Summary

• Cisco Cache Engines and Content Engines provide a transparent cache for world wide web pages retrieved via HTTP. These products also can be configured to transparently intercept requests to proxy servers supporting various protocols such as HTTPS. The default configuration of the proxy feature can be abused to open a TCP connection to any reachable destination IP address and hide the true IP source address of the connection. This behavior has been implicated in a variety of undesirable and possibly illegal activities such as transmitting unsolicited commercial e-mail, unauthorized network scanning, and denial of service attacks.

  There are two vulnerabilities that may cause this problem.

  • The vulnerability for the HTTP proxy can be resolved by upgrading the code to a fixed version.
    
  • The vulnerability for the HTTPS proxy can be resolved in the field by changing the configuration of the affected device.
    

  Fixed versions of the software have been modified to provide a more secure configuration by default.

  This advisory is available at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020515-transparent-cache-tcp-relay .

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  The following Cisco Cache Engine and Content Engine products are affected if they are running the specified versions of software:

  • Content Engine 507, 510, 560, 565, 590, 7305, 7320 or 7325 running cache software 2.x, 3.1, 4.0.x, 4.1.x, 4.2.x, 5.0.x, 5.1.x
    
  • Cache Engine 505, 550, or 570 running software version 2.2.0 or above
    
  • Content Router CR-4430 running ACNS 4.x
    
  • Content Distribution Manager CDM-4630 or CDM-4650 running ACNS 4.x
    
  • Content Engine Module for Cisco Routers 2600, 3600 and 3700
    

  Products Confirmed Not Vulnerable

  No other Cisco products are currently known to be affected by these vulnerabilities.

Details

• The vulnerability for the HTTPS proxy has been assigned Cisco bug ID CSCdx05705, which modifies the default settings to ensure the administrator must specify permitted traffic.

  The ability to handle proxied requests was added in version 2.2.0 of the Cache Engine software. More details are provided in the Release Notes at /en/US/products/sw/conntsw/ps547/prod_release_note09186a00800ea835.html#xtocid71711.

  In addition to caching pages from remote web servers, the cache software also has the ability to cache data for other proxy servers using a variety of supported protocols such as FTP and HTTPS. This function is enabled by default. Since proxied HTTPS services may be available on a variety of ports, the device can be instructed by a client to open a TCP connection to any reachable IP address and port.

  The following warning is displayed during configuration and the boot process when the Cache Engine running version 2.x is configured as an HTTPS proxy server without transparent redirection:

  It is recommended to set restrictions that allow or deny HTTPS traffic to
  Destination Ports. Default settings may not provide the desired security level.

  This warning is not displayed when the device operated in transparent mode and is not shown in any case when running software versions 3.x and 4.x.

  This issue has been resolved by changing the default behavior when HTTPS proxy is enabled so that connections are limited based on the destination port numbers and connections to ports less than 1024 (excluding 443 and 563) are denied.

  The vulnerability for the HTTP proxy has been assigned Cisco bug ID CSCeb19815, which introduces the new "http destination-port <deny|allow> <all|port ranges>" command and modifies the default settings to ensure the administrator must specify permitted traffic.

  The HTTP proxy vulnerability has been resolved by changing the default behavior so that the HTTP connections are limited based on the destination port numbers and connections to reserved ports (1-79 and 88-1024) are denied.

Workarounds

• There is no workaround for the HTTP proxy vulnerability below 4.2(11.3), 5.0(5.2) and 5.1(0.190). The default behavior has been changed in these versions, so no other configuration is needed for these versions and above.

  The problem for the HTTPS proxy can be solved by a configuration command, which blocks the use of redirected proxy requests for any port other than 443.

  https destination-port allow 443
  https destination-port deny all

  If the HTTPS proxy is not necessary to an installation, then the command "https destination-port allow 443" can be excluded from the above workaround.

Fixed Software

• The vulnerability for the HTTP proxy can be corrected by customers by upgrading to ACNS 4.2(11.3), 5.0(5.2) and 5.1(0.190). A new command has been introduced in these versions to control HTTP destinations ports and the default configuration has been corrected.

  The vulnerability for HTTPS proxy can be corrected by customers in the field by modifying the configuration of the device. A software upgrade is not required to address the HTTPS vulnerability.

  The default behavior for both vulnerabilities are corrected in ACNS 4.2(11.3), 5.0(5.2), 5.1(0.190) and will be carried forward into all future versions.

Exploitation and Public Announcements

• The Cisco PSIRT is aware of several instances in which Cisco Cache Engines or Content Engines have been abused to transmit unsolicited commercial e-mail and hide the true source of the message.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020515-transparent-cache-tcp-relay

Revision History

• Revision 2.0	2004-Jan-05	Added the HTTP vulnerability (CSCeb19815). Updated the Summary, Details, Fixed Software and Workarounds sections.
  Revision 1.1	2002-May-28	Updated Details section.
  Revision 1.0	2002-May-15	Initial public release

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