How were Linux kernel servers rooted four years ago?

On August 28, 2011, the servers of the Linux kernel project were breached, a fact that was discovered only 17 days later. News of this leaked out in September and it became known that the intrusion had been effected by stealing some user’s credentials. But how this intrusion was elevated to root status was never revealed. Indeed, four years and three months later, we still don’t know.