An authentication bypass vulnerability was present in the GitHub Enterprise Server (GHES) when utilizing SAML single sign-on authentication with the optional encrypted assertions feature. This vulnerability allowed an attacker to forge a SAML response to provision and/or gain access to a user with site administrator privileges. Exploitation of this vulnerability would allow unauthorized access to the instance without requiring prior authentication. This vulnerability affected all versions of GitHub Enterprise Server prior to 3.13.0 and was fixed in versions 3.9.15, 3.10.12, 3.11.10 and 3.12.4. This vulnerability was reported via the GitHub Bug Bounty program.