# CVE-2020-27721: Denial of service in BIG-IP GTM & DNS

> Attackers can cause BIG-IP DNS to stop using LTM virtual servers for DNS responses, leading to service disruption in GSLB deployments.

- **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:** 24 Dec 2020
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-7456-9w6m-v94r

## What is CVE-2020-27721?

CVE-2020-27721 is a high-severity denial of service vulnerability in F5 Big-Ip Domain Name System, published on 24 Dec 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In versions 16.0.0-16.0.0.1, 15.1.0-15.1.1, 14.1.0-14.1.3, 13.1.0-13.1.3.5, 12.1.0-12.1.5.2, and 11.6.1-11.6.5.2, in a BIG-IP DNS / BIG-IP LTM GSLB deployment, under certain circumstances, the BIG-IP DNS system may stop using a BIG-IP LTM virtual server for DNS response.

## Which versions are affected by CVE-2020-27721?

It affects 2 products.


Products: F5 Big-Ip Domain Name System, F5 Big-Ip Global Traffic Manager.

## How do I fix CVE-2020-27721?

No fix has been published for CVE-2020-27721 yet. Limit exposure of the affected software until one is available.

## Is CVE-2020-27721 being exploited?

There is no known exploitation of CVE-2020-27721 in the wild and no public exploit code.

## Other vulnerabilities in F5 Big-Ip Domain Name System

- [CVE-2022-23013](https://insights.sbomify.com/CVE-2022-23013/): Cross-site scripting in BIG-IP DNS & GTM (high 8.8)
- [CVE-2026-40061](https://insights.sbomify.com/CVE-2026-40061/): iControl REST and tmsh vulnerability (high 8.5)
- [CVE-2022-41787](https://insights.sbomify.com/CVE-2022-41787/): BIG-IP DNS Express vulnerability CVE-2022-41787 (high 7.5)
- [CVE-2021-23032](https://insights.sbomify.com/CVE-2021-23032/): Denial of service in BIG-IP DNS (high 7.5)
- [CVE-2020-27717](https://insights.sbomify.com/CVE-2020-27717/): Denial of service in BIG-IP DNS (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-27721 and fix it if it is affected.

What it is: CVE-2020-27721 is a high-severity denial of service vulnerability in F5 Big-Ip Domain Name System, published on 24 Dec 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-2020-27721 in the wild and no public exploit code.

Affected product: F5 Big-Ip Domain Name System. 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-2020-27721/ (Markdown: https://insights.sbomify.com/CVE-2020-27721/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-27721
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-27721
- OSV: https://osv.dev/vulnerability/CVE-2020-27721

---
From sbomify Insights: https://insights.sbomify.com/CVE-2020-27721/
Summaries are generated and may contain mistakes; the official description above is the record.