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Last Updated on May 12, 2011

Cisco ASA 5510 Adaptive Security Appliance Cluster
Image by Audric Leperdi via Flickr

Update May 12: TCP Split Handshake: Why Cisco ASA is not susceptible

On May, the 9th 2011, nearly in contemporary, Cisco Systems and Fortinet, the last two security vendors involved in the TCP Split Handshake affair, which had not yet released a fix for the encountered issue, released two separate posts indicating the result of a second session of tests performed with NSS Labs.

As you will probably know, Cisco Systems was not able to reproduce the issue on its labs and decided to perform a second joint session with NSS Labs on April, the 21st 2011 promising a definitive, resolutive post for the same day. I must confess I have been waiting for a while for the promised post, eager to know the outcome and the likely happy ending of the story. I still did not know I would have had to curb my hunger for knowledge for nearly 20 days (much more than initially expected), and (unfortunately) I would also have had to renounce to the happy ending as well (at least for Cisco).

Analyzing singularly (and in alphabetical order) the two vendors:

In an update to its initial post, dated May, the 9th 2001, Cisco stated that after a thorough investigation of the TCP Split Handshake issue raised by NSS Labs, the company has confirmed that the Cisco ASA firewall is not susceptible to this issue. In all test cases examined, the ASA operates as expected, providing protection in its default configuration against the Split-Handshake as defined in the original TCP Split Handshake paper. As a result, the Cisco PSIRT (Product Security Incident Response Team) closed this investigation on May 4th.

Moreover, during the two recent visits to NSS Labs, Cisco was presented with a number of scenarios, including new test cases that deviated from the original Split-Handshake scenario. The Cisco PSIRT collected traces and provided feedback to NSS Labs on all scenarios. In each case, Cisco demonstrated successful network protection through the default ASA configuration or the implementation of firewall policies that are fully supported, documented and used pervasively in enterprise deployments.

Similarly, in a nearly contemporary update to its initial post, Fortinet announced the release, on April the 20th 2011, of an update for their FortiGate platform to correctly handle and block the TCP split handshake attack technique. This fix was subsequently tested by NSS Labs, which recognized its effectiveness on permanently addressing the TCP split handshake issue with just the FortiGate firewall function enabled. The patch applies to FortiOS 4.0 MR2 and is available for download, for customers with a forticare contract, on the Company FortiCare support portal. An update to FortiOS 4.0 MR3 is scheduled in the near future.

All’s well that ends well?

Not really! NSS labs has released an update to their remediation guidance freely available, upon registration, at this link. If the document states that, after the update, the Fortinet platform is no more vulnerable to the TCP Split Handshake:

Update: On April 21, 2011 Fortinet provided NSS Labs FortiOS 4.0 MR2 Patch 6. NSS Labs has confirmed that with the patch applied, Fortinet provides protection against the TCP Split Handshake.

In case of Cisco the situation is not so univocally resolved:

Update on May 6: Over the past several weeks, NSS Labs has worked with Cisco, providing numerous configurations, PCAPs and live demonstrations of the TCP Split Handshake getting past a Cisco ASA. Our original results are unchanged, and ultimately Cisco did offer some mitigation steps. Unlike every other tested vendor, Cisco’s approach to defend against the TCP Split Handshake is based upon Access Control Lists (ACLs). An ACL centric approach is suboptimal since it requires firewall administrators to follow best practices as well as have a low-level understanding of how the TCP Split Handshake works in order to avoid an accidental “misconfiguration” that enables the attack. And there are some firewall configurations in which using an ACL will not be possible.

In practice, according to NSS Labs, it looks like (but this is my personal interpretation since the above phrase does not provide enough details), Cisco devices block the TCP Split Handshake if a proper Access Control List is in place. Unfortunately it is not specified if the ACL must permit the traffic (that is an allowed connection showing the TCP Split Handshake pattern is blocked) or must deny it (that is a blocked connection showing the TCP Split Handshake pattern is blocked as it should be). In any case the ACL-based approach is not considered optimal since it requires direct intervention (and configuration) from the Administrators (and a good knowledge of how TCP Split Handshake).

I must confess, if both assumptions are correct, that in any caseI do not completely agree with NSS Labs conclusions. Firstly in my ideal world firewall should be managed by skilled administrators knowing what they do and moreover which could be the impact of configuration changes on possible attack vectors (ok I did not know the occurrence of TCP Split Handshake before the NSS Labs affair, but nobody’s perfect!). Secondly if a critical resource should lack an ACL (or should be the unintended victim of an accidental “misconfiguration”), this could potentially be more dangerous than a “simple” TCP Split Handshake attack since in that case the target resource could be exposed to a pretty much wider range of threats…

Meanwhile I was too curious and I kindly asked to Cisco to provide more details… I will update the post as soon as I will have any…

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