Cisco Security Advisory-
The Cisco Secure PIX Firewall cannot distinguish between a forged TCP Reset (RST) packet and a genuine TCP RST packet. Any TCP/IP connection established through the Cisco Secure PIX Firewall can be terminated by a third party from the untrusted network if the connection can be uniquely determined. This vulnerability is independent of configuration. There is no workaround.
This vulnerability exists in all Cisco Secure PIX Firewall software releases up to and including 4.2(5), 4.4(4), 5.0(3) and 5.1(1). The defect has been assigned Cisco bug ID CSCdr11711.
This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20000711-pix-tcp-reset.
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This section provides details on affected products.
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When the Cisco Secure PIX Firewall receives a TCP Reset (RST) packet, it evaluates that packet based on data contained in the TCP packet header: source IP address, source port, destination IP address, and destination port. If these four values match an entry in the stateful inspection table, the associated connection will be reset. This affects only TCP sessions. Data exchange based on any other protocol is not affected.
To exploit this vulnerability, an attacker would need to have or infer:
- Detailed knowledge of the connection table in the Cisco Secure PIX Firewall prior to launching the attack, or
- Detailed knowledge of the source and destination IP Address and ports associated with a particular connection to be attacked
This particular vulnerability only affects the connection table (which keeps state regarding the connections being made through the device). It does not affect the translation table (in which address mappings are stored).
Cisco Secure PIX Firewall software has been fixed so that it now checks for a valid sequence number before removing a connection from the connection state table.
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There are no workarounds for this defect. Customers are urged to upgrade to the versions of code containing the fix for CSCdr11711.
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For the version listed in the left-most column below, customers should upgrade to at least the version shown in the center column. Please note the hardware requirements following the table.
Affected Version
Projected First Fixed Regular Release *
Date Available
All versions of Cisco Secure PIX up to version 4.2(5), including 2.7, 3.0, 3.1, 4.0, 4.1
4.4(5)
2000-06-09
All 4.3x and 4.4x versions up to and including version 4.4(4)
4.4(5)
2000-06-09
Version 5.0.x up to and including version 5.0(3)
5.1(2)
2000-06-09
Version 5.1.1
5.1(2)
2000-06-09
* Fix will carry forward into all later versions
A 128MB upgrade for the PIX Firewall is necessary if:
- Version 4.3 or 4.4 is used on a PIX 'Classic' (excluding PIX10000, PIX-510, PIX-520, and PIX-515)
- Version 5.0 is used on a PIX 'Classic', PIX10000, or PIX-510 (excluding PIX-520 and PIX-515)
As with any new software installation, customers planning to upgrade should carefully read the release notes and other relevant documentation before beginning any upgrade. Also, it is important to be certain that the new version of Cisco Secure PIX Firewall software is supported by your hardware and especially that enough memory is available.
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Cisco has received no reports of malicious exploitation of this vulnerability. The vulnerability was reported to Cisco by a customer and has been discussed on BUGTRAQ, a public full-disclosure security mailing list.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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