The MIT Kerberos team reports:
MIT krb5 (releases incorrectly accepts RFC 3961 key-derivation checksums using RC4 keys when verifying AD-SIGNEDPATH and AD-KDC-ISSUED authorization data. An authenticated remote attacker that controls a legitimate service principal has a 1/256 chance of forging the AD-SIGNEDPATH signature if the TGT key is RC4, allowing it to use self-generated "evidence" tickets for S4U2Proxy, instead of tickets obtained from the user or with S4U2Self. Configurations using RC4 for the TGT key are believed to be rare. An authenticated remote attacker has a 1/256 chance of forging AD-KDC-ISSUED signatures on authdata elements in tickets having an RC4 service key, resulting in privilege escalation against a service that relies on these signatures. There are no known uses of the KDC-ISSUED authdata container at this time.