CVE-2023-20215: Cisco Security Advisory: Cisco Secure Web Appliance Content Encoding Filter Bypass Vulnerability

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Summary

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• A vulnerability in the scanning engines of Cisco AsyncOS Software for Cisco Secure Web Appliance could allow an unauthenticated, remote attacker to bypass a configured rule, allowing traffic onto a network that should have been blocked.

  This vulnerability is due to improper detection of malicious traffic when the traffic is encoded with a specific content format. An attacker could exploit this vulnerability by using an affected device to connect to a malicious server and receiving crafted HTTP responses. A successful exploit could allow the attacker to bypass an explicit block rule and receive traffic that should have been rejected by the device.

  There are no workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-wsa-bypass-vXvqwzsj

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Affected Products

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• At the time of publication, this vulnerability affected Cisco Secure Web Appliance, both virtual and hardware versions, when the deflate, lzma, or brotli content-encoding type was enabled.

  Note: The deflate content-encoding type is disabled by default, but the lzma and brotli content-encoding types are enabled by default.

  Determine the Content-Encoding Type

  To determine whether the deflate, lzma, or brotli content-encoding type is enabled on Cisco Secure Web Appliance, run the advancedproxyconfig command followed by the CONTENT-ENCODING command as an administrator in the CLI. If the CONTENT-ENCODING command returns deflate, lzma, or br under Currently allowed content-encoding type(s), that content-encoding type is enabled, as shown in the following example:

  cisco-wsa> advancedproxyconfig
  cisco-wsa> CONTENT-ENCODING

  Enter values for the CONTENT-ENCODING options:
  Currently allowed content-encoding type(s): deflate, lzma, br

  For information about which Cisco software releases were vulnerable at the time of publication, see the Fixed Software section of this advisory. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.

  Cisco has confirmed that this vulnerability does not affect the following Cisco products:

  • Secure Email Gateway, formerly Email Security Appliance (ESA), both virtual and hardware appliances
  • Secure Email and Web Manager, both virtual and hardware appliances

  Attention: Simplifying the Cisco portfolio includes the renaming of security products under one brand: Cisco Secure. For more information, see Meet Cisco Secure.

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Workarounds

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• There are no workarounds that address this vulnerability. However, administrators can mitigate this vulnerability by disabling the deflate, lzma, and brotli content-encoding types if they are not required.

  Disable the Content-Encoding Type

  To disable a specific content-encoding type, use the following steps:

  1. Log in to the admin console interface for the device.

  2. Choose advancedproxyconfig > CONTENT-ENCODING.

  3. Enter the number associated with the specific content-encoding type.

  4. If the following message is displayed, enter Y at the prompt:

     The encoding type <"content-encoding type"> is currently allowed
     Do you want to block it? [N]> Y

     If the following message is displayed, enter N at the prompt:

     The encoding type <"content-encoding type"> is currently blocked
     Do you want to allow it? [N]> N

  5. Run the Commit command.

  While this mitigation has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations. Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.

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Fixed Software

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• When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories page, to determine exposure and a complete upgrade solution.

  In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

  Fixed Releases

  At the time of publication, the release information in the following table(s) was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  The left column lists Cisco software releases, and the right column indicates whether a release was affected by the vulnerability that is described in this advisory and which release included the fix for this vulnerability.

  Cisco AsyncOS for Secure Web Appliance Software Release

  First Fixed Release

  14.0 and earlier

  Migrate to a fixed release.

  14.5

  Release no. TBD (future release)

  The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.

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Exploitation and Public Announcements

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• The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

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Source

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• Cisco would like to thank q.beyond AG for reporting this vulnerability.

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Cisco Security Vulnerability Policy

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• To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.

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Related to This Advisory

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URL

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Revision History

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• Version

  Description

  Section

  Status

  Date

  1.1

  Updated source information.

  Source

  Interim

  2023-AUG-03

  1.0

  Initial public release.

  -

  Interim

  2023-AUG-02

  Show Less

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Legal Disclaimer

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• 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. CISCO EXPECTS TO UPDATE THIS DOCUMENT AS NEW INFORMATION BECOMES AVAILABLE.

  A standalone copy or paraphrase of the text of this document that omits the distribution URL is an uncontrolled copy and may lack important information or contain factual errors. The information in this document is intended for end users of Cisco products.

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