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                 MIT krb5 Security Advisory 2007-002

Original release: 2007-04-03
Last update: 2007-04-03

Topic: KDC, kadmind stack overflow in krb5_klog_syslog

Severity: CRITICAL

CVE: CVE-2007-0957
CERT: VU#704024

SUMMARY
=======

The library function krb5_klog_syslog() can write past the end of a
stack buffer.  The Kerberos administration daemon (kadmind) as well as
the KDC, are vulnerable.  Exploitation of this vulnerability is
probably simple.

This is a vulnerability in the the kadm5 library, which is used by the
KDC and kadmind, and possibly by some third-party applications.  It is
not a bug in the MIT krb5 protocol libraries or in the Kerberos
protocol.

IMPACT
======

An authenticated user may be able to cause a host running kadmind to
execute arbitrary code.

An authenticated user may be able to cause a KDC host to execute
arbitrary code.  Also, a user controlling a Kerberos realm sharing a
key with the target realm may be able to cause a KDC host to execute
arbitrary code.

Successful exploitation can compromise the Kerberos key database and
host security on the host running these programs.  (kadmind and the
KDC typically run as root.)  Unsuccessful exploitation attempts will
likely result in the affected program crashing.

Third-party applications which call krb5_klog_syslog() may also be
vulnerable.

AFFECTED SOFTWARE
=================

* MIT krb5 releases through krb5-1.6

FIXES
=====

* The upcoming krb5-1.6.1 release will contain a fix for this
  vulnerability.

Prior to that release you may:

* apply the patch

  The patch is available at

  http://web.mit.edu/kerberos/advisories/2007-002-patch.txt

  A PGP-signed patch is available at

  http://web.mit.edu/kerberos/advisories/2007-002-patch.txt.asc

  Systems which definitely provide vsnprintf() may not need the entire
  patch; see "DETAILS".

  Please note that releases prior to krb5-1.5 will require additional
  changes to the configure script src/lib/kadm5/configure in order to
  correctly detect the presence of vsnprintf().  krb5-1.5 and later
  releases already check for vsnprintf() in the top-level configure
  script, and do not have a separate src/lib/kadm5/configure script.

REFERENCES
==========

This announcement is posted at:

  http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2007-002-syslog.txt

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

        http://web.mit.edu/kerberos/advisories/index.html

The main MIT Kerberos web page is at:

        http://web.mit.edu/kerberos/index.html

CVE: CVE-2007-0957
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-0957

CERT: VU#704024
http://www.kb.cert.org/vuls/id/704024

ACKNOWLEDGMENTS
===============

We thank iDefense Labs for notifying us of this vulnerability.
iDefense credits an anonymous discoverer.

DETAILS
=======

krb5_klog_syslog() uses vsprintf() to format text into a fixed-length
stack buffer.  Format specifiers such as "%s" used in calls to
krb5_klog_syslog() may allow formatting of strings of sufficient
length to overwrite memory past the end of the stack buffer.

Certain strings received from the client by the kadmin daemon are not
truncated prior to logging.  Among these strings is the target
principal for the kadmin operation.

The KDC truncates most client-originated strings prior to logging.
One sort of string which is not truncated is a transited-realms
string.  A malicious KDC sharing a key with the target realm may issue
tickets with specially-crafted transited-realms strings to exploit
this vulnerability.  There are other places where an authenticated
user may cause the KDC to log a string which triggers the
vulnerability.

On a system where vsnprintf() is confirmed to be available, the
patches to files other than src/lib/kadm5/logger.c may not be
necessary to prevent a buffer overflow; these patches are still useful
to prevent malicious users from causing vsnprintf() to obliterate
useful log information by means of truncation.

REVISION HISTORY
================

2007-04-03      original release

Copyright (C) 2007 Massachusetts Institute of Technology
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