CVE-2026-39976

Impact

Authentication Bypass for client_credentials tokens. the league/oauth2-server library sets the JWT sub claim to the client identifier (since there's no user). The token guard then passes this value to retrieveById() without validating it's actually a user identifier, potentially resolving an unrelated real user. Any machine-to-machine token can inadvertently authenticate as an actual user.

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Usage of EnsureClientIsResourceOwner middleware together with Passport::$clientUuids set to false, can result in resolving the user instead, as stated in the documentation. 

The underlying OAuth2 server sets the token's sub claim to the client's identifier for client credentials tokens. By default, Passport uses UUIDs for clients, so this cannot collide with a user's integer primary key. However, if you have set Passport::$clientUuids to false, a client credentials token may inadvertently resolve a user whose ID matches the client's ID. In such cases, using this middleware cannot guarantee that the incoming token is a client credentials token.

Patches

Patched in v13.7.1

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

Disallow usage of client_credentials. 

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References

• https://github.com/laravel/passport/issues/1900
• https://github.com/laravel/passport/pull/1901
• https://github.com/laravel/passport/pull/1902
• https://github.com/thephpleague/oauth2-server/issues/1456#issuecomment-2734989996