Vulnerability GHSA-8cfq-jgf7-8m27

In OAuth2 silent authorization (prompt=none), when the scope parameter was empty or omitted, all scopes registered for the OAuth client became eligible for issuance. However, the prior consent check was performed against the empty requested scope set. As a result, an attacker could obtain an access token with broader scopes than the user had previously consented to, without requiring additional consent.

Impact

If an attacker controls an OAuth client capable of receiving authorization codes, and a user has already consented to only a subset of that client’s scopes, the attacker may be able to obtain a token containing all scopes registered for that client by sending an authorization request with prompt=none and an empty or omitted scope.

If the user has not consented to the client at all, consent_required is returned as usual.

Patches

This issue has been fixed in v3.28.3. After the fix, the consent check for prompt=none verifies the scopes that will actually be issued, rather than the empty requested scope set. If the user has not consented to all scopes after expansion, consent_required is returned.

traQ also recommends revoking any existing OAuth tokens created before the fix if they were potentially affected.

Workarounds

If users cannot update immediately, they should apply the following mitigations:

• Disable untrusted OAuth clients, minimize registered client scopes, and revoke existing tokens
• Reject authorization requests with prompt=none where scope is empty or omitted