Attack Chain

1. An attacker logs in to the wger application as a gym trainer with limited privileges.
2. The trainer initiates a legitimate switch to a lower-privileged user account using the /en/user/<user_id>/trainer-login endpoint.
3. Upon successful switch, the application sets the trainer.identity flag in the user’s session, identifying the original trainer.
4. The attacker, now operating under the context of the lower-privileged user, makes another call to /en/user/<manager_id>/trainer-login, this time targeting a gym manager account.
5. Due to the presence of the trainer.identity flag, the permission check at wger/core/views/user.py:169 is bypassed, allowing the trainer-login to proceed without validating if the current user has gym_trainer permissions.
6. Because the user is no longer a trainer, the check on line 173 in wger/core/views/user.py is not reached, which would normally block escalation to manage_gym or manage_gyms permissions.
7. The attacker’s session is now elevated to that of the gym manager, granting them full administrative privileges.
8. The attacker can now access sensitive data, modify gym settings, and perform other actions as a gym manager.

Impact

Successful exploitation of this vulnerability (CVE-2026-43978) allows a malicious gym trainer to gain unauthorized access to sensitive information and administrative functions within the wger application. This includes the ability to view member data, modify contracts, manage gym configurations, and access other trainers’ and managers’ personal information. The attacker can effectively take over the gym manager’s account, potentially impacting all gym operations and member data.

Recommendation

• Apply the patch provided in the advisory to wger/core/views/user.py to fix the permission check logic (see the “How to fix” section in the advisory).
• Upgrade wger to a version greater than 2.5 to remediate CVE-2026-43978.
• Deploy the Sigma rule “Detect wger Trainer Login Privilege Escalation Attempt” to your SIEM to detect exploitation attempts.
• Monitor web server logs for unusual activity related to the /en/user/<user_id>/trainer-login endpoint, which may indicate attempts to exploit this vulnerability.