GHSA-mm7c-rhg6-qr4r Gitea: Authorization Bypass via "Allow edits from maintainers" allows unauthorized commits to any readable repo

• Go code.gitea.io/gitea: < 1.26.2 → fixed in 1.26.2

Summary Any authenticated low-privilege user with read access to a repository can push arbitrary commits directly to that repository, bypassing all write-access checks. Vulnerability Gitea's "Allow edits from maintainers" PR option can be abused via reverse-fork PRs: 1. The web UI PR-create endpoint binds `allow_maintainer_edit=true` without verifying that the submitter has write access to the HEAD repository. 2. Gitea allows creating a PR where BASE = attacker's fork and HEAD = upstream target. The attacker is "maintainer" of the BASE (their own fork), so the flag is set against the upstream HEAD. 3. On `git push` over HTTP/SSH, Gitea relaxes the required access mode to `Read` when `SupportProcReceive` is enabled (`routers/web/repo/githttp.go`, `routers/private/serv.go`) and defers enforcement to the pre-receive hook. 4. The pre-receive hook calls `CanMaintainerWriteToBranch` (`models/issues/pull_list.go`), which finds the malicious PR, sees `AllowMaintainerEdit=true`, and checks whether the pusher has write access to the BASE repo. Since BASE is the attacker's own fork, the check passes and the push is authorized against the upstream. Exploitation 1. Attacker forks the target repository. 2. Attacker visits the web compare endpoint and creates a PR with `BASE = their_fork`, `HEAD = upstream`, and "Allow edits from maintainers" checked. 3. Attacker clones their fork, makes a commit, and runs `git push ` — the push is accepted. Reproduction ```bash python3 poc.py --repo http://gitea:3000/victim/repo --user attacker --password attacker_pass ``` poc.py Expected output: ``` [+] target: victim/my_repo default branch: main [*] forking -> attacker/my_repo_pocfork (202) [+] fork ready [+] malicious PR created (BASE=attacker fork, HEAD=upstream) remote: . Processing 1 references remote: Processed 1 references in total To http://192.168.101.20:3000/victim/my_repo.git e5c07b3..9a0b884 main -> main [+] latest commit on victim/my_repo@main: 'PoC: unauthorized commit via maintainer-edit bypass' [+] CONFIRMED: unauthorized push to upstream succeeded. ``` A `PWNED.txt` file will appear on the target repo's default branch, committed by the attacker who has no write access. Impact Full repository compromise. Any logged-in user can backdoor any repository they can read, including all public repositories on the instance. Suggested Fix Two independent checks are missing; both should be added for defense in depth: 1. At PR creation: before setting `AllowMaintainerEdit = true`, verify the submitter has write access to the HEAD repository. 2. In `CanMaintainerWriteToBranch`: verify that the PR's HEAD repo matches the repository being pushed to, and that the PR was opened by a legitimate owner/writer of the HEAD repository. Do not trust `AllowMaintainerEdit` solely based on BASE write access.