Some GitHub user accounts with weak passwords were recently compromised due to a brute force password-guessing attack. I want to take this opportunity to talk about our response to this specific incident and account security in general.
We sent an email to users with compromised accounts letting them know what to do. Their passwords have been reset and personal access tokens, OAuth authorizations, and SSH keys have all been revoked. Affected users will need to create a new, strong password and review their account for any suspicious activity. This investigation is ongoing and we will notify you if at any point we discover unauthorized activity relating to source code or sensitive account information.
Out of an abundance of caution, some user accounts may have been reset even if a strong password was being used. Activity on these accounts showed logins from IP addresses involved in this incident.
The Security History page logs important events involving your account. If you had a strong password or GitHub's two factor authentication enabled you may have still seen attempts to access your account that have failed.
While we aggressively rate-limit login attempts and passwords are stored properly, this incident has involved the use of nearly 40K unique IP addresses. These addresses were used to slowly brute force weak passwords or passwords used on multiple sites. We are working on additional rate-limiting measures to address this. In addition, you will no longer be able to login to GitHub.com with commonly-used weak passwords.
If you have any questions or concerns please let us know.