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

> Unauthenticated remote attackers can cause a denial of service on Cisco Email Security Appliances by sending crafted emails that exhaust CPU resources.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 4 Nov 2021
- **Updated:** 7 Nov 2024
- **Also known as:** GHSA-mcrm-m53h-jvp2

## What is CVE-2021-34741?

CVE-2021-34741 is a high-severity denial of service vulnerability in Cisco Asyncos, published on 4 Nov 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2021-34741?

It affects 1 product.


Products: Cisco Asyncos.

## How do I fix CVE-2021-34741?

No fix has been published for CVE-2021-34741 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-34741 being exploited?

There is no known exploitation of CVE-2021-34741 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-2021-34741 and fix it if it is affected.

What it is: CVE-2021-34741 is a high-severity denial of service vulnerability in Cisco Asyncos, published on 4 Nov 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-2021-34741 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-2021-34741/ (Markdown: https://insights.sbomify.com/CVE-2021-34741/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-34741
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34741
- OSV: https://osv.dev/vulnerability/CVE-2021-34741

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-34741/
Summaries are generated and may contain mistakes; the official description above is the record.