A flaw was found in Keycloak's Account REST API session binding mechanism. The issue exists because the sid session ID claim is not included in the blocked claim list for Dynamic Client Registration DCR protocol mappers. A malicious DCR client owner can configure mappers to substitute the azp authorized party and sid claims in their access token with values derived from user-controlled profile fields. When the Account REST API processes this manipulated token, the identity token verification resolves the user session based on the provided sid without verifying that the subject sub claim in the token matches the user associated with that session. Consequently, the API returns the victim's session data. An attacker with low privileges can exploit this to read and write a victim's account profile, change the recovery email, and perform a full account takeover via password reset.