[c-nsp] Cisco Security Advisory: Vulnerability in Cisco Secure Access Control Server EAP-TLS Authentication

Tue Nov 2 10:20:00 EST 2004

A Cisco Secure Access Control Server (ACS) that is configured to use
Extensible Authentication Protocol-Transport Layer Security (EAP-TLS)
to authenticate users to the network will allow access to any user that
uses a cryptographically correct certificate as long as the user name is
valid. Cryptographically correct means that the certificate is in the
appropriate format and contains valid fields. The certificate can be
expired, or come from an untrusted Certificate Authority (CA) and still
be cryptographically correct.

Only version 3.3.1 of the Cisco Secure ACS for Windows and Cisco Secure
ACS Solution Engine is affected by this vulnerability. Cisco has made
free software available to address this problem.

This vulnerability has no effect, that is, user authentication is not
impacted, if EAP-TLS is configured in the Cisco Secure ACS with binary
comparison of user certificates as the only comparison method and if the
user entry in Lightweight Directory Access Protocol/Active Directory
(LDAP/AD) contains only valid certificates.

No exploitations of this vulnerability have been reported.

This advisory is available at
http://www.cisco.com/warp/public/707/cisco-sa-20041102-acs-eap-tls.shtml.

Affected Products
=================

Vulnerable Products
- -------------------

Only version 3.3.1 of the Cisco Secure ACS for Windows and Cisco Secure
ACS Solution Engine is affected by the vulnerability described in this
document.

To determine your Cisco Secure ACS software version you can log into the
Cisco Secure ACS. The first screen that is presented after a successful
login will show the version number in the following format: CiscoSecure
ACS Release 3.3(1) Build 16..

ACS versions may also be displayed as 003.003(001.16), where "16" is
the build number referenced on the ACS Administration Graphical User
Interface (GUI).

Products Confirmed Not Vulnerable
- ---------------------------------

Cisco Secure ACS for Unix and versions of Cisco Secure ACS for Windows
and Cisco Secure ACS Solution Engine prior to, and later than, 3.3.1 are
not affected by this vulnerability. Version 3.3.1 is the first version
in the 3.3.x series and version 3.3.2 is the first one that is not
affected by this vulnerability. No other Cisco products are currently
known to be affected by this vulnerability.

Details
=======

Cisco Secure Access Control Server provides centralized authentication,
authorization, and accounting (AAA) services to network devices
that function as AAA clients, such as a network access servers, PIX
firewalls, routers and switches. With Cisco Secure ACS, network
administrators can quickly administer accounts and globally change
levels of service offerings for entire groups of users.

EAP is a general protocol for authentication that supports multiple
authentication methods, such as token cards, Kerberos, one-time
passwords, certificates, public key authentication and smart cards.

TLS is a protocol that provides privacy and data integrity between
client/ server applications communicating over an unsecure network such
as the Internet.

EAP and TLS are both IETF RFC standards. The EAP protocol carries
initial authentication information, specifically EAPOL (the
encapsulation of EAP over LANs as established by IEEE 802.1X). TLS uses
certificates both for user authentication and for dynamic ephemeral
session key generation. The EAP-TLS authentication protocol uses the
certificates of Cisco Secure ACS and of the end-user client, enforcing
mutual authentication of the client and of Cisco Secure ACS. More
detailed information on EAP, TLS, and EAP-TLS can be found in the
following IETF RFCs: RFC 2284 (PPP Extensible Authentication Protocol),
RFC 2246 (The TLS Protocol), and RFC 2716 (PPP EAP TLS Authentication
Protocol).

The vulnerability described in this document affects user authentication
in the following way: when the EAP-TLS protocol is enabled in version
3.3.1 of Cisco Secure ACS for Windows or Cisco Secure ACS Solution
Engine, and network devices and services are configured to authenticate
users via the ACS, access will be granted to any user that uses a
certificate that is cryptographically correct as long as the user
name is valid and regardless of whether the certificate is from a
trusted Certificate Authority or whether the certificate has expired.
Cryptographically correct means that the certificate is in the
appropriate format and contains valid fields.

If EAP-TLS is configured (through the ACS global authentication page)
to perform binary comparison of user certificates as the only user
certificate comparison method, user authentication is not affected by
this vulnerability, as long as the user entry in LDAP/AD contains only
valid certificates. The reason user authentication is not affected
under this scenario is that when using the binary comparison method,
the certificate that is sent by the user's machine during the EAP-TLS
conversation is also compared to the user certificate that is stored in
the user entry in the LDAP/AD.

The vulnerability described here is documented in the Cisco Bug ID
CSCef62913.

Impact
======

Successful exploitation of this vulnerability could allow unauthorized
access to the entire network, provided that the Cisco Secure ACS is
being used to control network access.

Software Versions and Fixes
===========================

The vulnerability described in this advisory is fixed in version 3.3.2
of the Cisco Secure ACS for Windows software and of the Cisco Secure ACS
Solution Engine. If you are currently running the identified vulnerable
software and are using EAP-TLS, you should obtain fixed software, as
detailed below.

