A double-free issue has been found in samba from 4.3.0 up to and including 4.9.2, where a user with a valid certificate or smart card can crash the Samba AD DC's KDC.
When configured to accept smart-card authentication, Samba's KDC willcall talloc_free() twice on the same memory if the principal in a validly signed certificate does not match the principal in the AS-REQ. This is only possible after authentication with a trusted certificate. talloc is robust against further corruption from a double-free with talloc_free() and directly calls abort(), terminating the KDC process. There is no further vulnerability associated with this issue, merely a denial of service.