Cisco Email Security Appliance URL Filtering Denial of Service Vulnerability

Summary

• A vulnerability in the email message filtering feature of Cisco AsyncOS Software for Cisco Email Security Appliances (ESA) could allow an unauthenticated, remote attacker to cause the CPU utilization to increase to 100 percent, causing a denial of service (DoS) condition on an affected device.

  The vulnerability is due to improper filtering of email messages that contain references to whitelisted URLs. An attacker could exploit this vulnerability by sending a malicious email message that contains a large number of whitelisted URLs. A successful exploit could allow the attacker to cause a sustained DoS condition that could force the affected device to stop scanning and forwarding email messages.

  Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190109-esa-url-dos

Affected Products

• Vulnerable Products

  This vulnerability affects all software versions prior to the first fixed release of Cisco AsyncOS Software for Cisco ESAs, both virtual and hardware, if the URL Filtering as Global Setting feature is enabled and a URL whitelist is in use. By default, the URL Filtering as Global Setting feature is disabled.

  To determine whether a vulnerable release of Cisco AsyncOS Software is running on an ESA, administrators can use the version command in the ESA CLI. The following example shows the output of the command for an ESA that is running Cisco AsyncOS Software Release 10.0.1-087:
  

  ciscoesa> version
  
  Current Version
  ===============
  Product: Cisco C100V Email Security Virtual Appliance
  Model: C100V
  Version: 10.0.1-087
  .
  .
  .

  To check whether a Cisco ESA is configured with URL Filtering and uses a URL whitelist, administrators can navigate to GUI > Security Services > URL Filtering. A vulnerable device will have URL Category and Reputation Filters set to Enabled and URL whitelist set to any value other than None.

  Products Confirmed Not Vulnerable

  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 products:

  • Content Security Management Appliance, virtual and hardware versions
  • Web Security Appliance, virtual and hardware versions

Workarounds

• There are two possible workarounds to address this vulnerability:

  1. If the whitelist through Global URL Filtering is not needed, administrators can disable it by following these steps:

  • On the ESA, navigate to Security Services > URL Filtering.
  • Click on Edit Global Settings and set Use a URL whitelist to None. This disables the global whitelist.
  • Commit the Changes.

  2. If the whitelist through Global URL Filtering feature is needed, the same level of filtering can be achieved by implementing a single whitelist per Content Filter, after disabling the whitelist through Global URL Filtering, as described in step 1. Administrators can choose a specific URL Reputation range or set of URL Categories to apply the whitelist to via the Content Filter. The required steps are as follows:

  1. On the ESA, navigate to Mail Policies > Incoming Content Filters
  2. [Optionally] Create a new Content Filter by clicking Add Filter
  3. Click on the desired Content Filter > Add Action > URL Reputation > Select Custom Range
     
  4. Input the desired range
  5. [Alternatively] Add Action > URL Categories > Add desired Categories set
  6. Choose the desired whitelist
  7. Click OK > Submit
  8. Commit the Changes

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 Technical Assistance Center (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

  In the following table, the left column lists major releases of Cisco AsyncOS Software for ESA. The right column indicates whether a major release is affected by the vulnerability described in this advisory and the first minor release that includes the fix for this vulnerability.
  
  Customers should upgrade to an appropriate release as indicated in the following table:

  Cisco AsyncOS Software for ESA Major Release	First Fixed Release for This Vulnerability
  Prior to 9.0	Affected; Migrate to 11.0.2-044 MD
  9.x	Affected; Migrate to 11.0.2-044 MD
  10.x	Affected; Migrate to 11.0.2-044 MD
  11.0.x	11.0.2-044 MD1
  11.1.x	11.1.2-023 MD2
  12.x	Not affected

  ¹ 11.0.2-044 will run on all legacy hardware for ESA models x70, x80, x90 and the virtual appliances.
  ² 11.1.2-023 will run on ESA models x80, x90 and the virtual appliances.

  Note that Cisco provides regular maintenance of products in the Cisco Cloud Email Security (CES) service solution, which includes Cisco Email Security Appliances and Cisco Content Security Management Appliances. Customers can also request a software upgrade by contacting Cisco TAC.

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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190109-esa-url-dos

Revision History

• Version	Description	Section	Status	Date
  1.1	Added details regarding the URL filtering-related workaround.	Workarounds	Final	2019-March-13
  1.0	Initial public release.	-	Final	2019-January-09

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