A session fixation vulnerability was found in Keycloak's login-actions endpoints. An unauthenticated attacker could exploit this flaw by pre-creating an authentication session and tricking a victim into visiting a maliciously crafted link. By leveraging the /login-actions/restart endpoint-which processes session handles without adequate CSRF protection or cookie ownership validation-an attacker can reset the authentication flow state.

This causes Single Sign-On (SSO) to authenticate the victim transparently upon clicking the link, allowing the attacker to hijack the required-action form without needing the victim's credentials. A successful exploit could lead to complete account takeover, including highly privileged administrative accounts.