you can of course come up with ways it could have been caught, but the code doesn't stand out as abnormal in context. that's all that really matters, unless your build system is already rigid enough to prevent it, and has no exploitable flaws you don't know about.
finding a technical overview is annoyingly tricky, given all the non-technical blogspam after it, but e.g. https://securelist.com/xz-backdoor-story-part-1/112354/ looks pretty good from a skim.
There's no good reason to have opaque, non generated data in the repository and it should certainly be a red flag going forwards.
should not have been in any commit, which is basically necessary to prevent this case, almost definitely not. it's normal, and requiring all data to be generated just means extremely complicated generators for precise trigger conditions... where you can still hide malicious data. you just have to obfuscate it further. which does raise the difficulty, which is a good thing, but does not make it impossible.
I completely agree that it's a good/best practice, but hard-requiring everywhere it has significant costs for all the (overwhelmingly more common) legitimate cases.
The xz exploit depended on the absence of that explanation but accepting that it was necessary for unstated reasons.
Whereas it's entirely reasonable to have a test that says something like: "simulate an error where the header is corrupted with early nulls for the decoding logic" or something - i.e. an explanation, and then a generator which flips the targeted bits to their values.
Sure: you _could_ try inserting an exploit, but now changes to the code have to also surface plausible data changes inline with the thing they claim is being tested.
I wouldn't even regard that as a lot of work: why would a test like that exist, if not because someone has an explanation for the thing they want to test?
Instead of committing blobs, why not commit documented code which generates those blobs? For example, have a script compress a bunch of bytes of well-known data, then have it manually corrupt the bytes belonging to file size in the archive header.
1. Build environments may not be adequately sandboxed. Some distributions are better than others (Gentoo being an example of a better approach). The idea is that the package specification specifies the full list of files to be downloaded initially into a sandboxed build environment, and scripts in that build environment when executed are not able to then access any network interfaces, filesystem locations outside the build environment, etc. Even within a build of a particular software package, more advanced sandboxing may segregate test suite resources from code that is built so that a compromise of the test suite can't impact built executables, or compromised documentation resources can't be accessed during build or eventual execution of the software.
2. The open source community as a whole (but ultimately in the hands of distribution package maintainers) are not being alerted to and apply caution for unverified high entropy in source repositories. Similar in concept to nothing-up-my-sleeve numbers.[1] Typical examples of unverified high entropy where a supply chain attack can hide payload: images, videos, archives, PDF documents etc in test suites or bundled with software as documentation and/or general resources (such as splash screens in software). It may also include IVs/example keys in code or code comments, s-boxes or similar matrices or arrays of high entropy data which may not be obvious to human reviewers how the entropy is low (such as a well known AES s-box) rather than high and potentially undifferentiated from attacker shellcode. Ideally when a package maintainer goes to commit a new package or package update, are they alerted to unexplained high entropy information that ends up in the build environment sandbox and required to justify why this is OK?
[1] https://en.wikipedia.org/wiki/Nothing-up-my-sleeve_number