MITKRB5-SA-2011-007 KDC null pointer dereference in TGS handling [CVE-2011-1530] Dec 06 2011 07:07PM
Tom Yu (tlyu MIT EDU)
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MITKRB5-SA-2011-007

MIT krb5 Security Advisory 2011-007
Original release: 2011-12-06
Last update: 2011-12-06

Topic: KDC null pointer dereference in TGS handling

CVE-2011-1530
KDC null pointer dereference in TGS handling

CVSSv2 Vector: AV:N/AC:L/Au:S/C:N/I:C/A:C/E:H/RL:OF/RC:C

CVSSv2 Base Score: 6.8

Access Vector: Network
Access Complexity: Low
Authentication: Single
Confidentiality Impact: None
Integrity Impact: None
Availability Impact: Complete

CVSSv2 Temporal Score: 5.9

Exploitability: High
Remediation Level: Official Fix
Report Confidence: Confirmed

SUMMARY
=======

In releases krb5-1.9 and later, the KDC can crash due to a null
pointer dereference in code that handles TGS (Ticket Granting Service)
requests. The trigger condition is trivial to produce using
unmodified client software, but requires the ability to authenticate
as a principal in the KDC's realm.

IMPACT
======

An authenticated remote attacker can crash a KDC via null pointer
dereference.

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

* The KDC in krb5-1.9 and later is vulnerable. Earlier releases
predate the internal interface changes that led to this
vulnerability.

FIXES
=====

* Workaround: restart the KDC when it crashes, possibly using an
automated monitoring process.

* Apply the patch:

diff --git a/src/kdc/Makefile.in b/src/kdc/Makefile.in
index f46cad3..102fbaa 100644
- --- a/src/kdc/Makefile.in
+++ b/src/kdc/Makefile.in
@@ -67,6 +67,7 @@ check-unix:: rtest

check-pytests::
$(RUNPYTEST) $(srcdir)/t_workers.py $(PYTESTFLAGS)
+ $(RUNPYTEST) $(srcdir)/t_emptytgt.py $(PYTESTFLAGS)

install::
$(INSTALL_PROGRAM) krb5kdc ${DESTDIR}$(SERVER_BINDIR)/krb5kdc
diff --git a/src/kdc/do_tgs_req.c b/src/kdc/do_tgs_req.c
index c169c54..840a2ef 100644
- --- a/src/kdc/do_tgs_req.c
+++ b/src/kdc/do_tgs_req.c
@@ -243,7 +243,8 @@ tgt_again:
if (!tgs_1 || !data_eq(*server_1, *tgs_1)) {
errcode = find_alternate_tgs(request, &server);
firstpass = 0;
- - goto tgt_again;
+ if (errcode == 0)
+ goto tgt_again;
}
}
status = "UNKNOWN_SERVER";
diff --git a/src/kdc/t_emptytgt.py b/src/kdc/t_emptytgt.py
new file mode 100644
index 0000000..1760bcd
- --- /dev/null
+++ b/src/kdc/t_emptytgt.py
@@ -0,0 +1,8 @@
+#!/usr/bin/python
+from k5test import *
+
+realm = K5Realm(start_kadmind=False, create_host=False)
+output = realm.run_as_client([kvno, 'krbtgt/'], expected_code=1)
+if 'not found in Kerberos database' not in output:
+ fail('TGT lookup for empty realm failed in unexpected way')
+success('Empty tgt lookup.')

This patch is also available at

http://web.mit.edu/kerberos/advisories/2011-007-patch.txt

A PGP-signed patch is available at

http://web.mit.edu/kerberos/advisories/2011-007-patch.txt.asc

REFERENCES
==========

This announcement is posted at:

http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2011-007.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

CVSSv2:

http://www.first.org/cvss/cvss-guide.html
http://nvd.nist.gov/cvss.cfm?calculator&adv&version=2

CVE: CVE-2011-1530
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-1530

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

Simo Sorce discovered this vulnerability.

CONTACT
=======

The MIT Kerberos Team security contact address is
<krbcore-security (at) mit (dot) edu [email concealed]>. When sending sensitive information,
please PGP-encrypt it using the following key:

pub 2048R/56CD8F76 2010-12-29 [expires: 2012-02-01]
uid MIT Kerberos Team Security Contact <krbcore-security (at) mit (dot) edu [email concealed]>

DETAILS
=======

The process_tgs_req() function in the KDC has logic that attempts to
find an alternative service principal if the service principal in the
client's TGS-REQ is unknown. If the find_alternate_tgs() helper
function returns an error that is not KRB5_KDB_NOENTRY, it leaves the
server variable holding a null pointer. The process_tgs_req()
function improperly ignores that error, and proceeds to call functions
that dereference the null pointer.

Prior to krb5-1.9, the krb5_db_get_principal() function and related
interfaces had output parameters "more" and "nprincs". The krb5-1.9
release includes changes to these interfaces so that they no longer
have those outputs. Prior to krb5-1.9, the find_alternate_tgs()
function in the KDC had a void return type, and indicated failure by
setting its "more" and "nprincs" outputs appropriately. Its interface
changed in krb5-1.9 to instead return an error code, with
corresponding changes to process_tgs_req(); these changes to
process_tgs_req() were flawed and allow errors other than
KRB5_KDB_NOENTRY to cause a null pointer dereference.

The vulnerable code executes after the KDC authenticates the request,
so an attacker must have first obtained valid initial Kerberos
credentials for the target realm.

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

2011-12-06 original release

Copyright (C) 2011 Massachusetts Institute of Technology
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