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MITKRB5-SA-2010-005

MIT krb5 Security Advisory 2010-005
Original release: 2010-05-18

Topic: GSS-API library null pointer dereference

CVE-2010-1321

CVSSv2 Vector: AV:N/AC:L/Au:S/C:N/I:N/A:C/E:POC/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:  5.3

Exploitability:         Proof-of-Concept
Remediation Level:      Official Fix
Report Confidence:      Confirmed

SUMMARY
======
Certain invalid GSS-API tokens can cause a GSS-API acceptor (server)
to crash due to a null pointer dereference in the GSS-API library.

This is an implementation vulnerability in MIT krb5, and not a
vulnerability in the Kerberos protocol.

IMPACT
=====
An authenticated remote attacker can cause a GSS-API application
server (including the Kerberos administration daemon kadmind) to crash
by sending a malformed GSS-API token that induces a null pointer
dereference.

AFFECTED SOFTWARE
================
* kadmind and other GSS-API server applications in all known releases
  of MIT krb5, up to and including krb5-1.8.1

* third-party GSS-API server applications that link link against the
  GSS-API library in all known releases of MIT krb5, up to and
  including krb5-1.8.1

* Independent implementations of the krb5 GSS-API mechanism may be
  vulnerable, as the underlying bug is based on plausible (but
  invalid) assumptions about the Kerberos protocol.

FIXES
====
* The upcoming krb5-1.8.2 release and an upcoming krb5-1.7 series
  release will contain a fix for this vulnerability.

* Apply the following patch.  The patch was generated against
  krb5-1.8.1, but should also apply to krb5-1.7 series releases.

diff --git a/src/lib/gssapi/krb5/accept_sec_context.c b/src/lib/gssapi/krb5/accept_sec_context.c
index ce3075f..6241055 100644
- --- a/src/lib/gssapi/krb5/accept_sec_context.c
+++ b/src/lib/gssapi/krb5/accept_sec_context.c
@@ -607,6 +607,13 @@ kg_accept_krb5(minor_status, context_handle,
     }
 #endif
 
+    if (authdat->checksum == NULL) {
+        /* missing checksum counts as "inappropriate type" */
+        code = KRB5KRB_AP_ERR_INAPP_CKSUM;
+        major_status = GSS_S_FAILURE;
+        goto fail;
+    }
+
     if (authdat->checksum->checksum_type != CKSUMTYPE_KG_CB) {
         /* Samba does not send 0x8003 GSS-API checksums */
         krb5_boolean valid;



  This patch is also available at

http://web.mit.edu/kerberos/advisories/2010-005-patch.txt 

  A PGP-signed patch is available at

http://web.mit.edu/kerberos/advisories/2010-005-patch.txt.asc 


For the krb5-1.6 release:

http://web.mit.edu/kerberos/advisories/2010-005-patch_r16.txt 

PGP-signed patch for krb5-1.6:

http://web.mit.edu/kerberos/advisories/2010-005-patch_r16.txt.asc 

  Earlier releases may require minor porting.


REFERENCES
=========
This announcement is posted at:

http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2010-005.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-2010-1321
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2010-1321 

ACKNOWLEDGMENTS
==============
Thanks to Shawn Emery (Oracle) for reporting this vulnerability.

CONTACT
======
The MIT Kerberos Team security contact address is
. When sending sensitive information, 
please PGP-encrypt it using the following key:

pub   2048R/8B8DF501 2010-01-15 [expires: 2011-02-01]
uid MIT Kerberos Team Security Contact  

DETAILS
======
The krb5 GSS-API mechanism specification requires that the checksum
field in the authenticator of the Kerberos AP-REQ (which is optional
in the base Kerberos protocol) be present and contain specific
contents.  If the checksum field is missing, the decoded structure
contains a null pointer, which code called through
krb5_gss_accept_sec_context() dereferences without first checking for
a null pointer.

Independent implementations of the krb5 GSS-API mechanism may be
vulnerable because a developer might reasonably make the invalid
assumption that the authenticator checksum field is not empty (and
hence, that the C representation would not contain a null pointer).

REVISION HISTORY
===============
2010-05-18      original release

Copyright (C) 2010 Massachusetts Institute of Technology
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