From upstream commit [1]:
"""
In MIT krb5, when a server process uses the krb5_recvauth function, an
unauthenticated remote attacker can cause a NULL dereference by
sending a zero-byte version string, or a read beyond the end of
allocated storage by sending a non-null-terminated version string.
The example user-to-user server application (uuserver) is similarly
vulnerable to a zero-length or non-null-terminated principal name
string.

The krb5_recvauth function reads two version strings from the client
using krb5_read_message(), which produces a krb5_data structure
containing a length and a pointer to an octet sequence.  krb5_recvauth
assumes that the data pointer is a valid C string and passes it to
strcmp() to verify the versions.  If the client sends an empty octet
sequence, the data pointer will be NULL and strcmp() will dereference
a NULL pointer, causing the process to crash.  If the client sends a
non-null-terminated octet sequence, strcmp() will read beyond the end
of the allocated storage, possibly causing the process to crash.

uuserver similarly uses krb5_read_message() to read a client principal
name, and then passes it to printf() and krb5_parse_name() without
verifying that it is a valid C string.

The krb5_recvauth function is used by kpropd and the Kerberized
versions of the BSD rlogin and rsh daemons.  These daemons are usually
run out of inetd or in a mode which forks before processing incoming
connections, so a process crash will generally not result in a
complete denial of service.
"""

[1]: https://github.com/krb5/krb5/commit/102bb6ebf20f9174130c85c3b052ae104e5073ec