Cisco Email Security Appliance URL Filtering Bypass Vulnerability [CVE-2023-20057]

CVE number = CVE-2023-20057

A vulnerability in the URL filtering mechanism of Cisco AsyncOS Software for Cisco Email Security Appliance (ESA) could allow an unauthenticated, remote attacker to bypass the URL reputation filters on an affected device.

This vulnerability is due to improper processing of URLs. An attacker could exploit this vulnerability by crafting a URL in a particular way.

A successful exploit could allow the attacker to bypass the URL reputation filters that are configured for an affected device, which could allow malicious URLs to pass through the device.

After additional investigation, it was determined that this vulnerability is not exploitable. 

Workarounds

Administrators should follow the Best Practice Guide for Anti-Spam, Anti-Virus, Graymail and Outbreak Filters and should ensure that the Cisco Anti-Spam feature is enabled because it contains rules designed to identify specially crafted URLs. It is also recommended that all positive verdicts be either dropped or quarantined.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.