xz

Over a period of over two years, an attacker using the name “Jia Tan” worked as a diligent, effective contributor to the xz compression library, eventually being granted commit access and maintainership. Using that access, they installed a very subtle, carefully hidden backdoor into liblzma, a part of xz that also happens to be a dependency of OpenSSH sshd on Debian, Ubuntu, Fedora, and other systemd-based Linux systems. That backdoor watches for the attacker sending hidden commands at the start of an SSH session, giving the attacker the ability to run an arbitrary command on the target system without logging in: unauthenticated, targeted remote code execution.