Last night GitHub logged out many users by invalidating their GitHub.com sessions to protect user accounts against a potentially serious security vulnerability. The vulnerability stemmed from a rare race condition vulnerability in which a GitHub user’s login session was misrouted to the web browser of another logged-in user, giving the latter an authenticated session cookie of and access to the former user’s account. The company states that the underlying bug was present for a cumulative period of under two weeks at certain points in time between February 8th and March 5th.
Source: https://www.bleepingcomputer.com/news/security/github-fixes-bug-causing-users-to-log-into-other-accounts/