Gitea: Authorization Bypass via "Allow edits from maintainers" allows unauthorized commits to any readable repo

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 <upstream_url> <branch> — the push is accepted.

Reproduction

python3 poc.py --repo http://gitea:3000/victim/repo --user attacker --password attacker_pass