Advisory ID: cisco-sa-20040831-krb5
For Public Release 2004 August 31 18:30 UTC (GMT)
Two vulnerabilities in the
of Technology (MIT) Kerberos 5
implementation that affect Cisco VPN 3000
Series Concentrators have been announced by the MIT Kerberos Team.
Cisco VPN 3000 Series Concentrators authenticating users against a
Kerberos Key Distribution Center (KDC) may be vulnerable to remote code
execution and to Denial of Service (DoS) attacks. Cisco has made free software
available to address these problems.
Cisco VPN 3000 Series Concentrators not authenticating users against a
Kerberos Key Distribution Center (KDC) are not impacted.
No exploitations of these vulnerabilities have been reported.
This advisory is available at
This section provides details on affected products.
The following products have their Kerberos 5 implementation based on
MIT Kerberos code and are affected by these vulnerabilities:
Cisco VPN 3000 Series Concentrators. All 4.0.x software versions
prior to 4.0.5.B and all 4.1.x software versions prior to 4.1.5.B are
vulnerable. Versions prior to 4.0.x are not vulnerable since
they do not support Kerberos authentication.
Note that vulnerable products are impacted only if
they are configured to authenticate users against a Kerberos KDC.
The following products have Kerberos 5 support, but their
implementation is not based on MIT Kerberos, and therefore are not
affected by the vulnerabilities discussed in this advisory:
Cisco IOS® (Kerberos support available in
release 11.2 or later)
The following products do not have Kerberos 5 support and therefore
are not affected by these vulnerabilities:
Cisco PIX Firewall
Cisco Firewall Services Module (FWSM) for the Cisco Catalyst 6500
Series and Cisco 7600 Series routers
No other Cisco products are currently known to be affected by these
Kerberos is a secret-key network authentication protocol developed at
the Massachusetts Institute of Technology (MIT) that uses the Data Encryption
Standard (DES) cryptographic algorithm for encryption and authentication.
Kerberos was designed to authenticate requests for network resources. Kerberos,
like other secret-key systems, is based on the concept of a trusted third party
that performs secure verification of users and services. In the Kerberos
protocol, this trusted third party is called the Key Distribution Center (KDC).
The primary use of Kerberos is to verify that users and the network
services they use are really who and what they claim to be. To accomplish this,
a trusted Kerberos server issues tickets to users. These tickets, which have a
limited lifespan, are stored in a user's credential cache and can be used in
place of the standard username-and-password authentication mechanism.
The Kerberos credential scheme embodies a concept called "single
logon." This process requires authenticating a user once, and then allows
secure authentication (without encrypting another password) wherever that
user's credential is accepted.
Vulnerable Cisco devices using versions of Kerberos based on the MIT
implementation to authenticate users are affected by two vulnerabilities. The
first vulnerability consists of a double-free error that can happen under
certain error conditions, and that can potentially allow a remote attacker to
execute arbitrary code.
The second vulnerability consists of an infinite loop in the Abstract
Syntax Notation (ASN) 1 decoder that can be entered upon receipt of an ASN.1
SEQUENCE type with invalid Basic Encoding Rules (BER) encoding. This
vulnerability can be exploited by an attacker impersonating a legitimate
Kerberos KDC or application server to cause a client program to hang inside an
infinite loop, and thus creating a Denial of Service condition. This
vulnerability can also be exploited to cause a KDC or application server to
hang inside an infinite loop.
More information about these MIT Kerberos vulnerabilities is available
The information in these links is provided by MIT.
The two vulnerabilities described above are documented for the
Cisco VPN 3000 Series Concentrators in Bug IDs
registered customers only)
customers only) and
registered customers only)
Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.
CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.
Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.
Cisco has provided an FAQ to answer additional questions regarding CVSS at
Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
An exploitation of the double-free vulnerability could potentially give
an attacker control of the Cisco device and potentially compromise an entire
An exploitation of the "infinite loop in the ASN.1 decoder"
vulnerability could potentially take out of service an affected product. The
vulnerability could potentially be repeatedly exploited to keep the product out
of service until an upgrade can be performed.
The vulnerabilities described in this advisory are fixed in software
versions 4.0.5.B and later and 4.1.5.B and later of the Cisco VPN 3000
Series Concentrators. If you are currently running the identified
vulnerable software, you should obtain fixed software, as detailed below.
There is no workaround available to mitigate the effects of this
vulnerability Affected users should upgrade to a fixed version of the affected
Cisco has made free software available to address this vulnerability
for affected customers. Prior to deploying software, customers should consult
their maintenance provider or check the software for feature set compatibility
and known issues specific to their environment.
Customers may only install and expect support for the feature sets they
have purchased. By installing, downloading, accessing or otherwise using such
software upgrades, customers agree to be bound by the terms of Cisco's software
license terms found at
or as otherwise set forth at Cisco.com Downloads at
Do not contact either "firstname.lastname@example.org" or "email@example.com"
for software upgrades.
Customers with contracts should obtain upgraded software 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
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 guidance and assistance with the appropriate course of
action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific
customer situations such as product mix, network topology, traffic behavior,
and organizational mission. Due to the variety of affected products and
releases, customers should consult with their service provider or support
organization to ensure any applied workaround or fix is the most appropriate
for use in the intended network before it is deployed.
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 in obtaining fixed software through their point of sale should obtain software patches and bug fixes 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)
Have your product serial number available and provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
The Cisco PSIRT is not aware of any malicious exploitation of the
vulnerabilities described in this advisory.
These vulnerabilities were reported by the MIT Kerberos Team in
concert with the CERT Coordination Center.
These vulnerabilities may impact other products that are not provided
by Cisco. CERT/CC is coordinating the public disclosure of the impact these
vulnerabilities may have on other, non-Cisco products. This Cisco Security
Advisory is being published in coordination with CERT/CC.
The MIT Kerberos Team advisories for these vulnerabilities can be
found at http://web.mit.edu/kerberos/www/advisories/
(MITKRB5-SA-2004-002 and MITKRB5-SA-2004-003).
The CERT/CC advisories for these vulnerabilities can be found at
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A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.
This advisory will be posted on Cisco's worldwide website at
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.
firstname.lastname@example.org (includes CERT/CC)
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.
Initial public release.