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CVE-2021-1566

24
FAUCET Score

CVE-2021-1566 is a high-severity vulnerability affecting Cisco AsyncOS for Cisco Email Security Appliance (ESA) and Web Security Appliance (WSA) when integrated with Cisco AMP for Endpoints. This flaw stems from improper certificate validation during TLS connections, allowing an unauthenticated, remote attacker to perform a man-in-the-middle (MitM) attack. A successful exploit could lead to spoofing trusted hosts, extracting sensitive information, or altering API requests. While no public exploit code or active exploitation is reported, the vulnerability has garnered some community discussion and media coverage.

Impacted Technologies

VendorProductVersion(s)CPE
Range not provided by sourceCPE matchmatch criteria
cpe:2.3:a:cisco:email_security_appliance:-:*:*:*:*:*:*:*
< 12.5.3-035CPE matchmatch criteria
cpe:2.3:o:cisco:asyncos:*:*:*:*:*:*:*:*
>= 13.0, < 13.0.0-030CPE matchmatch criteria
cpe:2.3:o:cisco:asyncos:*:*:*:*:*:*:*:*
>= 13.5, < 13.5.3-010CPE matchmatch criteria
cpe:2.3:o:cisco:asyncos:*:*:*:*:*:*:*:*
Range not provided by sourceCPE matchmatch criteria
cpe:2.3:a:cisco:web_security_appliance:-:*:*:*:*:*:*:*

CVSS Data

CVSS version used by this source: 3.1

7.4HIGH

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack Vector
NETWORK
Attack Complexity
HIGH
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
NONE
Exploitability Score
2.2
Impact Score
5.2
CvssVersion
3.1

Exploit Intelligence

EPSS Score
0.67%
Probability of exploitation in next 30 days
EPSS Percentile
48.2%
Percentile rank of EPSS score among Peer Group
As of 2026-07-27
Model: v2026.06.15
This CVE's current EPSS score of 0.0067 is in the 19th percentile among its peer group of 8,918 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.1 Reddit, 0.1 Bluesky, 0.1 Mastodon, and 0.2 GitHub mentions.

Media Mentions

The average CVE in this peer group has 0.1 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Vendor Patches (1)

ciscovendor investigatingvia nvd_reference
View patch

References

tools.cisco.com / security/center/content/CiscoSecurityAdvisory/cisco-sa-esa-wsa-cert-vali-n8L97RW
Vendor Advisory