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Given that a malicious party gains access to a GitHub repository, would it be possible for the adversary to delete or modify versioning information / history after she altered code stored on the repo? Is it possible to always detect such code changes after a compromise of a GitHub repo?

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The most damaging thing someone can do to a GitHub repository is to delete it. With enough access they could then create a new repository with the same name. After that, anything they wanted can be different from before.

Git lets you sign commits. Making and verifying those digital signatures is a good way to assure yourself that the changes retain integrity.

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    Git currently uses SHA-1, which is vulnerable to collision attacks. A sophisticated attacker could create a benign commit with the same fingerprint as a malicious change. – forest May 22 '18 at 4:42
  • Would you consider it rather realistic or unrealistic that an adversary with access to a GitHub repo, with commits that were not signed, is able to modify the commit history and hence is able to attack the integrity of the stored data without being noticed? – user1192748 May 22 '18 at 10:20
  • The tricky part to answer is “without being noticed”. I can certainly contrive a scenario with an undetected attack. Git makes it easy to spot when someone has altered the history of a repository underneath the work you're doing. On the other hand, Git also provides all the tools you'd want to completely change the history of that repo. – Tim Bannister May 23 '18 at 8:50

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