The MIT Kerberos team reports:
MIT krb incorrectly accepts an unkeyed checksum with DES session keys for version 2 (RFC 4121) of the GSS-API krb5 mechanism. An unauthenticated remote attacker can forge GSS tokens that are intended to be integrity-protected but unencrypted, if the targeted pre-existing application session uses a DES session key. MIT krb5 KDC incorrectly accepts RFC 3961 key-derivation checksums using RC4 keys when verifying the req-checksum in a KrbFastArmoredReq. An unauthenticated remote attacker has a 1/256 chance of swapping a client-issued KrbFastReq into a different KDC-REQ, if the armor key is RC4. The consequences are believed to be minor.