Episode 224

Posted on Friday, Apr 5, 2024
It’s been an absolutely manic week in the Linux security community as the news and reaction to the recent announcement of a backdoor in the xz-utils project was announced late last week, so we dive deep into this issue and discuss how it impacts Ubuntu and give some insights for what this means for the open source and Linux communities in the future.

Show Notes


It’s been an absolutely manic week in the Linux security community as the news and reaction to the recent announcement of a backdoor in the xz-utils project was announced late last week, so we dive deep into this issue and discuss how it impacts Ubuntu and give some insights for what this means for the open source and Linux communities in the future.

This week in Ubuntu Security Updates

20 unique CVEs addressed

[USN-6718-2] curl vulnerability

  • 1 CVEs addressed in Xenial ESM (16.04 ESM), Bionic ESM (18.04 ESM)

[USN-6719-1] util-linux vulnerability

  • 1 CVEs addressed in Focal (20.04 LTS), Jammy (22.04 LTS), Mantic (23.10)

[USN-6686-5] Linux kernel (Intel IoTG) vulnerabilities

[USN-6715-1] unixODBC vulnerability

  • 1 CVEs addressed in Xenial ESM (16.04 ESM), Bionic ESM (18.04 ESM), Focal (20.04 LTS), Jammy (22.04 LTS), Mantic (23.10)

