GitProxy Approval Bypass When Pushing Multiple Branches

Summary

This vulnerability allows a user to push to the remote repository while bypassing policies and explicit approval. Since checks and plugins are skipped, code containing secrets or unwanted changes could be pushed into a repository.

Because it can allow policy violations to go undetected, we classify this as a High impact vulnerability.

Details

The source of the vulnerability is the push parser action parsePush.ts. It reads the first branch and parses it, while ignoring subsequent branches (silently letting them go through).

Although the fix involves multiple improvements to the commit and push parsing logic, the core solution is to prevent multiple branch pushes from going through in the first place:

if (refUpdates.length !== 1) {
  step.log('Invalid number of branch updates.');
  step.log(`Expected 1, but got ${refUpdates.length}`);
  step.setError('Your push has been blocked. Please make sure you are pushing to a single branch.');
  action.addStep(step);
  return action;
}

PoC

1. Make a commit on a branch:

git checkout -b safe-branch
echo "Approved code" > file.txt
git add .
git commit -m "Approved code"
git push proxy safe-branch

2. Wait for approval of safe-branch.

3. Make a commit on a separate branch with a secret, for example:

git checkout -b bad-branch
echo "SECRET=abc123" > .env
git add .
git commit -m "Bad code"

4. Push both at the same time:

git push proxy safe-branch bad-branch

Expected Result

Ideally, this would force checks to run for the second branch while sending it out for approval. Meanwhile, the first branch would be pushed to the remote. A simpler solution is to simply prevent multiple branch pushes.

Actual Result

Both branches get pushed to the remote, and second branch bypasses the proxy.

Impact

Attackers with push access can bypass review policies, potentially inserting unwanted/malicious code into a GitProxy-protected repository.

The vulnerability impacts all users or organizations relying on GitProxy to enforce policies and prevent unapproved changes. It requires no elevated privileges beyond regular push access, and no extra user interaction. It does however, require a GitProxy administrator or designated user (canUserApproveRejectPush) to approve the first push. It is much more likely that a well-meaning user would trigger this accidentally.