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Home » Security Advisories» NetBSD » NetBSD 2003-006: kerberos v4 cryptographic weakness

NetBSD 2003-006: kerberos v4 cryptographic weakness

A vulnerability has been found Kerberos protocol version 4, which may allow an attacker to impersonate any principal in a realm and therefore subvert a site's entire Kerberos authentication infrastructure.

  • Vendor: NetBSD
  • Vendor ID: 2003-006
  • Date: April 06, 2003


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NetBSD Security Advisory 2003-006
=================================

Topic: Cryptographic weaknesses in Kerberos v4 protocol


Version: NetBSD-current: source prior to March 20, 2003
NetBSD 1.6: affected
NetBSD-1.5.3: affected
NetBSD-1.5.2: affected
NetBSD-1.5.1: affected
NetBSD-1.5: affected
pkgsrc: prior to kth-krb4-1.2.1 or heimdal-0.5.1

Severity: Every user on a Kerberos 4 network can be compromised

Fixed: NetBSD-current: March 20, 2003
NetBSD-1.6 branch: March 22, 2003 (1.6.1 will include the fix)
NetBSD-1.5 branch: April 1, 2003
pkgsrc: kth-krb4-1.2.2, heimdal-0.5.2


Abstract
========

A cryptographic weakness in version 4 of the Kerberos protocol allows
an attacker to use a chosen-plaintext attack to impersonate any
principal in a realm. This attack subverts a site's entire Kerberos
authentication infrastructure.

Kerberos version 5 does not contain this cryptographic vulnerability.

Sites are not vulnerable if they have Kerberos v4 completely disabled,
including the disabling of any krb5 to krb4 translation services.


Technical Details
=================

An attacker controlling a krb4 shared cross-realm key can
impersonate any principal in the remote realm to any service in the
remote realm. This can lead to a root-level compromise of a KDC,
along with compromise of any hosts that rely on authentication
provided by that KDC.

This attack may be performed against cross-realm principals, thus
allowing an attacker to hop realms and compromise any realm that
transitively shares a cross-realm key with the attacker's local
realm.

Related, but more difficult attacks may be possible without
requiring the control of a shared cross-realm key. At the very
least, an attacker capable of creating arbitrary principal names in
the target realm may be able to perform the attack.

A leak has occurred of an unpublished paper containing enough
details about the vulnerability that an attacker familiar with the
krb4 protocol can easily construct an exploit. No exploit is known
to be circulating at this time, though.

These are PROTOCOL vulnerabilities; fixes inherently involve
restricting the functionality of the protocol.

The fixes are required for the KDC machine - patches are not needed
on the clients, if v4 is disabled on the server.


Solutions and Workarounds
=========================

If you can't upgrade to a newer version, make sure you disable all
cross-realm functionality, remove or randomize the cross-realm key.

You can use ``kinit --version'' do determine if you have a vulnerable system

current:

kinit (Heimdal 0.5nb2, KTH-KRB 1.2)
Copyright (c) 1999-2002 Kungliga Tekniska Högskolan
Send bug-reports to [email protected]

is secure/safe.


The following instructions describe how to upgrade your affected
binaries by updating your source tree and rebuilding and
installing a new version of Heimdal.

* NetBSD-current:

Systems running NetBSD-current dated from before 2003-03-20
should be upgraded to NetBSD-current dated 2003-03-21 or later.

The following directories need to be updated from the
netbsd-current CVS branch (aka HEAD):
crypto/dist/heimdal/kdc
include/heimdal

To update from CVS, re-build, and re-install your KDC binaries.
# cd src
# cvs update -d -P crypto/dist/heimdal/kdc include/heimdal
# cd crypto/dist/heimdal/kdc

# make USETOOLS=no cleandir dependall
# make USETOOLS=no install


* NetBSD 1.6:

The binary distribution of NetBSD 1.6 is vulnerable.

Systems running NetBSD 1.6 sources dated from before
2003-03-22 should be upgraded from NetBSD 1.6 sources dated
2003-03-23 or later.

NetBSD 1.6.1 will include the fix.

The following directories need to be updated from the
netbsd-1-6 CVS branch:
crypto/dist/heimdal/kdc
include/heimdal

To update from CVS, re-build, and re-install your KDC binaries.

# cd src
# cvs update -d -P -r netbsd-1-6 crypto/dist/heimdal/kdc include/heimdal
# cd crypto/dist/heimdal/kdc

# make USETOOLS=no cleandir dependall
# make USETOOLS=no install


* NetBSD 1.5, 1.5.1, 1.5.2, 1.5.3:

The binary distribution of NetBSD 1.5.3 is vulnerable.

Systems running NetBSD 1.5, 1.5.1, 1.5.2, or 1.5.3 sources dated
from before 2003-03-31 should be upgraded from NetBSD 1.5.*
sources dated 2003-04-01 or later.

The following directories need to be updated from the
netbsd-1-5 CVS branch:
crypto/dist/heimdal/kdc
include/heimdal

To update from CVS, re-build, and re-install your KDC binaries.

# cd src
# cvs update -d -P -r netbsd-1-5 crypto/dist/heimdal/kdc include/heimdal
# cd crypto/dist/heimdal/kdc

# make cleandir dependall
# make install



Thanks To
=========

Sam Hartman and Tom Yu for notifying us in the first place and
providing text for the advisory.

Steve Bellovin provided some hints that led MIT people to discover
this vulnerability.

Love Hornquist-Astrand for coordination of information exchange.


Revision History
================

2003-04-04 Initial release


More Information
================

Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at
ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2003-006.txt.asc

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.ORG/ and http://www.NetBSD.ORG/Security/.


Copyright 2003, The NetBSD Foundation, Inc. All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$NetBSD: NetBSD-SA2003-006.txt,v 1.6 2003/04/04 06:12:17 wiz Exp $

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