# CVE-2020-3421: Cisco IOS XE Software Zone-Based Firewall Denial of Service Vulnerabilities

> Remote attackers can cause Cisco IOS XE devices to reload or stop forwarding traffic by sending specific Layer 4 packet sequences.

- **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:** 24 Sep 2020
- **Updated:** 13 Nov 2024
- **Also known as:** GHSA-9gg8-xvhp-2q7g

## What is CVE-2020-3421?

CVE-2020-3421 is a high-severity denial of service vulnerability in Cisco Ios Xe, published on 24 Sep 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Multiple vulnerabilities in the Zone-Based Firewall feature of Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause the device to reload or stop forwarding traffic through the firewall. The vulnerabilities are due to incomplete handling of Layer 4 packets through the device. An attacker could exploit these vulnerabilities by sending a certain sequence of traffic patterns through the device. A successful exploit could allow the attacker to cause the device to reload or stop forwarding traffic through the firewall, resulting in a denial of service. For more information about these vulnerabilities, see the Details section of this advisory.

Weakness: CWE-754 (Improper Check for Unusual or Exceptional Conditions)

## Which versions are affected by CVE-2020-3421?

It affects 1 product.


Products: Cisco Ios Xe.

## How do I fix CVE-2020-3421?

No fix has been published for CVE-2020-3421 yet. Limit exposure of the affected software until one is available.

## Is CVE-2020-3421 being exploited?

There is no known exploitation of CVE-2020-3421 in the wild and no public exploit code.

## Other vulnerabilities in Cisco Ios Xe

- [CVE-2023-20198](https://insights.sbomify.com/CVE-2023-20198/): Cisco IOS XE Web UI Privilege Escalation (critical 10.0, exploited in the wild)
- [CVE-2018-0151](https://insights.sbomify.com/CVE-2018-0151/): Cisco IOS Software and Cisco IOS XE Software Quality of Service Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2023-20273](https://insights.sbomify.com/CVE-2023-20273/): Cisco IOS XE Web UI Command Injection (high 7.2, exploited in the wild)
- [CVE-2025-20188](https://insights.sbomify.com/CVE-2025-20188/): Hardcoded credentials in Cisco IOS XE Software (critical 10.0)
- [CVE-2021-34770](https://insights.sbomify.com/CVE-2021-34770/): Cisco IOS XE Software for Catalyst 9000 Family Wireless Controllers CAPWAP Remote Code Execution Vulnerability (critical 10.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-3421 and fix it if it is affected.

What it is: CVE-2020-3421 is a high-severity denial of service vulnerability in Cisco Ios Xe, published on 24 Sep 2020. 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-2020-3421 in the wild and no public exploit code.

Affected product: Cisco Ios Xe. 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-2020-3421/ (Markdown: https://insights.sbomify.com/CVE-2020-3421/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-3421
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-3421
- OSV: https://osv.dev/vulnerability/CVE-2020-3421

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From sbomify Insights: https://insights.sbomify.com/CVE-2020-3421/
Summaries are generated and may contain mistakes; the official description above is the record.