What is CVE-2021-1566?

CVE-2021-1566 is a high-severity vulnerability in Cisco Asyncos, classified under Improper Following of a Certificate's Chain of Trust. CVSS score: 7.4/10. Published 2021-06-16.

How severe is CVE-2021-1566?

High severity. CVSS v3 base score is 7.4 out of 10.

Vulnerability in Cisco Asyncos

CVE-2021-1566

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.

Published 2021-06-16 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 7.4 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N.

EPSS exploit prediction

EPSS: 0.007 (51.1th percentile), scored 2026-10-21.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 51st percentile — 51.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000

Affected products

Weakness classification (CWE)

References

tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-esa-wsa-… (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2021-1566?: CVE-2021-1566 is a high-severity vulnerability in Cisco Asyncos, classified under Improper Following of a Certificate's Chain of Trust. CVSS score: 7.4/10. Published 2021-06-16.
How severe is CVE-2021-1566?: High severity. CVSS v3 base score is 7.4 out of 10.