether to install it or not). If the package is not
in the Packages.gz and the administrator has configured the system
to only install checked packages it will not be installed either.
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/.
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[52] 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.
This section describes how the distribution release check mechanism works, it
was written by Joey Hess and is also available at the Debian Wiki.
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.
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.
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 [53] , 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-----
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[54] 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.
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) <[email protected]>
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.
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).[55]
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
[email protected]>" 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.
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 [56].
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.
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) <[email protected]>
sig!3 2D230C5F 2006-01-03 Debian Archive Automatic Signing Key
(2006) <[email protected]>
sig! 2A4E3EAA 2006-01-03 Anthony Towns <[email protected]>
sig! 4F368D5D 2006-01-03 Debian Archive Automatic Signing Key
(2005) <[email protected]>
sig! 29982E5A 2006-01-04 Steve Langasek <[email protected]>
sig! FD6645AB 2006-01-04 Ryan Murray <[email protected]>
sig! AB2A91F5 2006-01-04 James Troup <[email protected]>
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.
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.
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) <[email protected]>
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.
In case you want to add now the additional security checks and don't want or cannot run the latest apt version[57] 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:
get the keys the archive software uses to sign Release files,
http://ftp-master.debian.org/ziyi_key_2006.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_2006.asc
remove any /etc/apt/sources.list lines that don't use the normal
"dists" structure, or change the script so that it works with them.
be prepared to ignore the fact that Debian security updates don't have signed
Release files, and that Sources files don't have
appropriate checksums in the Release file (yet).
be prepared to check that the appropriate sources are signed by the appropriate keys.
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 <[email protected]>
#
# 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"
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
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.
[ previous ] [ Contents ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ] [ 10 ] [ 11 ] [ 12 ] [ A ] [ B ] [ C ] [ D ] [ E ] [ F ] [ G ] [ H ] [ next ]
Securing Debian Manual
[email protected]