Advisory ID: cisco-sa-20011003-pix-firewall-auth-flood
For Public Release 2001 October 3 16:00 UTC (GMT)
The Cisco Secure PIX Firewall AAA authentication feature, introduced in
version 4.0, is vulnerable to a Denial of Service (DoS) attack initiated by
authenticating users on the system. This vulnerability affects specific
configurations and has been resolved in released versions of the PIX Firewall.
This vulnerability has been assigned Cisco bug ID CSCdt92339.
The complete notice will be available at
This section provides details on affected products.
All users of Cisco Secure PIX Firewalls with software versions 4.0 up
to and including 4.4(8), 5.0(3), 5.1(3), 5.2(2), and 5.3(1) with configurations
using AAA authentication are at risk.
Affected configurations will have configuration lines that begin:
pixfirewall# aaa authentication ...
Configurations not including aaa
authentication are not affected. PIX
Firewall software versions 6.0(1) and later are not affected.
The IOS Firewall feature set is not affected by the above defect.
No other Cisco products are currently known to be affected by these
When AAA authentication services are configured on the Cisco Secure PIX
Firewall, it is possible for a single source address to consume all of the
authentication resources, preventing other legitimate users from
authenticating. This is a denial of service strictly for the authentication
resources; other established traffic continues unaffected, and only new
authentication requests are prevented.
This vulnerability has been resolved in the recent versions of the PIX
Firewall by creating a maximum limit of three open authentication requests per
Cisco Bug ID CSCdv09731 amends this limitation to allow system
administrators to modify this limit, depending on their network requirements.
Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.
CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.
Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.
Cisco has provided an FAQ to answer additional questions regarding CVSS at
Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
This issue causes a PIX Firewall to be vulnerable to a DoS attack in
which the availability of the unit is degraded. This does not result in a loss
in confidentiality or in loss of integrity of the traffic being
When considering software upgrades, also consult
and any subsequent advisories to determine exposure and a complete upgrade
In all cases, customers should exercise caution to be certain the
devices to be upgraded contain sufficient memory and that current hardware and
software configurations will continue to be supported properly by the new
release. If the information is not clear, contact the Cisco Technical
Assistance Center ("TAC") or your contracted maintenance provider for
Fix carries forward into all later versions
Fixed Regular Release (available now)
Fix carries forward into all later versions
All versions of Cisco Secure PIX up to and including version
Upgrade to 5.2(6)
Version 5.0 and Version 5.1
Upgrade to 5.2(6)
Version 5.3.x up to and including version
There is no known workaround for all authentication types.
If authentication is configured in conjunction with the Virtual HTTP
feature, a limit of three concurrent authentication attempts per user exists,
which could prevent this denial of service for HTTP traffic only. This would
have no effect on authentication attempts for FTP or Telnet if the PIX is
configured for authentication of those services.
Cisco has made free software available to address this vulnerability
for affected customers. Prior to deploying software, customers should consult
their maintenance provider or check the software for feature set compatibility
and known issues specific to their environment.
Customers may only install and expect support for the feature sets they
have purchased. By installing, downloading, accessing or otherwise using such
software upgrades, customers agree to be bound by the terms of Cisco's software
license terms found at
or as otherwise set forth at Cisco.com Downloads at
Do not contact either "email@example.com" or "firstname.lastname@example.org"
for software upgrades.
Customers with contracts should obtain upgraded software through their
regular update channels. For most customers, this means that upgrades should be
obtained through the Software Center on Cisco's worldwide website at
Customers whose Cisco products are provided or maintained through prior
or existing agreement with third-party support organizations such as Cisco
Partners, authorized resellers, or service providers should contact that
support organization for guidance and assistance with the appropriate course of
action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific
customer situations such as product mix, network topology, traffic behavior,
and organizational mission. Due to the variety of affected products and
releases, customers should consult with their service provider or support
organization to ensure any applied workaround or fix is the most appropriate
for use in the intended network before it is deployed.
Customers who purchase direct from Cisco but who do not hold a Cisco
service contract and customers who purchase through third-party vendors but are
unsuccessful at obtaining fixed software through their point of sale should get
their upgrades by contacting the Cisco Technical Assistance Center (TAC). TAC
contacts are as follows.
+1 800 553 2447 (toll free from within North America)
+1 408 526 7209 (toll call from anywhere in the world)
Have your product serial number available and give the URL of this
notice as evidence of your entitlement to a free upgrade. Free upgrades for
non-contract customers must be requested through the TAC.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
This vulnerability has been publicly discussed on the Bugtraq list
maintained by SecurityFocus at
. Cisco has no
evidence or knowledge of exploitation of this vulnerability; this was
discovered when a program on a customer site utilized all authentication
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A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.
This advisory will be posted on Cisco's worldwide website at
In addition to worldwide web posting, a text version of this notice is
clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail
and Usenet news recipients.
email@example.com (includes CERT/CC)
Various internal Cisco mailing lists
Future updates of this advisory, if any, will be placed on Cisco's
worldwide website, but may or may not be actively announced on mailing lists or
newsgroups. Users concerned about this problem are encouraged to check the
above URL for any updates.
For public release.