Arhont Ltd - Information Security

Arhont Advisory by:  Konstantin V. Gavrilenko (http://www.arhont.com)
             http://www.hackingciscoexposed.com
Arhont ref:    arh200511-1
Advisory:    Cisco PIX embryonic state machine TTL(n-1) 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 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 ------ Router ------ Server

By sending a legitimate packet and specifying TTL equal to n-1 of the
destination value, 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.

In order for the attack to succeed, an additional hop (router) should be
present between the PIX and the server, that would timeout the packet
returning the ICMP time exceeded in-transit.

Such setups can be easily identified using the TCPTraceroute to the open
port and returning repeating destination IP in the last two hops. e.g.

TCPTraceroute
5  xxx.xxx.xxx.32  18.952 ms  19.396 ms  20.438 ms
6  xxx.xxx.xxx.7  19.667 ms  22.174 ms  20.629 ms
7  xxx.xxx.xxx.68  29.286 ms  21.401 ms  19.935 ms
8  xxx.xxx.xxx.100  108.143 ms  42.783 ms *
9  xxx.xxx.xxx.100 [open]  32.268 ms  26.037 ms  23.569 ms

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 -t 8 $TARGET_IP

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 -t 8 $TARGET_IP


The attack was tested on two PIX 535 firewalls with 1Gb of RAM each
performing static permanent mapping and running in 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:  04/11/2005
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 info@arhont.com

-- 
Respectfully,
Konstantin V. Gavrilenko

Arhont Ltd - Information Security

web:    http://www.arhont.com
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e-mail: k.gavrilenko@arhont.com

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