Cisco Umbrella Secure Web Gateway File Decryption Bypass Vulnerability

A vulnerability in the automatic decryption process in Cisco Umbrella Secure Web Gateway (SWG) could allow an authenticated, adjacent attacker to bypass the SSL decryption and content filtering policies on an affected system. This vulnerability is due to how the decryption function uses the TLS Sever Name Indication (SNI) extension of an HTTPS request to discover the destination domain and determine if the request needs to be decrypted. An attacker could exploit this vulnerability by sending a crafted request over TLS from a client to an unknown or controlled URL. A successful exploit could allow an attacker to bypass the decryption process of Cisco Umbrella SWG and allow malicious content to be downloaded to a host on a protected network. There are workarounds that address this vulnerability. This advisory is available at the following link:https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-uswg-fdbps-xtTRKpp6

As a workaround, use the instructions in the Cisco Umbrella Secure Internet Gateway (SIG) User Guide https://docs.umbrella.com/umbrella-user-guide/docs to create a rule to block traffic that could bypass the SSL decryption and content filtering policies.

Step 1: Follow the instructions in the Add a Web Destination List https://docs.umbrella.com/umbrella-user-guide/docs/add-a-web-policy-destination-list section to create destination lists to control identity access to websites.

Step 2: Follow the instructions in the Add a Ruleset to the Web Policy https://docs.umbrella.com/umbrella-user-guide/docs/add-a-rules-based-policy to create a ruleset.

Step 3: Follow the instructions in the Add Rules to a Ruleset https://docs.umbrella.com/umbrella-user-guide/docs/add-rules-to-a-ruleset section to create a Block rule.

From the Rule Action drop-down list, choose Block.
Under Destinations, click Add Destination and then click Destination Lists. From the drop-down menu, choose the destination lists that were created in Step 1.

While this workaround 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.