An authentication bypass flaw was found in the way krb5's certauth interface before 1.16.1 handled the validation of client certificates. A remote attacker able to communicate with the KDC could potentially use this flaw to impersonate arbitrary principals under rare and erroneous circumstances.
https://github.com/krb5/krb5/pull/694/commits/b7af544e50a4d8291524f590e20dd44430bf627d
https://github.com/krb5/krb5/pull/694/commits/50fe4074f188c2d4da0c421e96553acea8378db2
https://github.com/krb5/krb5/pull/694/commits/1de6ca2f2eb1fdbab51f1549a25a6903aefcc196
https://github.com/krb5/krb5/pull/694
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2017-7562