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Ziggurat @sh.itjust.works

How does the xz incident impacts the average user ? #xz

My mastodon feed is full of IT security specialist talking about the xz affair where someone let a backdoor in some library.

But beside showing the two side of Free/Libre software (anybody can add a backdoor, and anybody can spot it), I have no idea how it impacts the average person. Is it a common library or something used only by specific application ? Would my home-grade router protects me ?

37 comments

  • Quick summary:

    • only impacts Debian and Linux distributions that utilize RPM for packages
    • only impacts cases where liblzma is compiled from a tarball, rather than cloned source repository or precompiled binary
    • only impacts x64 architecture
    • introduced in liblzma 5.6.0 which was released in late February so only impacts installs receiving updates to liblzma since then

    liblzma is a library for the lzma compression format. Loosely, this means it's used by various other pieces of software that need this type of compression, rather than being an application itself.

    It is very widely used. It comes installed on most major Linux distributions and is used by software like openssh, one of the standard remote connection packages.

    However, since it was only in the tarball, you wouldn't see it widely until debian, fedora, et al release a new version that includes the latest liblzma updates. This version hadn't been added to any of the stable release channels yet, so the typical user wouldn't have gotten it yet.

    I believe this would have gone out in debian 12.6 next week, and the attacker was actively petitioning fedora maintainers to get it added to fedora 40 & 41

    The interesting thing about this situation was how much effort the attacker put in to gain trust just to get to the point where they could do this, and how targeted the vulnerability seems to have been. They tried very hard to reduce the likelihood of being caught by only hitting a limited set of configurations

    • I'm still confused exactly what the circumstances would be where this worked as the attacker intended. Would simply having the infected liblzma version on the system create the vulnerability or does something have to happen to invoke it and then what? What's he chain of events that would have happened had this worked perfectly and gone undetected? I tried to read some of the more detailed analysis but the stuff went way over my head.

      Also, what about Mac OS? Can the package create any vulnerability there if installed via homebrew as it's reported to have done in some cases? Or is that environment also not right for it to work?

      • Here's how it was intended to work:

        • debian, fedora, or another RPM-based distribution updates references to liblzma to 5.6.x in their latest release
        • the package repository is updated (usually through automation) by getting the infected tarball and compiling it into an RPM or DEB which is added to the repo
        • if the package is built using glibc and the gnu linker, and for a system that uses systemd, the exploit is enabled during compilation of the x86-64 version of the package; otherwise the result is normal
        • when an application is installed that depends on liblzma, possibly during OS installation itself, the infected RPM/DEB package from the package repository is downloaded and installed (assuming the system matches the requirements above)
        • in this particular case, OpenSSH was the primary target; if the attacker wanted to, it could have targeted any web-facing service that uses liblzma such as OpenSSL + Apache/nginx, etc
        • when the OpenSSH server is started on an infected system, it loads the infected liblzma binary
        • the attacker starts an SSH connection to the infected server, having already known about the server or by scanning the internet for visible ssh servers
        • during creation of the SSH connection, the user has the option of trying to sign in using an RSA key. The attacker uses a specially formed RSA key only available to the attacker that also contains a chunk of code (the "payload") that they want executed on the server
        • liblzma is utilized to compress data in transit; when the infected liblzma decompresses the RSA key on the server, the exploit recognizes the attacker's special RSA key and executes the payload on the host system. Otherwise, the ssh session continues as normal

        This would not impact MacOS because you couldnt install the infected package, since it is only ever built for debian or RPM-based systems running systemd, using glibc and the gnu linker, and for x86-64. Unless I'm misunderstanding something, there is no way to get the compiled binaries that are infected to work on a MacOS system

        Additionally, I should note that I'm not exactly an expert on this stuff; I'm just in the security space and have been reading about this as it happens, so it's possible there are errors in my understanding. But that should at least give you the gist of the attack

  • I'm one of those security specialists (although not on mastodon). To be clear, if a vulnerable version of libxz were included in a distribution that we actually use; this would be an all hands on deck, drop everything until it is fixed emergency.

    Having said that, for an average user, it probably doesn't matter. First, many users just don't have the vulnerable version installed. All things considered, it was found very quickly; so only rolling release distros would have it. Additionally, it appears that only .deb or .rpm based distributions would have it. Not because they are particularly vulnerable, the attack explicitly tests for it.

    However, lets set all of this asside and assume a typical use is running a vulnerable system. In my assessment, the risk to them is still quite low. With most vulnerabilities, the hard part is discovering it. Once that happens, the barrier to exploiting it is relatively low, so you get a bunch of unrelated hackers trying to exploit any system they can find. This case is different; exploiting it requires the attackers private key. Even though the attack is now widely known, there is still only 1 organization capable of using it.

    Further, this attack was sophisticated. I'm not going to go as far as others in saying that only a state actor could do it. However, it is hard to think of anyone other than a state actor who would do it. Maybe a group of college kids doing it for the lolz research? But, if the motivation us lolz, I don't see them pivoting to do anything damaging with it. And even if they wanted to, there would still only be a handful of them. In short, this is one of those cases where obscurity works. Whoever did this attack does not know or care about Joe the Linux user; and they were probably never going to risk burning it by exploiting it on a large scale.

    However, setting all of that asside, suppose you were using vulnerable software, and someone with the private key is interested in your home system. First, you would need to be running OpenSSH on a remotely accessible interface. [0]. Second, you would need your firewall to allow remote SSH traffic. Third, you would need your router to have port forwarding enabled; and explicitly configured to forward traffic to your OpenSSH server [1].

    If all of that happens; then yes, you would be at risk.

    [0] Even though the attack itself is in the libxz library, it appears to specifically target OpenSSH.

    [1] Or, the attacker would need some other mechanism to get on the same network as you.

37 comments