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Re: Cisco VPN Concentrator IKE resource exhaustion DoS Advisory

From: Eloy Paris (elpariscisco.com)
Date: Fri Jul 28 2006 - 16:57:52 CDT


This is a Cisco PSIRT response to an advisory published on July 26, 2006
by an unaffiliated third party, Roy Hills, of NTA Monitor Ltd, entitled
"Cisco VPN Concentrator IKE resource exhaustion DoS", and available at:


This issue is being tracked by the following Cisco Bug IDs:

* CSCse70811 (Cisco IOS software)

* CSCdt92467 (Cisco VPN 3000 Concentrators)

* CSCsb51032 (Cisco PIX firewalls)

We thank Roy Hills from NTA Monitor Ltd for reporting this issue to
Cisco. We greatly appreciate the opportunity to work with researchers
on security vulnerabilities, and welcome the opportunity to review and
assist in product reports.

The attack against the Internet Key Exchange (IKE) protocol described
in the NTA Monitor advisory exploits the stateless nature of the IKE
version 1 protocol. The goal of such an attack is to deplete the
resources available on a device to negotiate IKE security associations,
and block legitimate users from establishing a new security association.

This vulnerability is not related to a specific vendor implementation,
but to underlying issues in the IKE protocol, and may affect any device
which implements IKE version 1. Cisco devices implementing IKE version 1
include the PIX and ASA security appliances, Cisco IOS software, and the
VPN 3000 Series Concentrators.

Customers running Cisco IOS software can mitigate this vulnerability by
implementing the feature "Call Admission Control for IKE". Additional
information on this feature can be found at:

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a0080229125.html .

There are no workarounds to mitigate this vulnerability for other
affected devices.

Cisco will continue to investigate the possibility of implementing
software workarounds to minimize the impact of this vulnerability.

Cisco's statement and further information are available on the Cisco
public website at:


Please contact psirtcisco.com with any questions regarding this issue.


Eloy Paris.-
Product Security Incident Response Team (PSIRT)
Cisco Systems, Inc.

On Wed, Jul 26, 2006 at 02:56:29PM +0100, Roy Hills wrote:
> Cisco VPN Concentrator IKE resource exhaustion DoS Advisory
> 1. Overview
> NTA Monitor discovered a denial of service vulnerability in the Cisco
> VPN 3000 series concentrator products while performing a VPN security
> test for a customer in July 2005.
> The vulnerability affects Phase-1 of the IKE protocol. Both Main Mode
> and Aggressive Mode over both UDP and TCP transports are affected.
> The vulnerability allows an attacker to exhaust the IKE resources on
> a VPN concentrator by sending a high rate of IKE requests, which will
> prevent valid clients from connected or re-keying. The attack does
> not require a high bandwidth, so one attacker could potentially
> target many concentrators.
> This mechanism behind this vulnerability is similar to the well-known
> TCP SYN flood vulnerability.
> 2. Vulnerability Details
> The vulnerability allows an attacker to exhaust the IKE resources on
> a remote VPN concentrator by starting new IKE sessions faster than
> the concentrator expires them from its queue. By doing this, the
> attacker fills up the concentrator's queue, which prevents it from
> handling valid IKE requests.
> The exploit involves sending IKE Phase-1 packets containing an
> acceptable transform. It is not necessary to have valid credentials
> in order to exploit this vulnerability, as the problem occurs before
> the authentication stage. The vulnerability affects both Main Mode
> and Aggressive Mode, and both normal IKE over UDP and Cisco
> proprietary TCP-encapsulated IKE.
> In order to exploit the vulnerability, the attacker needs to send IKE
> packets at a rate which exceeds the Concentrator's IKE session expiry
> rate. Tests show that the target concentrator starts to be affected
> at a rate of 2 packets per second, and is becomes unusable at 10
> packets per second. As a minimal Main Mode packet with a single
> transform is 112 bytes long, 10 packets per second corresponds to a
> data rate of slightly less than 9,000 bits per second.
> The concentrator will remain unable to process IKE requests as long
> as the flow of packets continues. Once the flow stops, the
> concentrator will return to normal operation as the negotiation queue
> drains.
> It is not normally possible to block public inbound access to the IKE
> service on the VPN concentrator, because it is required for remote
> access IPsec operation. As IKE normally uses the UDP transport
> protocol, the attacker may forge the packet's source IP address to
> avoid identification, or to prevent the victim from blocking the
> traffic with ingress filtering. In addition, IDS/IPS systems will
> probably not be able to detect the attack, because the packets are
> valid IKE packets.
> It is possible for attackers to detect and fingerprint Cisco VPN
> concentrators using the IKE fingerprinting techniques that we have
> previously published in VPN security white papers. Therefore users
> should not assume that their concentrator is invisible just because
> it's not published in the DNS and is not running any TCP services.
> The symptoms are that the target concentrator won't respond to IKE
> requests from any source when all the negotiation slots are filled.
> This means that new clients will be unable to connect, and Phase-1
> re-keying attempts will fail. It is not known if Phase-2 re-keying is
> also affected. Traffic over existing VPN tunnels should not be
> affected until they need to re-key.
> The mechanism behind this vulnerability is similar to that behind the
> well-known TCP SYN flood issue. In both cases the target system has a
> stateful mechanism for recording outstanding negotiations, uses a
> fixed-size list to store negotiations in progress, and does not
> require any authentication in order to start a negotiation.
> 3. Example
> We are not planning to release examples of how to exploit this
> vulnerability until it has been addressed and users have had an
> opportunity to apply the fix or workaround.
> 4. Affected Versions
> The issue is believed to affect all models of Cisco VPN 3000
> Concentrator: 3005, 3015, 3020, 3030, 3060 and 3080. It is suspected
> that other cisco products that support IKE may also be affected, but
> this has not been confirmed.
> 5. Solution
> There is no known fix or workaround at this time.
> 6. Timeline
> The vulnerability was first discovered on 4th July 2005, and was
> reported to Cisco's security team (PSIRT) the same day. Cisco
> responded on 9th August 2005, but no further progress has been made.
> 7. References
> NTA Monitor advisory
> http://www.nta-monitor.com/posts/2006/07/cisco-concentrator-dos.html
> Roy Hills
> NTA Monitor Ltd
> --
> Roy Hills Tel: +44 1634 721855
> NTA Monitor Ltd FAX: +44 1634 721844
> 14 Ashford House, Beaufort Court,
> Medway City Estate, Email:
> Roy.Hillsnta-monitor.com
> Rochester, Kent ME2 4FA,
> UK WWW: http://www.nta-monitor.com/

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