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:
```go
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:
```go
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" buttons
Suggested Fix
Change the validation in internal/database/repo_collaboration.go line 129 from:
```go
if mode <= AccessModeNone || mode > AccessModeOwner {
```
to:
```go
if mode <= AccessModeNone || mode >= AccessModeOwner {
```
