CVE-2021-1566: Cisco Asyncos

High severity, CVSS 7.4. EPSS: 0.7% chance of exploitation in the next 30 days.

A vulnerability in the Cisco Advanced Malware Protection (AMP) for Endpoints integration of Cisco AsyncOS for Cisco Email Security Appliance (ESA) and Cisco Web Security Appliance (WSA) could allow an unauthenticated, remote attacker to intercept traffic between an affected device and the AMP servers. This vulnerability is due to improper certificate validation when an affected device establishes TLS connections. A man-in-the-middle attacker could exploit this vulnerability by sending a crafted TLS packet to an affected device. A successful exploit could allow the attacker to spoof a trusted host and then extract sensitive information or alter certain API requests.

Affected products

• Cisco Asyncos: before 12.5.3-035 (fixed in 12.5.3-035); from 13.0, before 13.0.0-030 (fixed in 13.0.0-030); from 13.5, before 13.5.3-010 (fixed in 13.5.3-010); before 11.8.3-021 (fixed in 11.8.3-021); from 12.0.0, before 12.0.3-005 (fixed in 12.0.3-005); from 12.5.0, before 12.5.1-043 (fixed in 12.5.1-043)
• Cisco Email Security Appliance: affected versions not specified
• Cisco Web Security Appliance: affected versions not specified

Published 2021-06-16. Last modified 2026-06-17.