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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20001204-cbos.

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.

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.

Fixed Software

• 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.

  Major Release	Description or Platform	Availability of Repaired Releases*
  		Patch release**	General Availability (GA)
  All releases	627, 633, 673, 675, 677, 678	2.3.5.015 2000-DEC-15	-
  2.3.7.001	677i	2.3.7.002 2000-DEC-15	-
  All releases	All platforms	-	2.3.9 2001-MARCH 19
  All releases	All platforms	-	2.4.1 2000-DEC-15
  Notes
  * All dates are estimated and subject to change. ** Patch releases are subjected to less rigorous testing than regular GA releases, and may have serious bugs.

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.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20001204-cbos

Revision History

• 1.5	2001-August-08	Updated Workarounds Section.
  1.4	2001-March-19	Updated GA for release 2.3.9; changed status from interim to final.
  1.3	2000-December-19	Removed instructions for temporarily accessing fixed software images; removed text under CSCdr98772 about Web access on Cisco 600 routers.
  1.2	2000-December-13	Updated software availability date, and added instructions for obtaining software.
  1.1	2000-December-10	Updated software availability date, and added instructions on requesting assistance from third-party support organizations in the Obtaining Fixed Software section.
  1.0	2000-December-03	Draft for initial public release.

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