GHSA-4565-r4x7-hg8j: Gogs Vulnerable to Privilege Escalation via Collaboration Access Mode Validation

Summary A repository admin collaborator can escalate their privileges to owner-level access by exploiting an off-by-one error in the ChangeCollaborationAccessMode function. Vulnerable Code In internal/database/repo_collaboration.go, line 129: func (r *Repository) ChangeCollaborationAccessMode(userID int64, mode AccessMode) error { // Discard invalid input if mode <= AccessModeNone || mode > AccessModeOwner { return nil } AccessModeOwner has value 4. The check mode > AccessModeOwner evaluates to 4 > 4 = false, allowing AccessModeOwner to pass through. The correct check should be mode >= AccessModeOwner. The web route at internal/route/repo/setting.go:413-416 takes the mode as a raw integer from query parameters: func ChangeCollaborationAccessMode(c *context.Context) { if err := c.Repo.Repository.ChangeCollaborationAccessMode( c.QueryInt64("uid"), database.AccessMode(c.QueryInt("mode"))); err != nil { This allows an admin collaborator to POST mode=4 and escalate to owner. Impact A repository admin collaborator (AccessModeAdmin = 3) can escalate to owner-level access (AccessModeOwner = 4), gaining the ability to: - Delete the repository - Transfer repository ownership to another user - Erase wiki data - Perform all other owner-only operations The access table is also updated (line 181), so the escalated permissions persist across sessions. Contrast The API route at internal/route/api/v1/repo_collaborators.go:46 uses ParseAccessMode() which only returns Read, Write, or Admin - never Owner. The API endpoint is not affected. Steps to Reproduce 1. User A creates a private repository 2. User A adds User B as a collaborator with Admin access (mode=3) 3. User B logs in and navigates to the repository settings collaboration page 4. User B sends a POST request: POST /{owner}/{repo}/settings/collaboration/access_mode?uid={B_uid}&mode=4 5. User B now has Owner access - the "Danger Zone" section appears with "Delete This Repository" and "Transfer Ownership" button