ZITADEL is an open source identity management platform. Starting in version 2.50.0 and prior to versions 2.71.19, 3.4.4, and 4.6.6, a vulnerability in ZITADEL's federation process allowed auto-linking users from external identity providers to existing users in ZITADEL even if the corresponding IdP was not active or if the organization did not allow federated authentication. This vulnerability stems from the platform's failure to correctly check or enforce an organization's specific security settings during the authentication flow. An Organization Administrator can explicitly disable an IdP or disallow federation, but this setting was not being honored during the auto-linking process. This allowed an unauthenticated attacker to initiate a login using an IdP that should have been disabled for that organization. The platform would incorrectly validate the login and, based on a matching criteria, link the attacker's external identity to an existing internal user account. This may result in a full Account Takeover, bypassing the organization's mandated security controls. Note that accounts with MFA enabled can not be taken over by this attack. Also note that only IdPs create on an instance level would allow this to work. IdPs registered on another organization would always be denied in the (auto-)linking process. Versions 4.6.6, 3.4.4, and 2.71.19 resolve the issue by correctly validating the organization's login policy before auto-linking an external user. No known workarounds are available aside from upgrading.