# CVE-2021-1566: Cisco Email Security Appliance and Cisco Web Security Appliance Certificate Validation Vulnerability

> A man-in-the-middle attacker can bypass TLS certificate validation to intercept traffic, extract sensitive information, or alter API requests on Cisco security appliances.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 16 Jun 2021
- **Updated:** 7 Nov 2024
- **Also known as:** GHSA-9pg8-7479-p4jw

## What is CVE-2021-1566?

CVE-2021-1566 is a high-severity vulnerability in Cisco Asyncos, published on 16 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-295 (Improper Certificate Validation), CWE-296 (Improper Following of a Certificate's Chain of Trust)

## Which versions are affected by CVE-2021-1566?

It affects 3 products.


Products: Cisco Asyncos, Cisco Email Security Appliance, Cisco Web Security Appliance.

## How do I fix CVE-2021-1566?

No fix has been published for CVE-2021-1566 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-1566 being exploited?

There is no known exploitation of CVE-2021-1566 in the wild and no public exploit code.

## Other vulnerabilities in Cisco Asyncos

- [CVE-2025-20393](https://insights.sbomify.com/CVE-2025-20393/): Cisco Multiple Products Improper Input Validation (critical 10.0, exploited in the wild)
- [CVE-2026-76519](https://insights.sbomify.com/CVE-2026-76519/): Cisco Secure Email Gateway Relative Path Traversal (critical 9.8, exploited in the wild)
- [CVE-2026-76461](https://insights.sbomify.com/CVE-2026-76461/): Cisco Secure Email Gateway SQL Injection (critical 9.8, exploited in the wild)
- [CVE-2024-20435](https://insights.sbomify.com/CVE-2024-20435/): Command injection in Cisco Secure Web Appliance (high 8.8)
- [CVE-2019-15956](https://insights.sbomify.com/CVE-2019-15956/): Cisco Web Security Appliance Unauthorized Device Reset Vulnerability (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-1566 and fix it if it is affected.

What it is: CVE-2021-1566 is a high-severity vulnerability in Cisco Asyncos, published on 16 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-1566 in the wild and no public exploit code.

Affected product: Cisco Asyncos. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-1566/ (Markdown: https://insights.sbomify.com/CVE-2021-1566/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-1566
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-1566
- OSV: https://osv.dev/vulnerability/CVE-2021-1566

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-1566/
Summaries are generated and may contain mistakes; the official description above is the record.