Cisco Security Advisory

Contents

Summary
Affected Products
Details
Vulnerability Scoring Details
Impact
Software Versions and Fixes
Workarounds
Obtaining Fixed Software
Exploitation and Public Announcements
Status of This Notice: Final
Distribution
Revision History
Cisco Security Procedures

This section provides details on affected products.

Vulnerable Products

Products Confirmed Not Vulnerable

The HTTP server was introduced in IOS release 11.0 to extend router management to the worldwide Web. The "?" (question mark) character is defined in the HTML specifications as a delimiter for CGI arguments. It is also interpreted by the IOS command-line interface as a request for help.

As of Cisco IOS Software Release 12.0T, the meaning of a question mark when it appears adjacent to a "/" (slash) character cannot be determined properly by the URI parser in affected versions of Cisco IOS software. When a URI containing "?/" is presented to the HTTP service on the router and a valid enable password is supplied, the router enters an infinite loop. A watchdog timer expires two minutes later and forces the router to crash and reload. The router continues to be vulnerable to this defect as long as it is running an affected IOS software release and the enable password is known.

This vulnerability may only be exploited if the enable password is not set, it is well known, or it can be guessed.

In rare cases, an affected device fails to reload, which means an administrator must physically cycle the power to resume operation.

The HTTP server is not enabled by default except on unconfigured Cisco model 1003, 1004, and 1005 routers. Once initial access is granted to configure the router, the customer may set an enable password, and disable or limit access to the HTTP server by changing the configuration. Once the new configuration has been saved, the HTTP server will not be enabled when the router restarts.

The Cisco Bug ID CSCdr91706 relates to fixes for IOS; the fixes for the affected switch product lines are listed with Cisco Bug ID CSCds57774 and CSCdv38391.

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
http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
http://intellishield.cisco.com/security/alertmanager/cvss .

An affected Cisco IOS device that is operating with the HTTP service enabled and is not protected by having the enable password configured can be forced to halt for up to two minutes and then reload. The vulnerability can be exercised repeatedly, possibly creating a denial of service (DOS) attack, unless the service is disabled, the enable password is set, or the router is upgraded to a fixed release.

In instances in which a router at a remote location fails to reload, an administrator must visit the site to enable the device to recover from the defect.

The following table summarizes the Cisco IOS software releases affected by the defect described in this notice and scheduled dates on which the earliest corresponding fixed releases will be available. Dates are tentative and subject to change.

Each table row shows the earliest release that contains the fix in the "Rebuild", "Interim", or "Maintenance" columns, presented in release number order.

A Maintenance Release is the most heavily tested and highly recommended release.

A Rebuild Release is constructed from a previous maintenance or mainline release and contains a code fix for a specific defect. Although it receives less testing than a maintenance release, it is built from a previous maintenance release and includes minimum changes to address a specific defect.

An Interim Release has much less testing than a maintenance release and should be selected only if no other suitable release fixes the defect.

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.

In lieu of an upgrade, the threat may be eliminated or reduced by taking any of the following measures:

• Select and configure strong enable passwords on networking devices.
  Or
  
• Disable the HTTP server using the command no ip http server while in global configuration mode.
  Or
  
• If the HTTP server must remain enabled while unrepaired, network access to it can be controlled by applying a standard access list to the HTTP service itself. For example, if the router's HTTP service should be reachable only from a browser running on a computer at IP address 10.1.2.3, then use the following commands in global configuration mode to create a standard access list and apply it to the HTTP server:
  

       access-list 1 permit 10.1.2.3 
       ip http access-class 1
  

  If access list 1 is already in use, then choose another number in the range 0-99. The implicit deny rule added to the end of every access list will prevent access from other IP addresses.
  Or
  
• Prevent network access to a vulnerable HTTP server by blocking traffic in the network path to the server's port with an extended access list. Such a list would be applied on an interface of the vulnerable router itself or on another Cisco router in the path of a potential attack, e.g., applied inbound on the outside interface of an edge router. The port number used in the extended access list statement must be the default port used by the HTTP server, port 80, or equal to whatever value it may have been set via the ip http port command. Use this workaround with great care; it cannot be recommended confidently without knowledge of specific customer network configurations.
  

Save the resulting configuration in memory so that protection of the server is not inadvertently removed after a reload.

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 "psirt@cisco.com" or "security-alert@cisco.com" for software upgrades.

Customers with Service Contracts

Customers Using Third-Party Support Organizations

Customers Without Service Contracts

The Cisco PSIRT was alerted to this issue by CORE SDI, which discovered the issue during routine security audits on equipment. The security audit included a check for common CGI vulnerabilities against a Cisco device without a configured password; the audit attempted to browse to "http://<router-ip>/cgi-bin/source-help?/", which caused the device to crash and reload.

The Cisco PSIRT has received no reports of malicious exploitation of this vulnerability.

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

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 notice will be posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20001025-ios-http-server-query.

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.

• cust-security-announce@cisco.com
  
• first-teams@first.org
  
• bugtraq@securityfocus.com
  
• vulnwatch@vulnwatch.org
  
• cisco@spot.colorado.edu
  
• cisco-nsp@puck.nether.net
  
• full-disclosure@lists.grok.org.uk
  
• comp.dcom.sys.cisco@newsgate.cisco.com
  

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.

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.