[ottoke]

How did the changes in the binary test files tests/files/bad-3-corrupt_lzma2.xz and tests/files/good-large_compressed.lzma, and the makefile change in m4/build-to-host.m4) manifest to the Debian maintainer? Was there a chance of noticing something odd?

[Groxx]

mostly no, from my reading - it was a multi-stage chain of relatively normal looking things that added up to an exploit. helped by the tests involved using compressed data that wasn't human-readable.

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.

[sanjams]

The article references a technical write-up: https://research.swtch.com/xz-script

[XorNot]

Compression algorithms are deterministic over fixed data though (possibly with some effort).

There's no good reason to have opaque, non generated data in the repository and it should certainly be a red flag going forwards.

[dhx]

There's a few obvious gaps, seemingly still unsolved today:

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