Arhont Ltd - Information Security
Arhont Advisory by: Konstantin V. Gavrilenko (http://www.arhont.com) http://www.hackingciscoexposed.com Arhont ref: arh200601-1 Advisory: Cisco PIX embryonic state machine 1b data DoS Class: design bug? Version: Tested on PIX535, PIX OS ver 6.3(4) Tested on PIX515E, PIX OS ver 7.0(4) Model Specific: Other versions might have the same bug
DETAILS Further to the advisory from Arhont Information Security released on 22/11/2005 and named Cisco PIX TCP Connection Prevention, I would like to report that it is possible to perform an additional DoS attack utilising the same flaw in the embryonic connection mechanism on the PIX, but from the outside interface.
It is possible to prevent new communication establishment to a specific port on a server located behind the PIX firewall, when a permanent static mapping is applied between a local and a global ip address, similar to the Network setup diagram below.
Network Setup Attacker ------ Internet ------ PIX ------ Server
By sending a legitimate packet and specifying a meaningless data in the data field of the packet, it is possible to disable communication between the source and destination port pair for the duration of approximately 120 seconds on PIXOS version 6 and 30 seconds on PIXOS version 7.
Although, it would take a lot of packets to disrupt the communication between the hosts completely, we assume that the attacker's aim is to prevent the communication to a specific service located on the machine behind the PIX firewall (e.g. HTTP/S, SMTP) and some other host on the Internet, whose source address can be spoofed. Depending on the bandwidth, it might take as little as 15 seconds to generate and send out 65535 packets with a custom source port. The attack can be performed using the interactive packet constructors such as hping, e.g.
if you want to prevent new communication establishment between SOURCE_IP source port 31337 and TARGET_IP destination port 80, execute: arhontus / # hping2 -a $SOURCE_IP -S -c 1 -s 31337 -p 80 $TARGET_IP -d 1
if you want to prevent new communication establishment between SOURCE_IP port ranges 0-63535 and TARGET_IP destination port 80, execute: arhontus / # hping2 -a $SOURCE_IP -S -s 0 -p 80 --faster $TARGET_IP -d 1
The attack was tested on two PIX 535 firewalls with 1Gb of RAM each performing static permanent mapping and running in the failover mode with PIXOS ver 6.3(4), and on a single PIX 515E with 64Mb of RAM running PIXOS ver 7.0(4)
RISK FACTOR: Medium
WORKAROUNDS: PSIRT response with workarounds to follow this disclosure
COMMUNICATION HISTORY: Issue discovered: 24/01/2006 PSIRT notified: 24/01/2006 Public disclosure: 07/03/2006
ADDITIONAL INFORMATION: *According to the Arhont Ltd. policy, all of the found vulnerabilities and security issues will be reported to the manufacturer at least 7 days before releasing them to the public domains (such as CERT and BUGTRAQ).
If you would like to get more information about this issue, please do not hesitate to contact Arhont team on firstname.lastname@example.org
-- Respectfully, Konstantin V. Gavrilenko
Managing Director Arhont Ltd - Information Security
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