Advisory ID: cisco-sa-20020515-transparent-cache-tcp-relay
For Public Release 2002 May 15 18:00 UTC (GMT)
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
This section provides details on affected 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
Content Router CR-4430 running ACNS 4.x
Content Distribution Manager CDM-4630 or CDM-4650 running ACNS
Content Engine Module for Cisco Routers 2600, 3600 and
No other Cisco products are currently known to be affected by these
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
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)
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.
Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.
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
Cisco Cache Engines and Content Engines can be used to forward
unexpected traffic, and to obscure the true originator of undesirable
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
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
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
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
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
or as otherwise set forth at Cisco.com Downloads at
Do not contact either "email@example.com" or "firstname.lastname@example.org"
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
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)
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.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
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.
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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
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.
Added the HTTP vulnerability (CSCeb19815). Updated the Summary,
Details, Fixed Software and Workarounds sections.
Updated Details section.
Initial public release