ZITADEL 4.x before 4.17.3 and 3.x through 3.4.15 protects IdP intent tokens with unauthenticated, malleable encryption, allowing authenticated users to tamper with their own token so it is accepted for another user's external login intent. An attacker who predicts a victim's in-flight intent identifier and wins a timing race can call /v2/idp_intents or /v2/sessions to steal the victim's IdP tokens or hijack their session.