CVE-2021-20612
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedLack of administrator control over security vulnerability 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 denial-of-service (DoS) condition in communication function of the product or other unspecified effects by sending specially crafted packets to an unnecessary opening of TCP port. Control by MELSEC-F series PLC is not affected by this vulnerability, but system reset is required for recovery.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceMELSEC-F series Ethernet communication modules (FX3U-ENET, FX3U-ENET-L, FX3U-ENET-P502) firmware 1.14 and prior contain an unnecessary open TCP port that lacks proper administrative access controls. Remote unauthenticated attackers can send specially crafted packets to trigger a denial-of-service condition in the communication function, requiring a system reset for recovery.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data<= 1.14<= 1.14<= 1.14CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify the Ethernet module modelLocate the hardware model label or check system configuration to confirm the exact model number is one of: FX3U-ENET, FX3U-ENET-L, or FX3U-ENET-P502Affected if The model is not one of these three listed variants, then this CVE does not apply
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Check the installed firmware versionAccess the module web interface, check via configuration software (such as GX Works2 or GX Developer), or query the device directly to retrieve the firmware versionAffected if Firmware version cannot be determined or device does not support version query
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Verify the Ethernet communication function is enabledConfirm via the module configuration that the Ethernet communication function is actively configured and operational, not merely installed but unusedAffected if The Ethernet communication function is enabled and the module is actively running
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Assess network exposure of the moduleScan the network segment where the module is deployed to identify open TCP ports, or review firewall/network segmentation configurations to determine if the module is accessible from untrusted networksAffected if The module TCP ports are reachable from untrusted or external network segments without proper access controls
A user is affected if they have an FX3U-ENET, FX3U-ENET-L, or FX3U-ENET-P502 module running firmware version 1.14 or lower with the Ethernet communication function enabled and exposed to accessible TCP ports.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpdate firmware to version 1.15 or later if available, and implement network segmentation or firewall rules to block unauthorized access to the exposed TCP port until the update can be applied.
- Consultation3.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,408.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-20612 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-20612 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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