Software Engineering Institute

As described on the MIT Kerberos web site: "Kerberos is a network authentication protocol. It is designed to provide strong authentication for client/server applications by using secret-key cryptography." MIT Kerberos code is used in network applications from a variety of different vendors and is included in many UNIX and Linux distributions.

The MIT Kerberos krb524d daemon converts Kerberos 5 service tickets into Kerberos 4 service tickets. There is a double-free vulnerability in krb524d that can be triggered during the conversion of a cross-realm ticket. From MITKRB5-SA-2004-002:

The patch (introduced in krb5-1.2.8 and present in all subsequent
releases) for disabling krb4 cross-realm authentication in krb524d
introduced a double-free vulnerability.  If handle_classic_v4() denies
the conversion of a cross-realm ticket, v5tkt->enc_part2 gets freed
but not nulled, so do_connection() double-frees many things when it
subsequently calls krb5_free_ticket().

An unauthenticated, remote attacker to could execute arbitrary code on a system running krb524d. In many cases, this system also operates a KDC, so this vulnerability could allow an attacker to gain the master secret for a Kerberos realm, leading to compromise of the entire realm. An attacker may also be able to crash a system running krb524d, causing a denial of service.

Apply a patch

Apply the appropriate patch(es) referenced in MITKRB5-SA-2004-002 or specified by your vendor.

Upgrade

According to MITKRB5-SA-2004-002, "The upcoming krb5-1.3.5 release will contain fixes for these problems."

Restrict access

Depending on network architecture, it may be practical to restrict access to systems running krb524d (4444/udp) from untrusted networks such as the Internet. While this will help to limit the source of attacks, it will not prevent attacks from trusted hosts or networks or attackers who can successfully spoof their source addresses.