# CVE-2021-34791: Security feature bypass in Cisco Adaptive Security Appliance (ASA) Software

> Unauthenticated remote attackers can bypass the Application Level Gateway to open unauthorized connections to hosts behind the firewall.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 27 Oct 2021
- **Updated:** 7 Nov 2024
- **Also known as:** GHSA-qhq7-rm64-qh84

## What is CVE-2021-34791?

CVE-2021-34791 is a medium-severity vulnerability in Cisco Adaptive Security Appliance, published on 27 Oct 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Multiple vulnerabilities in the Application Level Gateway (ALG) for the Network Address Translation (NAT) feature of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to bypass the ALG and open unauthorized connections with a host located behind the ALG. For more information about these vulnerabilities, see the Details section of this advisory. Note: These vulnerabilities have been publicly discussed as NAT Slipstreaming.

Weakness: CWE-20 (Improper Input Validation), CWE-358 (Improperly Implemented Security Check for Standard)

## Which versions are affected by CVE-2021-34791?

It affects 11 products.


Products: Cisco Adaptive Security Appliance, Cisco Adaptive Security Appliance Software, Cisco Asa 5505 Firmware, Cisco Asa 5512-X Firmware, Cisco Asa 5515-X Firmware, Cisco Asa 5525-X Firmware, Cisco Asa 5545-X Firmware, Cisco Asa 5555-X Firmware, Cisco Asa 5580 Firmware, Cisco Asa 5585-X Firmware, Cisco Secure Firewall Threat Defense.

## How do I fix CVE-2021-34791?

No fix has been published for CVE-2021-34791 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-34791 being exploited?

There is no known exploitation of CVE-2021-34791 in the wild and no public exploit code.

## Other vulnerabilities in Cisco Adaptive Security Appliance

- [CVE-2021-1573](https://insights.sbomify.com/CVE-2021-1573/): Denial of service in Cisco Adaptive Security Appliance Software (high 8.6)
- [CVE-2021-34793](https://insights.sbomify.com/CVE-2021-34793/): Denial of service in Cisco Adaptive Security Appliance (ASA) Software (high 8.6)
- [CVE-2021-40117](https://insights.sbomify.com/CVE-2021-40117/): Denial of service in Cisco Adaptive Security Appliance (ASA) Software (high 8.6)
- [CVE-2021-40118](https://insights.sbomify.com/CVE-2021-40118/): Denial of service in Cisco Adaptive Security Appliance (ASA) Software (high 8.6)
- [CVE-2020-3304](https://insights.sbomify.com/CVE-2020-3304/): Denial of service in Cisco Adaptive Security Appliance (ASA) Software (high 8.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-34791 and fix it if it is affected.

What it is: CVE-2021-34791 is a medium-severity vulnerability in Cisco Adaptive Security Appliance, published on 27 Oct 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-34791 in the wild and no public exploit code.

Affected product: Cisco Adaptive Security Appliance. 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-2021-34791/ (Markdown: https://insights.sbomify.com/CVE-2021-34791/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-34791
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34791
- OSV: https://osv.dev/vulnerability/CVE-2021-34791

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-34791/
Summaries are generated and may contain mistakes; the official description above is the record.