Smuggling content in PR with refs/replace in GitHub

Summary

An incorrect comparison vulnerability was identified in GitHub Enterprise Server that allowed commit smuggling by displaying an incorrect diff within the GitHub pull request UI. To do so, an attacker would need write access to the repository. This vulnerability affected GitHub Enterprise Server versions 3.7.0 and above and was fixed in versions 3.7.9, 3.8.2, and 3.9.1. This vulnerability was reported via the GitHub Bug Bounty program.