# CVE-2021-20596: Null dereference in MELSEC-F Series FX3U-ENET; FX3U-ENET-L; FX3U-ENET-P502

> Remote unauthenticated attackers can cause a denial of service in Mitsubishi MELSEC-F series PLC communication modules by sending specially crafted packets.

- **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:** 22 Jul 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-wrr3-chw5-r88x

## What is CVE-2021-20596?

CVE-2021-20596 is a high-severity vulnerability in MELSEC-F Series FX3U-ENET; FX3U-ENET-L; FX3U-ENET-P502, published on 22 Jul 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

NULL Pointer Dereference in MELSEC-F Series FX3U-ENET firmware version 1.14 and prior, FX3U-ENET-L firmware version 1.14 and prior and FX3U-ENET-P502 firmware version 1.14 and prior allows a remote unauthenticated attacker to cause a DoS condition in communication by sending specially crafted packets. Control by MELSEC-F series PLC is not affected and system reset is required for recovery.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2021-20596?

It affects 3 products.


Products: Mitsubishielectric Fx3u-Enet-L Firmware, Mitsubishielectric Fx3u-Enet-P502 Firmware, Mitsubishielectric Fx3u-Enet Firmware.

## How do I fix CVE-2021-20596?

No fix has been published for CVE-2021-20596 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-20596 being exploited?

There is no known exploitation of CVE-2021-20596 in the wild and no public exploit code.

## Other vulnerabilities in MELSEC-F Series FX3U-ENET; FX3U-ENET-L; FX3U-ENET-P502

- [CVE-2021-20612](https://insights.sbomify.com/CVE-2021-20612/): Denial of service in MELSEC-F series FX3U-ENET; MELSEC-F series FX3U-ENET-L; MELSEC-F series FX3U-ENET-P502 (high 7.5)
- [CVE-2021-20613](https://insights.sbomify.com/CVE-2021-20613/): Denial of service in MELSEC-F series FX3U-ENET; MELSEC-F series FX3U-ENET-L; MELSEC-F series FX3U-ENET-P502 (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-20596 and fix it if it is affected.

What it is: CVE-2021-20596 is a high-severity vulnerability in MELSEC-F Series FX3U-ENET; FX3U-ENET-L; FX3U-ENET-P502, published on 22 Jul 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-2021-20596 in the wild and no public exploit code.

Affected product: MELSEC-F Series FX3U-ENET; FX3U-ENET-L; FX3U-ENET-P502. 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-2021-20596/ (Markdown: https://insights.sbomify.com/CVE-2021-20596/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-20596
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-20596
- OSV: https://osv.dev/vulnerability/CVE-2021-20596

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-20596/
Summaries are generated and may contain mistakes; the official description above is the record.