Malicious code was discovered in the upstream tarballs of xz, starting with version 5.6.0.
Through a series of complex obfuscations, the liblzma build process extracts a prebuilt object file from a disguised test file existing in the source code, which is then used to modify specific functions in the liblzma code. This results in a modified liblzma library that can be used by any software linked against this library, intercepting and modifying the data interaction with this library.
Credits
Red Hat would like to thank Andres Freund for reporting this issue.
