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

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.