Cisco Security Advisory: Multiple Vulnerabilities in CBOS

Document ID: 10925

Advisory ID: cisco-sa-20001204-cbos

http://www.cisco.com/warp/public/707/cisco-sa-20001204-cbos.shtml

Revision 1.5

Last Updated 2001 August 08 2100 UTC (GMT)

For Public Release 2000 December 04 0800 UTC (GMT)

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Contents

Summary
Affected Products
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

Summary

Multiple vulnerabilities have been identified and fixed in CBOS, an operating system for the Cisco 600 family of routers.

• Any router in the Cisco 600 family that is configured to allow Web access can be locked by sending a specific URL. Web access is disabled by default, and it is usually enabled in order to facilitate remote configuration. This defect is documented as Cisco bug ID CSCdr98772.
  
  
• By sending a stream of TCP SYN packets to the router, it is possible to exhaust all available TCP sockets. The consequence is that no new TCP sessions addressed to the router will be established. The difference between this vulnerability and a SYN Denial-of-Service attack is that this one can be accomplished by a slow stream of packets (one per second). This defect is documented as Cisco bug ID CSCds59206.
  
  
• Invalid login attempts using the Web interface are not logged. This defect is documented as Cisco bug ID CSCds19142.
  
  
• It is possible to lock up the router by sending a large ICMP ECHO (PING) packet to it. This defect is documented as Cisco bug ID CSCds23921.
  

The following releases of CBOS are vulnerable to all defects: 2.0.1, 2.1.0, 2.1.0a, 2.2.0, 2.2.1, 2.2.1a, 2.3, 2.3.2, 2.3.5, 2.3.7 and 2.3.8.

These defects will be fixed in the following CBOS releases: 2.3.5.015, 2.3.7.002, 2.3.9 and 2.4.1. Customers are urged to upgrade to releases that are not vulnerable to this defect as shown in detail in the section Software Versions and Fixes below.

This advisory is posted at http://www.cisco.com/warp/public/707/cisco-sa-20001204-cbos.shtml.

Affected Products

This section supplies details on affected products.

Vulnerable Products

The affected models are: 627, 633, 673, 675, 675E, 677, 677i and 678.

These models are vulnerable if they run any of the following, or earlier, CBOS releases: 2.0.1, 2.1.0, 2.1.0a, 2.2.0, 2.2.1, 2.2.1a, 2.3, 2.3.2, 2.3.5, 2.3.7 and 2.3.8.

These defects will be fixed in the following CBOS releases: 2.3.5.015, 2.3.7.002, 2.3.9 and 2.4.1.

Products Confirmed Not Vulnerable

No other releases of CBOS software are affected by this vulnerability. No other Cisco products are affected by this vulnerability.

Details

CSCdr98772

The behavior is caused by inadequate URL parsing in CBOS. Each URL was expected to terminate with a minimum of a single space character (ASCII code 32, decimal). Sending a URL that does not terminate with a space causes CBOS to enter an infinite loop. It is necessary to power cycle the router to resume operation. To exploit this vulnerability, a router must be configured to accept Web connections. Having a Web access password configured does not provide protection against this vulnerability.

CSCds59206

By sending a stream of SYN packets addressed to the router, it is possible to exhaust all available TCP sockets within CBOS. This is due to the memory leak in CBOS. When a router is set into a state where it cannot accept a new connection, it can be maintained in this state by a slow stream of SYN packets until the router is rebooted. The stream can be as slow as one packet per second, so one machine with a 64 KB connection can hold up approximately 150 routers.

Note: This does not effect non-TCP traffic. All User Datagram Protocol (UDP) and Internet Control Message Protocol (ICMP) packets can be handled by a router without any problems. All existing and new TCP sessions through the router will not be affected.

When an attacking stream is terminated, a router recovers itself within a few minutes.

CSCds19142

Using the Cisco Web Management interface, it is possible to keep guessing an access password without those password attempts being logged. A password may be either "exec-only" or "enable". A user with an "exec-only" password cannot change a router configuration.

