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Cisco VPN 3000 Kerberos Authentication Implementation Remote Code
Execution And DoS
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SUMMARY
<http://www.cisco.com/en/US/products/hw/vpndevc/ps2284/> The Cisco VPN
3000 Series Concentrators are "purpose-built, remote access virtual
private network (VPN) platforms that incorporate high availability, high
performance, and scalability with the most advanced encryption and
authentication techniques available today. Supported connectivity
mechanisms include IP security (IPSec), Point-to-Point Tunneling Protocol
(PPTP), Layer 2 Tunneling Protocol (L2TP) over IPSec, and Cisco WebVPN
(clientless secure sockets layer [SSL] browser-based connectivity)."
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.
DETAILS
Vulnerable Systems:
* Cisco VPN 3000 Series Concentrators versions 4.0.x, all versions prior
to 4.0.5.B
* Cisco VPN 3000 Series Concentrators versions 4.1.x all versions prior
to 4.1.5.B
Immune Systems:
* Cisco IOS (Kerberos support available in release 11.2 or later)
* Cisco CatOS
* Cisco PIX Firewall (no Kerberos 5 support)
* Cisco Firewall Services Module (FWSM) for the Cisco Catalyst 6500
Series and Cisco 7600 Series routers (no Kerberos 5 support)
Note that vulnerable products are impacted only if they are configured to
authenticate users against a Kerberos KDC.
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. An
attacker impersonating a legitimate Kerberos KDC or application server to
cause a client program to hang inside an infinite loop, thus creating a
Denial of Service condition, can exploit this vulnerability. This
vulnerability can also be exploited to cause a KDC or application server
to hang inside an infinite loop.
Impact
An exploitation of the double-free vulnerability could potentially give an
attacker control of the Cisco device and potentially compromise an entire
Kerberos realm. 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.
Vendor Status:
Cisco has already released updated software revisions for the affected
devices that fix the abovementioned vulnerabilities.
ADDITIONAL INFORMATION
The information has been provided by <mailto:psirt@cisco.com> Cisco
Systems Product Security Incident Response Team.
The original article can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20040831-krb5.shtml>
http://www.cisco.com/warp/public/707/cisco-sa-20040831-krb5.shtml
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