Xz/liblzma: Bash-stage Obfuscation Explained

gynvael.coldwind//vx

2024-03-30: xz/liblzma: Bash-stage Obfuscation Explained

Yesterday Andres Freund emailed oss-security@ informing the community of the discovery of a backdoor in xz/liblzma, which affected OpenSSH server (huge respect for noticing and investigating this). Andres' email is an amazing summary of the whole drama, so I'll skip that. While admittedly most juicy and interesting part is the obfuscated binary with the backdoor, the part that caught my attention – and what this blogpost is about – is the initial part in bash and the simple-but-clever obfuscation methods used there. Note that this isn't a full description of what the bash stages do, but rather a write down of how each stage is obfuscated and extracted.

P.S. Check the comments under this post, there are some good remarks there.

Before we begin

We have to start with a few notes.

First of all, there are two versions of xz/liblzma affected: 5.6.0 and 5.6.1. Differences between them are minor, but do exist. I'll try to cover both of these.

Secondly, the bash part is split into three (four?) stages of interest, which I have named Stage 0 (that's the start code added in m4/build-to-host.m4) to Stage 2. I'll touch on the potential "Stage 3" as well, though I don't think it has fully materialized yet.

Please also note that the obfuscated/encrypted stages and later binary backdoor are hidden in two test files: tests/files/bad-3-corrupt_lzma2.xz and tests/files/good-large_compressed.lzma.

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