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Re: [Full-Disclosure] Potential denial of service bug in Cisco Pix Firewall IOS 6.2.2 a nd 6.3.(3.102)
From: Ilker Temir (itemircisco.com)
Date: Fri Oct 03 2003 - 13:21:14 CDT
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This is in response to the e-mail posted by John Airey. The original
e-mail is available at
Cisco's Product Security Incident Response Team (PSIRT) was not
previously aware of this issue. Thank you for bringing it to our attention.
Cisco bug ID CSCec47609 has been opened to investigate this issue. We
have updated the Security Notice about the "Nachi Worm Mitigation
to reflect this information.
We are always open for vulnerability reports regarding any Cisco
products. Such reports can be directly sent to us at psirtcisco.com or
to security-alertcisco.com in case of an emergency.
| Brief Description
| Users of Cisco Pix Firewalls may discover that their pool of NAT'ted IP
| addresses is running out, and that a reboot or reload of the firewall
| the problem.
| The problem is caused by the Firewall being swamped by incoming ICMP
| on the global pool IP addresses. If these are not intercepted by a router
| beforehand, the incoming echo requests (that are emanating from
| Nachi/Welchia worm infected machines) are preventing the release of the
| address translation. ie, the Pix is detecting the blocked traffic as
| indication that the translation is still in use.
| I believe that this bug also affects the recent security update version
| 6.3(3.102) detailed at
| I have been unable (and unwilling) to test this, but given that a
| fix is being worked on it is undoubtedly the case.
| For those who are unable to block incoming ICMP echo requests at their
| router (for whatever reason), Cisco have sent me the following details:
| "1- use PAT (a global pool with a single entry) this way although the
| will remain up but all your internal hosts will be multiplexed over
| address. single pat address can accomodate in theory 65535 connections.
| however this might break un-PATable traffic
| 2- use statics for your important servers that need NAT (1 to 1 mapping)
| 3- also instead of rebooting the whole pix you can simply log into it
| "clear xlate" this will clear all translations."
| It should be pointed out that "2" is not a solution to this problem. The
| others are not ideal either.
| Permanent fix
| I have been informed that Cisco are aware of this and that a bug fix is
| being worked on.
| I am releasing this notification as there may well be system adminstrators
| who are still suffering from this problem. Specifically the release of
| information cannot lead to any further attacks against systems that are
| already affected.
| Unfortunately Cisco have not updated their information regarding
| against the Nachi/Welchia worm at
| The mitigation information only covers outgoing connections. I have asked
| Cisco for a fix to for this twice, and at present I am still waiting for a
| resolution to my second request.
| Apologies for the evil Outlook word-wrapping, which may render URLs above
| Please be kind to me. This is my first security vulnerability I've ever
| John Airey, BSc (Jt Hons), CNA, RHCE
| Internet systems support officer, ITCSD, Royal National Institute of the
| Bakewell Road, Peterborough PE2 6XU,
| Tel.: +44 (0) 1733 375299 Fax: +44 (0) 1733 370848 John.Aireyrnib.org.uk
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