The setup: ChatGPT used the open-source redis-py library to cache session data. A bug in that library meant cancelled requests could corrupt connections, returning unrelated data from the cache.

What leaked: During a nine-hour window, some users could see chat titles from another active user's history. Separately, OpenAI disclosed that 1.2% of ChatGPT Plus subscribers active during that window may have had billing information exposed — first and last name, email, payment address, the last four digits of a card, and expiration date.

The fix: OpenAI patched the library bug and added redundant checks to confirm cached data actually belongs to the requesting user.