xz was not directly linked to ssh, and systemd itself was not providing the backdoor. The weakness is embedded into the architecture of glibc (which has spread to other systems like FreeBSD as well): https://github.com/robertdfrench/ifuncd-up

The entire argumentation here is ridiculous. There's a big jump from "IFUNC undermines RELRO" to "IFUNC is the issue". You could have gotten all but the same effect spawning a thread from a plain init or C++ constructor. No one should think that any relro, r^x or aslr or anything like this is going to deter anyone who can literally control the contents of the libraries which are linked in. They could, literally, spawn a copy of sshd with a patched config if necessary.

Sure, but distros not using systemd were not affected.

The only reason distros not using systemd were "not affected" is because this particular attack wasn't going after them. They were compromised nevertheless, their compromise was simply consequence-less due to attacker's choices of what to do after the compromise.

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