CVE-2021-34741: Cisco Asyncos

High severity, CVSS 7.5. EPSS: 1.3% chance of exploitation in the next 30 days.

A vulnerability in the email scanning algorithm of Cisco AsyncOS software for Cisco Email Security Appliance (ESA) could allow an unauthenticated, remote attacker to perform a denial of service (DoS) attack against an affected device. This vulnerability is due to insufficient input validation of incoming emails. An attacker could exploit this vulnerability by sending a crafted email through Cisco ESA. A successful exploit could allow the attacker to exhaust all the available CPU resources on an affected device for an extended period of time, preventing other emails from being processed and resulting in a DoS condition.

Affected products

• Cisco Asyncos: before 13.0.4 (fixed in 13.0.4); version 13.5.3-010 only; version 13.7.0-093 only

Published 2021-11-04. Last modified 2026-06-17.