"On the evening of March 8, we logged out all users from GitHub.com who had logged in prior to 12:03 UTC on March 8. This was done to protect users from an extremely rare, but potentially serious, security vulnerability affecting a very small number of GitHub.com sessions. No user accounts were affected"

I really couldn't simplify much without losing information. Logging out users isn't the same as invalidation of authenticated sessions because this probably revoked access to bots / API calls etc.