Josh wrote recently about a serious security bug that appeared in Debian Linux back in 2006, and whether it was really a backdoor inserted by the NSA. (He concluded that it probably was not.)
Today I want to write about another incident, in 2003, in which someone tried to backdoor the Linux kernel. This one was definitely an attempt to insert a backdoor. But we don’t know who it was that made the attempt—and we probably never will.
Back in 2003 Linux used a system called BitKeeper to store the master copy of the Linux source code. If a developer wanted to propose a modification to the Linux code, they would submit their proposed change, and it would go through an organized approval process to decide whether the change would be accepted into the master code. Every change to the master code would come with a short explanation, which always included a pointer to the record of its approval.
— source freedom-to-tinker.com | Ed Felten | Oct 9, 2013