CVE-2021-34741: Cisco Email Security Appliance Denial of Service Vulnerability

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-11-04 · updated 2026-06-17
Weakness: CWE-770

Description

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; 13.5.3-010, 13.7.0-093 — fixed in 13.0.4

How to fix

Upgrade to a fixed version: Cisco Asyncos 13.0.4. Check the vendor advisory for mitigations.

References

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