Simon Willison’s Weblog

18th March 2021 - Link Blog

How we found and fixed a rare race condition in our session handling. GitHub had a terrifying bug this month where a user reported suddenly being signed in as another user. This is a particularly great example of a security incident report, explaining how GitHub identified the underlying bug, what caused it and the steps they are taking to ensure bugs like that never happen in the future. The root cause was a convoluted sequence of events which could cause a Ruby Hash to be accidentally shared between two requests, caused as a result of a new background thread that was introduced as a performance optimization.