How does the xz incident impacts the average user ?

It doesn't.

Average person:

• not running Debian sid, Fedora nightly, ~~Arch~~, OpenSUSE Tumbleweed, or tbh any flavour of Linux. (Arch reportedly unafffected)
• ssh service not exposed publicly

The malicious code was discovered within ~~a day or two~~ a month of upload iirc and presumably very few people were affected by this. There's more to it but it's technical and not directly relevant to your question.

For the average person it has no practical impact. For those involved with or interested in software supply chain security, it's a big deal.

Edit:
Corrections:

• OpenSUSE Tumbleweed was affected; Arch received malicious package but due to how it is implemented did not result in compromised SSH service.
• Affected package was out in the wild for about a month, suggesting many more affected systems before malicious package was discovered and rolled back.

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

only someone running arch or debian sid or an bleeding edge rolling release on an internet exposed ssh port. the idea of that configuration would sound ludicrous. even so we should be building off git repos not tar balls.

the weird part this situation has made me feel safer. the amount of work that went into social engineering this and it only lasted a month tops for people that run distros that would just not be or should not be used as an exposed server ever.

it shows open source works. This is more embarrassing than anything and we deserve it. We need to pay core library devs and have a mechanism that core libraries can be handed off to a trusted org.while another upstream maintainer can be found or the project shut down and other projects move away from the un maintained project. When the person maintaining the project gets burned out or has other issues.

For exploits like this, most of the risk is in services you use. Websites, power company customer database, etc. are at risk of customer data dumps leaking.

Be extra alert for targeted scams and phishing, i.e. Phil from HBO Max customer department calling and asking to correct your payment details and he sounds very convincing because he knows your username, viewing history, etc.

And be extra fast with installing any security updates on all your stuff.

The average user? Likely doesn't. the package is for Linux, so that removes any windows users.

The version was only on "bleeding edge" releases, so if you are on a lts release you will likely be ok.

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.

I don't even have xz installed on my machine. Could anyone clue me in for what it is normally used?