CVE-2021-34741

Cisco Email Security Appliance Denial of Service Vulnerability

HIGH

7.5

CVSS Score

Published: November 4, 2021 at 03:40 PM UTC

Updated: June 17, 2026 at 03:56 AM UTC

13 products affected across 1 vendorAsyncosS395S195

Using Cisco Email Security Appliance (ESA) by Cisco?

Get instant alerts when new vulnerabilities affect Cisco Email Security Appliance (ESA) - before attackers find them.

Real-time alertsTrack your software stackFree plan available

Start Free Sign In

Severity

HIGH7.5

Exploit prediction

EPSS 1%

SSVC

No known exploit

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

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Asyncos · Cisco

95%

v < 13.0.413.5.3-10 13.7.0-93

Affected range stops below 13.0.4, so it looks safe — our inference, not a vendor statement

cisco-m170 (environment) · cisco-m190 (environment) · cisco-m380 (environment) · cisco-m390 (environment) · cisco-m390x (environment) · cisco-m680 (environment) · cisco-m690 (environment) · cisco-m690x (environment) · cisco-s195 (environment) · cisco-s395 (environment) · cisco-s695 (environment)

Email Security Appliance · Cisco

60%

All versions

M170 · Ciscodevice

80%

All versions

M190 · Ciscodevice

80%

All versions

M380 · Ciscodevice

80%

All versions