If you are running Cisco Secure ACS for Windows you can either upgrade
to version 3.3.2 or just replace the current CSCRL.dll Windows Dynamic
Link Library (DLL) in the Windows System32 folder with a fixed DLL
and restart Cisco Secure ACS for Windows. Replacing the DLL fixes the
problem and does not require a full upgrade.

The DLL fix can be downloaded from
http://www.cisco.com/pcgi-bin/tablebuild.pl/cs-acs-win. The file name
is CSCef62913-fix-ACSWIN-v3.3.1.16.zip. The accompanying Readme file
(available from the same location) contains detailed installation
instructions.

If you are using the Cisco Secure ACS Solution Engine you can also
upgrade to version 3.3.2 or run an upgrade package to replace the
affected DLL (an upgrade package is needed because there is no access to
the System32 directory when using the ACS Solution Engine.)

The upgrade package for the DLL fix can be downloaded from
http://www.cisco.com/pcgi-bin/tablebuild.pl/acs-soleng-3des. The file
name is CSCef62913-fix-ACSSE-v3.3.1.16.zip. The accompanying Readme
file (available from the same location) contains detailed installation
instructions.

Either upgrade method, a full upgrade to version 3.3.2, or just an
upgrade of the affected DLL, is provided free of charge.

Obtaining Fixed Software
========================

Customers with Service Contracts
- --------------------------------

Customers with contracts should obtain fixed software (free of charge)
through their regular update channels. For most customers, this means
that upgrades should be obtained through the Software Center on Cisco's
worldwide website at http://www.cisco.com.

Customers using Third-party Support Organizations
- -------------------------------------------------

Customers whose Cisco products are provided or maintained through prior
or existing agreement with third-party support organizations such as
Cisco Partners, authorized resellers, or service providers should
contact that support organization for assistance with the upgrade, which
should be free of charge.

Customers without Service Contracts
- -----------------------------------

Customers who purchase direct from Cisco but who do not hold a Cisco
service contract and customers who purchase through third-party vendors
but are unsuccessful at obtaining fixed software through their point of
sale should get their upgrades (free of charge) by contacting the Cisco
Technical Assistance Center (TAC). TAC contacts are as follows.

  * +1 800 553 2447 (toll free from within North America)
  * +1 408 526 7209 (toll call from anywhere in the world)
  * e-mail: tac at cisco.com

Please have your product serial number available and give the URL of
this notice as evidence of your entitlement to a free upgrade. Free
upgrades for non-contract customers must be requested through the TAC.

Please do not contact either "psirt at cisco.com" or
"security-alert at cisco.com" for software upgrades.

Workarounds
===========

If the user account resides in an LDAP/AD server and the user
certificate is stored in the user object in LDAP/AD, binary comparison
of user certificates can be configured in the ACS Global Authentication
page as the only allowed comparison method. This will work around the
vulnerability described in this document provided that only valid
certificates are stored in the user entry in LDAP/AD.

Please note that for this workaround to work, no other certificate
comparison methods can be enabled, that is, SAN and CN certificate
comparison must be disabled in the Global Authentication page.

Exploitation and Public Announcements
=====================================

The Cisco PSIRT is not aware of any public announcements or malicious
use of the vulnerability described in this advisory.

Status of This Notice: FINAL
============================

THIS ADVISORY IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY
KIND OF GUARANTEE OR WARRANTY. YOUR USE OF THE INFORMATION ON THE
ADVISORY OR MATERIALS LINKED FROM THE ADVISORY IS AT YOUR OWN RISK.
CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS NOTICE AT ANY TIME.

Distribution
============

This advisory will be posted on Cisco's worldwide website at
http://www.cisco.com/warp/public/707/cisco-sa-20041102-acs-eap-tls.shtml.

In addition to worldwide web posting, a text version of this notice is
clear-signed with the Cisco PSIRT PGP key and is posted to the following
e-mail and Usenet news recipients.

  * cust-security-announce at cisco.com
  * first-teams at first.org (includes CERT/CC)
  * bugtraq at securityfocus.com
  * vulnwatch at vulnwatch.org
  * cisco at spot.colorado.edu
  * cisco-nsp at puck.nether.net
  * full-disclosure at lists.netsys.com
  * comp.dcom.sys.cisco at newsgate.cisco.com

Future updates of this advisory, if any, will be placed on Cisco's
worldwide website, but may or may not be actively announced on mailing
lists or newsgroups. Users concerned about this problem are encouraged
to check the above URL for any updates.

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

+---------------------------------------------+
| Revision |                  | Initial       |
| 1.0      | 2004-November-02 | public        |
|          |                  | release.      |
+---------------------------------------------+

Cisco Security Procedures
=========================

Complete information on reporting security vulnerabilities
in Cisco products, obtaining assistance with security
incidents, and registering to receive security information
from Cisco, is available on Cisco's worldwide website at
http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html.
This includes instructions for press inquiries regarding
Cisco security notices. All Cisco security advisories are available at
http://www.cisco.com/go/psirt.
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