# CVE-2021-34781: Cisco Firepower Threat Defense Software SSH Connections Denial of Service Vulnerability

> Unauthenticated remote attackers can cause denial of service on Cisco FTD devices by exhausting resources via crafted SSH connections in multi-instance deployments.

- **Severity:** High 8.6 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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:** 27 Oct 2021
- **Updated:** 7 Nov 2024
- **Also known as:** GHSA-cg69-5r4f-47q5

## What is CVE-2021-34781?

CVE-2021-34781 is a high-severity denial of service vulnerability in Cisco Secure Firewall Threat Defense, published on 27 Oct 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A vulnerability in the processing of SSH connections for multi-instance deployments of Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on the affected device. This vulnerability is due to a lack of proper error handling when an SSH session fails to be established. An attacker could exploit this vulnerability by sending a high rate of crafted SSH connections to the instance. A successful exploit could allow the attacker to cause resource exhaustion, which causes a DoS condition on the affected device. The device must be manually reloaded to recover.

Weakness: CWE-755 (Improper Handling of Exceptional Conditions), CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer)

## Which versions are affected by CVE-2021-34781?

It affects 3 products.


Products: Cisco Firepower Management Center Virtual Appliance, Cisco Secure Firewall Threat Defense, Cisco Sourcefire Defense Center.

## How do I fix CVE-2021-34781?

No fix has been published for CVE-2021-34781 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-34781 being exploited?

There is no known exploitation of CVE-2021-34781 in the wild and no public exploit code.

## Other vulnerabilities in Cisco Secure Firewall Threat Defense

- [CVE-2021-34762](https://insights.sbomify.com/CVE-2021-34762/): Cisco Firepower Management Center Software Authenticated Directory Traversal Vulnerability (high 8.1)
- [CVE-2021-34755](https://insights.sbomify.com/CVE-2021-34755/): Cisco Firepower Threat Defense Software Command Injection Vulnerabilities (medium 6.7)
- [CVE-2021-34756](https://insights.sbomify.com/CVE-2021-34756/): Cisco Firepower Threat Defense Software Command Injection Vulnerabilities (medium 6.7)
- [CVE-2021-34763](https://insights.sbomify.com/CVE-2021-34763/): Cisco Firepower Management Center Software Cross-Site Scripting and Open Redirect Vulnerabilities (medium 4.8)
- [CVE-2021-34764](https://insights.sbomify.com/CVE-2021-34764/): Cisco Firepower Management Center Software Cross-Site Scripting and Open Redirect Vulnerabilities (medium 4.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-34781 and fix it if it is affected.

What it is: CVE-2021-34781 is a high-severity denial of service vulnerability in Cisco Secure Firewall Threat Defense, published on 27 Oct 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-2021-34781 in the wild and no public exploit code.

Affected product: Cisco Secure Firewall Threat Defense. 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-2021-34781/ (Markdown: https://insights.sbomify.com/CVE-2021-34781/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-34781
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34781
- OSV: https://osv.dev/vulnerability/CVE-2021-34781

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-34781/
Summaries are generated and may contain mistakes; the official description above is the record.