MITKRB5-SA-2011-007 KDC null pointer dereference in TGS handling [CVE-2011-1530]

[Tom Yu]

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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.edu>.  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.edu>

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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