# CVE-2021-21002: Denial of Service in Phoenix Contact FL COMSERVER UNI products

> An attacker can cause a temporary denial of service in Phoenix Contact FL COMSERVER UNI devices by sending invalid Modbus exception responses.

- **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:** 25 Jun 2021
- **Updated:** 17 Sep 2024
- **Also known as:** GHSA-wc86-7c83-q7g5

## What is CVE-2021-21002?

CVE-2021-21002 is a high-severity denial of service vulnerability in Phoenix Contact FL COMSERVER, published on 25 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In Phoenix Contact FL COMSERVER UNI in versions < 2.40 a invalid Modbus exception response can lead to a temporary denial of service.

Weakness: CWE-772 (Missing Release of Resource after Effective Lifetime)

## Which versions are affected by CVE-2021-21002?

It affects 2 products.


Products: Phoenixcontact Fl Comserver Uni 232\/422\/485-T Firmware, Phoenixcontact Fl Comserver Uni 232\/422\/485 Firmware.

## How do I fix CVE-2021-21002?

No fix has been published for CVE-2021-21002 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-21002 being exploited?

There is no known exploitation of CVE-2021-21002 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-21002 and fix it if it is affected.

What it is: CVE-2021-21002 is a high-severity denial of service vulnerability in Phoenix Contact FL COMSERVER, published on 25 Jun 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-2021-21002 in the wild and no public exploit code.

Affected product: Phoenix Contact FL COMSERVER. 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-2021-21002/ (Markdown: https://insights.sbomify.com/CVE-2021-21002/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21002
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21002
- OSV: https://osv.dev/vulnerability/CVE-2021-21002

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-21002/
Summaries are generated and may contain mistakes; the official description above is the record.