MITKRB5-SA-2003-001: Multiple vulnerabilities in old releases of MIT Kerberos

Ken Raeburn raeburn at MIT.EDU
Tue Jan 28 16:58:13 EST 2003

Hash: SHA1

	    MIT krb5 Security Advisory 2003-001

Original Release Date: 2003-01-28

Topic: Multiple vulnerabilities in old releases of MIT Kerberos

Severity: CRITICAL: Remote user can crash KDC, and may be able to
          forge non-local identities and compromise the KDC or
          application servers.


Multiple vulnerabilities have been found in MIT Kerberos 5 releases
prior to release 1.2.5.  MIT recommends updating to 1.2.7 if possible.


* A remote user can crash the KDC.

* A user authenticated in a remote realm may be able to claim to be
  other non-local users to an application server.

* It may be possible for a user to gain access to the KDC system and


* All releases of MIT Kerberos 5 before 1.2.5.


MIT recommends updating to release 1.2.5 or later, preferably to the
latest release.  Patches specifically to fix these problems are not
available at this time.

This announcement and related security advisories may be found on the
MIT Kerberos security advisory page at:

The main MIT Kerberos web page is at:


Thanks to greg pryzby, Joseph Sokol-Margolis, Gerald Britton, E. Larry
Lidz, and CERT for reporting these problems.


Problem 1: KDC null pointer dereferences

Certain protocol requests, compliant with the protocol encoding scheme
but indicative of a client system most likely configured incorrectly,
can crash a KDC with a null pointer dereference.  We do not believe
any exploit to gain access to the KDC or otherwise alter its behavior
is possible on systems without storage mapped at address zero.  We
have not explored the effects of this on a system with mapped memory
at address zero.

The fallback and retransmit algorithm used in the MIT krb5 library
will cause an application not receiving a reply from a KDC to try
other KDCs in the same realm; it will iterate through this list a few
times, or until it gets a response.  Thus, one client may take down
multiple KDCs.

We believe this vulnerability is limited to the TGS-REQ exchange, that
is, cases where the user has already authenticated to the KDC or one
with which it shares inter-realm keys.  So (ignoring cases of
well-known passwords) there is an audit trail of sorts, even if it has
to be dug out of a core file, and it is not a simple, scriptable
attack against KDCs in general.


 - Start your KDC from inittab or a loop in a shell script.  (The
   inittab approach may not work well if the KDC is crashed too often
   in a short span of time.)

Thanks to greg pryzby <GregPryzby at> for reporting this problem.

Problem 2: realm transit checks

Realms with shared keys can impersonate people in other non-local
realms in certain cases.  It may be exploitable in various ways if
non-local principal names are on critical ACLs.

This vulnerability affects both the KDC and Kerberos application

This problem was fixed in the 1.2.3 release.  That release also added
a flag to the KDC config file that can be set to refuse untrusted
cross-realm authentication, in case application servers cannot be
updated quickly enough.  This is not recommended as a long-term
solution, because the current model we use says that the application
server is responsible for doing this validation, which allows (for
example) a service on a specific machine (perhaps one set up for
software testing) to be configured to know about authentication paths
known to the maintainer of the service, even if the maintainer of the
KDC does not trust these paths for general use within the realm.
Enforcing this limitation in the KDC takes this option away from the
maintainers of individual machines.


 - Delete or change inter-realm keys so inter-realm authentication is

 - Remove all non-local principals from all critical ACLs in services
   using old MIT Kerberos code to validate the realm transit path

Thanks to Joseph Sokol-Margolis <seph at> and Gerald Britton
<gbritton at> for finding this problem.

Problem 3: format strings

Older versions of the MIT KDC used strings containing Kerberos
principal names as printf-style format strings in logging routines.

At least some cases do not require successful authentication, so this
can be used as a remote, anonymous attack.

It is easy to crash the KDC with this exploit.  We do not know of any
exploits to gain access to the host system, but we do not rule out the

Workarounds: See under problem 1.  ***However, these do not address
the host access possibility.***

Thanks to E. Larry Lidz <ellidz at> for discovering
this problem.

Problem 4: bounds checking on data sizes

Some of our code does not do bounds checking on lengths before
allocating storage.  On some systems, attempting to allocate large
negative amounts of storage can crash the program.  Thus, some bogus
packets may crash the KDC or an application server using Kerberos.  We
do not believe this can be exploited to gain access to the host


 - start KDC in a loop in a script, or from inittab

 - do likewise for any server processes that need to handle multiple
   client connections

Thanks to CERT for bringing this to our attention.


2003-01-28	original release

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