Attack Chain

1. Attacker obtains a valid client_credentials token issued by Laravel Passport (versions 13.0.0 - 13.7.0). This could be a token intended for machine-to-machine communication.
2. The attacker sends a request to a protected endpoint of the Laravel application, including the client_credentials token in the Authorization header.
3. The Laravel Passport token guard extracts the JWT sub claim from the token. In vulnerable versions, this sub claim contains the client identifier.
4. The token guard calls retrieveById() using the client identifier from the sub claim as the user ID.
5. Due to the lack of validation, retrieveById() queries the user database, potentially finding a user whose ID matches the client identifier.
6. If a user with the matching ID is found, the application authenticates the request as that user, granting the attacker their privileges.
7. The attacker can then access resources and perform actions as the authenticated user.
8. The attacker exploits the user’s privileges to compromise data or perform unauthorized actions within the application.

Impact

Successful exploitation of CVE-2026-39976 allows attackers to bypass authentication and gain unauthorized access to user accounts in Laravel applications using affected versions of Laravel Passport. This can lead to data breaches, privilege escalation, and other malicious activities, depending on the privileges of the compromised user accounts. The severity of the impact depends on the application’s functionality and the sensitivity of the data it handles. Potentially all applications using Laravel Passport for authentication are vulnerable.

Recommendation

• Upgrade Laravel Passport to version 13.7.1 or later to patch CVE-2026-39976.
• Implement additional validation within the application’s authentication logic to verify that the user ID extracted from the JWT sub claim corresponds to a valid user, especially when using client_credentials tokens.
• Monitor application logs for unexpected authentication events or API requests originating from machine-to-machine tokens that are being authenticated as users. The webserver log source can be used for this monitoring.
• Deploy the Sigma rule provided to detect requests to protected endpoints with client_credentials tokens that are incorrectly authenticated as users.