Gitea: Webhooks created by a collaborator keep firing after their repo access is revoked → ongoing real-time exfiltration of private repo content

Gitea — services/repository/collaboration.go (DeleteCollaboration) + webhook delivery

When a collaborator with admin permission on a private repo creates a webhook, that webhook keeps firing
after the collaborator's access is revoked. Gitea's revocation cleanup DeleteCollaboration removes the
collaboration record, recalculates accesses, drops watches, and unassigns issues — but it does not
remove or disable webhooks the user created, and webhook delivery never re-checks whether the creator still
has repo access. The former collaborator therefore receives the full payload (issue/comment bodies, commit
data) of all future repository events at their controlled endpoint, indefinitely and invisibly.

• services/repository/collaboration.go → DeleteCollaboration() — cleans watches/assignees only; no
  webhook cleanup.
• Webhook delivery path — fires on repo events without re-validating the creator's current access.

Using the provided reproduction materials:

1. Attacker (admin collaborator) creates a webhook → revoke access.
2. Control: GET /api/v1/repos/admin/wh-repo (attacker) → 404.
3. GET .../hooks → webhook still active=true.
4. Admin creates a new issue after revocation → the catcher receives action:"opened",
   issue.title:"CRITICAL SECRET: …", issue.body (sentinel private key), repository.private:true.
   (Runtime-confirmed on gitea/gitea:1.25.4. Catcher is an internal sentinel listener; the payload is a
   planted sentinel, not real data; nothing is sent to any external/metadata endpoint.)

Authenticated former admin-collaborator → ongoing real-time exfiltration of private content created after
revocation; invisible to the owner; scope crosses from the application boundary to data the user should no
longer access.

1. On revocation, delete/disable webhooks created by the removed collaborator (or hand them to the owner).
2. Re-validate the creator's current repo access before each webhook delivery.
3. At minimum, warn admins on revocation if the user created webhooks.

Reported as part of an incomplete-patch / authorization-residue measurement study (responsible disclosure).