CSCds23921 By sending a large (at least 65500 bytes in size) ICMP ECHO (PING) packet to the router itself, it is possible to overflow an internal variable and cause router lockup. The router is not affected by the packets which are routed through it.

Impact

CSCdr98772

By sending a tailored URL to a router, it is possible to cause a Denial-of-Service. Every affected router must be powered off and back on in order to restore its normal functionality.

CSCds59206

It is possible to prevent all TCP access to a router. This blocks all attempts at remote router administration.

CSCds19142

Long term, brute force password guessing can be performed without being noticed. When the correct password is guessed, it can be used to view or modify router configuration. This may be particularly dangerous in installations where multiple routers have the same password.

CSCds23921

It is possible to lock up the router thus causing Denial-of-Service. Every affected device must be powered off and back on in order to restore its normal functionality.

Software Versions and Fixes

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

Workarounds

CSCdr98772

There are two workarounds for this vulnerability. The potential for exploitation can be lessened by ensuring that Web access to the router is limited to a legitimate IP address.

This can be done by entering the following commands while in enable mode:

cbos# set web remote 10.0.0.1
cbos# set web enabled 

where 10.0.0.1 is the address of the host with a legitimate need for Web access to the router.

Alternatively, disabling the Web access completely will also prevent this vulnerability from being exploited. This can be done by entering the following command while in enable mode:

cbos# set web disabled

Another option is to change the Web port from the default port 80 to some arbitrary port. This will also help in cases where malicious program is making HTTP requests to the default port. This can be accomplished by the following commands:

cbos# set web port <arbitrary_port_greater_than_1024>
cbos# write
cbos# reboot 

You must reboot the router in order for changes to take effect. You should also take care to avoid well known ports that are described in RFC1700.

CSCds59206

There is no workaround for this vulnerability.

CSCds19142

The Web Management interface can be disabled by entering the following commands in enable mode:

cbos# set web disabled 

CSCds23921

All incoming ICMP ECHO (PING) packets destined to the router itself should be denied. That can be achieved by following commands:

cbos# set filter number on deny incoming all 0.0.0.0 0.0.0.0 <eth0_IP_address> 255.255.255.255 protocol ICMP
cbos# set filter number+1 on deny incoming all 0.0.0.0 0.0.0.0 <wan0_IP_address> 255.255.255.255 protocol ICMP

Where number is a free filter number between 0 and 17.

Obtaining Fixed Software

Cisco is offering free software upgrades to eliminate this vulnerability for all affected customers.

Customers with Service Contracts

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 Web site at http://www.cisco.com.

Customers using Third Party Support Organizations

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 assistance with the upgrade, which should be free of charge.

Customers without Service Contracts

Customers without contracts 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: tac@cisco.com
  

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. Please do not contact either "psirt@cisco.com" or "security-alert@cisco.com" for software upgrades.

Exploitation and Public Announcements

The vulnerability CSCdr98772 was discovered by several customers. It was also discussed at public forums. PSIRT has received reports that this vulnerability has been exploited in vivo.

The vulnerability CSCds23921 was discovered by a customer. The other two vulnerabilities (CSCds59206 and CSCds19142) were discovered during internal testing.

The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements of CSCds59206, CSCds19142 and CSCds23921.

Status of this Notice: FINAL

Distribution

This advisory is posted on Cisco's worldwide website at http://www.cisco.com/warp/public/707/cisco-sa-20001204-cbos.shtml. 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
  
• bugtraq@securityfocus.com
  
• first-teams@first.org (includes CERT/CC)
  
• cisco@spot.colorado.edu
  
• comp.dcom.sys.cisco
  
• firewalls@lists.gnac.com
  
• Various internal Cisco mailing lists
  

Future updates of this notice, if any, will be placed on Cisco's Worldwide Web server, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the URL given above for any updates.

Revision History

Cisco Security Procedures

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.

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