A flaw was found in the token endpoint of keycloak-services. The authorization code payload does not persist the issuing client identifier, and the endpoint relies on a mutable component of the code string to select the client session. An attacker who registers a client in the same realm, establishes an SSO session with a victim user, and intercepts an authorization code issued to a different client can rewrite the client identifier in the code. By redeeming this modified code at the token endpoint, the attacker can obtain tokens for their own client associated with the victim's identity. Successful exploitation allows an attacker to impersonate a user to an attacker-controlled client and access the victim's data exposed to that client.