Advisory ID: cisco-sa-20011003-pix-firewall-auth-flood
For Public Release 2001 October 3 16:00 UTC (GMT)
Vulnerability Scoring Details
Software Versions and Fixes
Obtaining Fixed Software
Exploitation and Public Announcements
Status of this Notice: Final
Cisco Security Procedures
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 http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20011003-pix-firewall-auth-flood.
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 vulnerabilities.
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 user.
Cisco Bug ID CSCdv09731 amends this limitation to allow system administrators to modify this limit, depending on their network requirements.
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 filtered.
When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.
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 assistance.
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 4.4(8)
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 5.3(1)
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 http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.
Do not contact either "firstname.lastname@example.org" or "email@example.com" 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 http://www.cisco.com.
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)
- e-mail: firstname.lastname@example.org
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.
Refer to http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.
This vulnerability has been publicly discussed on the Bugtraq list maintained by SecurityFocus at http://www.securityfocus.com/archive/1/174577 . Cisco has no evidence or knowledge of exploitation of this vulnerability; this was discovered when a program on a customer site utilized all authentication resources.
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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 http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20011003-pix-firewall-auth-flood.
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.
Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.