Summary

• A vulnerability in the HTTPS decryption feature of Cisco Web Security Appliance (WSA) could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition.

  The vulnerability is due to insufficient validation of Secure Sockets Layer (SSL) server certificates. An attacker could exploit this vulnerability by installing a malformed certificate in a web server and sending a request to it through the Cisco WSA. A successful exploit could allow the attacker to cause an unexpected restart of the proxy process on an affected device.

  Cisco has released software updates that address this vulnerability. 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-20190703-wsa-dos

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco AsyncOS Software for Cisco WSA, both virtual and hardware appliances, when the devices have the HTTPS Proxy feature enabled and have at least one decryption policy configured. The HTTPS Proxy feature is disabled by default.

  For information about which Cisco AsyncOS Software releases are vulnerable, see the Fixed Software section of this advisory.

  To determine whether the HTTPS Proxy feature is enabled for a WSA, administrators can log in to the web interface of the WSA and navigate to Security Services > HTTPS Proxy. The value in the HTTPS Proxy field indicates whether the feature is enabled or disabled.

  Products Confirmed Not Vulnerable

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

Indicators of Compromise

• Exploitation of this vulnerability will cause the proxy process to generate a core file with a particular backtrace. Contact the Cisco Technical Assistance Center (TAC) to review the core file and determine whether the device has been compromised by exploitation of this vulnerability and to identify the IP address of the server that is hosting the malformed certificate.

Workarounds

• There are no workarounds that address this vulnerability. It is possible to mitigate the impact of this vulnerability by bypassing decryption for web servers that are hosting malformed certificates that exploit this vulnerability. For details about how to configure the bypass, refer to AsyncOS User Guide.

Fixed Software

• Cisco has released free software updates that address the vulnerability described in this advisory. Customers may only install and expect support for software versions and feature sets for which they have purchased a license. By installing, downloading, accessing, or otherwise using such software upgrades, customers agree to follow the terms of the Cisco software license:
  https://www.cisco.com/c/en/us/products/end-user-license-agreement.html

  Additionally, customers may only download software for which they have a valid license, procured from Cisco directly, or through a Cisco authorized reseller or partner. In most cases this will be a maintenance upgrade to software that was previously purchased. Free security software updates do not entitle customers to a new software license, additional software feature sets, or major revision upgrades.

  When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories and Alerts 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 TAC or their contracted maintenance providers.

  Customers Without Service Contracts

  Customers who purchase directly from Cisco but do not hold a Cisco service contract and customers who make purchases through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain upgrades by contacting the Cisco TAC:
  https://www.cisco.com/c/en/us/support/web/tsd-cisco-worldwide-contacts.html

  Customers should have the product serial number available and be prepared to provide the URL of this advisory as evidence of entitlement to a free upgrade.

  Fixed Releases

  Customers are advised to upgrade to an appropriate fixed software release as indicated in the following table:

  Cisco AsyncOS Software Major Release	First Fixed Release
  Prior to 10.5	Migrate to 10.5.5-005
  10.5	10.5.5-005
  11.5	11.5.2-020
  11.7	Not vulnerable

  In most cases, the software can be upgraded over the network by using the System Upgrade options in the Cisco WSA web interface. To upgrade a device by using the web interface:

  1. Choose System Administration > System Upgrade.
  2. Click Upgrade Options.
  3. Choose Download and Install.
  4. Choose the release to upgrade to.
  5. In the Upgrade Preparation area, choose the appropriate options.
  6. Click Proceed to begin the upgrade. A progress bar displays the status of the upgrade.

  After the upgrade is complete, the device reboots.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

Source

• This vulnerability was found during the resolution of a Cisco TAC support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190703-wsa-dos

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2019-July-03

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