On Tuesday evening PST, we became aware of unauthorized attempts to access a large number of GitHub.com accounts. This appears to be the result of an attacker using lists of email addresses and passwords from other online services that have been compromised in the past, and trying them on GitHub accounts. We immediately began investigating, and found that the attacker had been able to log in to a number of GitHub accounts.
GitHub has not been hacked or compromised.
情報源: GitHub Security Update: Reused password attack