Specifically, SSH logins were consuming too many CPU cycles and were generating errors with valgrind, a utility for monitoring computer memory.

Through sheer luck and Freund’s careful eye, he eventually discovered the problems were the result of updates that had been made to xz Utils. On Friday, Freund took to the Open Source Security List to disclose the updates were the result of someone intentionally planting a backdoor in the compression software.

It is lucky that Andres Freund checked and found the issue in valgrind that was maybe intentionally or maybe unintentionally.

https://www.openwall.com/lists/oss-security/2024/03/29/4

I'm interested in figuring out what happened and more information on the contributor behind the attack.