[USN-6704-4] Linux kernel (Intel IoTG) vulnerabilities

[USN-6707-4] Linux kernel (Azure) vulnerabilities

[USN-6720-1] Cacti vulnerability

Goings on in Ubuntu Security Community

xz-utils backdoor and Ubuntu

  • https://www.openwall.com/lists/oss-security/2024/03/29/4
  • Late last week, 28th / 29th March backdoor in liblzma from xz-utils was disclosed to the open source community via oss-security mailing list - this was in the recent 5.6.0/5.6.1 releases from late Feb/early March this year
  • initially the impact was not entirely clear - assumed initially that it may impact only xz-utils and so only in handling of xz compressed data / files - but even with that assumption that perhaps it could then infect anything that got compressed/decompressed
  • within a few hours though became clear that the primary target was not xz-utils/liblzma itself but openssh - and the affect was to provide a backdoor into openssh that would allow the attacker to get remote access to any machine running ssh server with this backdoor liblzma installed
  • To paint a picture - this was all unfolding on late Thursday / Friday of the Easter break, so lots of folks were either EOD or on leave etc - and trying to grapple with a threat that we knew could possibly impact the impedending 24.04 LTS release
  • Good news:
  • TL;DR for Ubuntu, this version was only ever in the -proposed pocket for the currently in-development 24.04 release - not in any other Ubuntu versions - and was removed as soon as we became aware, so unless you are running the devel release AND you had manually opted to install this version from the -proposed pocket, you would not be affected - very lucky
  • https://discourse.ubuntu.com/t/xz-liblzma-security-update/43714
  • What do we know about this? A lot - there has been significant investigation (and speculation) since it was announced, both at the social side of things and the technical aspects of the backdoor itself
  • For the purpose of the podcast, we won’t go too deep into either but will try and cover the salient details
  • Regarding the inclusion of the backdoor itself - looks to have been a very long and patient campaign by the attacker, who slowly gained the trust of the upstream project over the last 2 years and likely pressured the maintainer via sock-puppet accounts to then get themselves added as an additional maintainer
  • Since then they seemed to be quite a good maintainer themselves - diligently adding new features and bug fixes etc over the past 2 years, but then suddenly introduced the backdoor into the most recent 2 releases
  • The method of introducing the backdoor was also interesting, in that it required 2 parts - the binary containing the backdoor and the code to get this compiled into the liblzma library at build time
    • The binary was committed into the upstream git repo disguised as an xz archive itself used as part of the test suite
    • The code to inject this into the build was NOT part of the git repo, but instead was just in the tarball prepared by the maintainer for the official release
    • And it used many levels of obfuscation to hide this backdoor within that fake test xz archive
  • So the attacker was not just patient but also very technically skilled - and not just multiple levels of obfuscation in the build process but the backdoor code itself contained many elements to try and make it harder to recognise and reverse engineer, presumably to allow it to hide in plain sight
    • although as we will see, this runtime obfuscation within the backdoor binary was what gave it away eventually
  • It’s often said that one of the advantages of Open Source is the huge community, which is summarised as Linus’ Law - with enough eyeballs all bugs are shallow - but sadly this wasn’t proven out in this case
  • Backdoor was not found by anyone doing review of the changes upstream or by the various distros like Debian / Fedora / OpenSUSE / Arch or even Ubuntu when incorporating this new version into their repos - but instead was found by Andres Freund, one of the maintainers of PostgreSQL, when they were looking to benchmark some new changes scheduled for the next PostgreSQL release
  • Luckily decided to use Debian unstable for this, and Debian had incorporated this new version into unstable a few weeks ago, and wanted to get the performance noise floor of the system as low as possible before doing benchmarking of PostgreSQL - noticed large transient CPU spikes in sshd and then eventually weird memory errors in sshd due to bugs in the initial version of the backdoor
  • After a lot of painstaking work was able to determine that liblzma was the culprit and appeared to contain some very strange code to hook into the authentication process of sshd when it was launched via systemd
  • Was able to trace that back to the aforementioned manually prepared tarballs of xz-utils on Github
  • The CPU spikes Andres observed were due to the use of things like a trie to lookup symbol names at runtime, rather than directly encoding them in the exploit binary, presumably to try and make reverse engineering of the binary harder (since you can’t just run strings on it and get any real sensible output)
  • Some excellent writeups have been done regarding both the technical aspects of the backdoor itself, as well as the process taken by the attacker to incorporate this into the xz-utils project - both from a community point of view and a technical point of view
  • From a technical point of view, the impact of this backdoor was to allow an attacker to get pre-authentication remote-code execution in sshd via a specific private key when connecting to the server - NOBUS
    • Hooked into the RSA certificate validation process in sshd, looking for a particular matching private key from the client - and if found would then proceed to execute arbitrary commands specified by the client without requiring usual authentication
    • By using this mechanism, nobody but the attacker can use the backdoor since they don’t have the matching private key - so the impact of the backdoor is somewhat limited
    • BUT the fact they targeted such a widely used and deployed package across a huge number of distros, means they essentially wanted a backdoor into any Linux machine in the future that only they could use
  • Interesting to try and speculate who the attacker could be (nation state?) and what their intended purpose was especially given this wide reaching goal of getting this into all the major Linux distros - but it would be just speculation
  • So rather than speculate, for the purpose of this podcast episode, interesting to look at the timeline as it concerned Ubuntu
  • The publishing history of the package is all visible in Launchpad
  • Packages in Ubuntu get inherited from Debian who also publish history
  • Upstream published the first backdoored version 5.6.0 via GitHub tarball on 24th Feb 2024
  • Debian incorporated this into unstable on 26th Feb
  • On 27th Feb, the Ubuntu Archive Auto-Sync bot copied this version into noble-proposed
  • Due to the ongoing time_t transition, sat it noble-proposed for the next month
  • Security team heard whispers of the possible backdoor just hours before it was publicly disclosed, and as soon as we heard of the possible backdoor, and realised that it only affected the version in-development noble-proposed we quickly notified the Archive Admin team who then deleted it from noble-proposed on 29th March, neutralising the main threat
  • Most important thing to know for Ubuntu, we have taken a very conservative approach - not only have we removed this version from noble-proposed as soon as we became aware, we are then rebuilding every binary package that got built since that compromised version was in noble-proposed originally - out of an abundance of caution - we don’t have any information that says this backdoor was doing anything other than what the various writeups have found so far, BUT we also can’t be certain that it didn’t have other functionality either - so being very cautious and rebuilding everything that was itself built since 27th Feb
  • As such, delayed the development of 24.04 so beta release is slipping by one week
  • One of the most interesting parts is the sheer luck that this was found - not by security researchers or maintainers but by a developer from Microsoft who happened to be looking for the right things at the right time and decided to be curious
  • Also for Ubuntu, luck that the time_t transition in Debian/Ubuntu caused many packages to be stuck in noble-proposed and not in the release pocket, else many Ubuntu users and developers would have been impacted if this had migrated to the noble release pocket
  • Also interesting that the attacker appears to have had quite a good grasp on OSS development practices and was quite persistent in trying to get this incorporated into distros - even urging for this new version to be synced to Ubuntu so that it would land in the upcoming noble release as recently as Thursday last week, just hours before the public disclosure
    • Not only could they do all the original social engineering work upstream, and technical work to develop and hide the backdoor, but could then interface with distros via their established practices to try and get them to incorporate their new backdoored version faster than they may have otherwise
  • Huge amount of work has been done to detail both the timeline of the attack as well as the technical details of both the code used to incorporate the backdoored code into the final liblzma binary during the build process, as well as the details of the backdoor itself and how it operates at runtime
  • In the end highlights both the challenges and strengths of OSS - lots of OSS projects have long dependency chains - in this case openssh when integrated with libsystemd which in turn used liblzma - and it is unclear who the maintainers and authors are or what procedures are in place for vetting and transferring ownership of OSS projects - all present significant challenges for OSS
  • However, significant strength of OSS is the visibility and ability for anyone to get involved, which is what we saw in the aftermath - despite all the advanced obfuscation techniques employed was able to be analysed in a matter of days by the community working together - and to analyse it in a huge amount of depth and in such an open way that it leaves little room for questioning the validity of the assessment - anyone can double check the work and come to the same conclusions
  • This isn’t the first software supply chain attack and likely isn’t even the first against an OSS project but it is a wake-up call to the OSS and Linux ecosystem

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