CVE-2021-20591
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 · uneditedUncontrolled Resource Consumption vulnerability in Mitsubishi Electric MELSEC iQ-R series CPU modules (R00/01/02CPU all versions, R04/08/16/32/120(EN)CPU all versions, R08/16/32/120SFCPU all versions, R08/16/32/120PCPU all versions, R08/16/32/120PSFCPU all versions) allows a remote unauthenticated attacker to prevent legitimate clients from connecting to the MELSOFT transmission port (TCP/IP) by not closing a connection properly, which may lead to a denial of service (DoS) condition.
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 confidenceA remote unauthenticated attacker can cause denial of service by improperly maintaining connections to the MELSOFT transmission port (TCP/IP), exhausting available connection slots and preventing legitimate clients from connecting to Mitsubishi Electric MELSEC iQ-R series CPU modules.
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 dataall versionsall versionsall versionsall versionsall versionsall versionsall versionsall versionsCVSS 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 Mitsubishi MELSEC iQ-R CPU modules on the networkUse network scanning or asset discovery to find devices with Mitsubishi Electric R00cpu, R01cpu, R02cpu, R04cpu, R08cpu, R16cpu, R32cpu, or R120cpu firmware. Check device model identification or consult asset inventory.Affected if Any of these CPU module models are present on the network
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Verify MELSOFT transmission port is accessibleCheck network exposure of the MELSOFT transmission port (TCP/IP) on identified MELSEC iQ-R devices. Use port scanning or review firewall rules to determine if the port accepts external connections.Affected if The MELSOFT transmission port is reachable from untrusted or external networks
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Check for connection exhaustion symptomsMonitor the affected CPU module for symptoms of connection exhaustion: inability of legitimate MELSOFT clients (GX Works3, etc.) to establish new sessions, connection timeout errors, or unusual latency when connecting.Affected if Legitimate clients cannot connect and connection attempts fail due to slot exhaustion
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Review network segmentation controlsEvaluate whether the MELSEC iQ-R devices with MELSOFT ports are placed behind network segmentation (VLANs, firewalls) that restrict access to trusted engineering stations and management networks only.Affected if The devices lack network segmentation and are directly exposed to untrusted networks
A defender is affected if they have any Mitsubishi MELSEC iQ-R series CPU modules (R00cpu through R120cpu) with the MELSOFT transmission port exposed to untrusted networks and are experiencing connection exhaustion.
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 dataImplement connection timeout mechanisms and connection rate limiting on the MELSOFT transmission port to automatically close idle or improperly terminated connections, and consider network segmentation to limit exposure to untrusted networks.
- Consultation3.0 h
- Implementation6.0 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 $3,984.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-20591 — 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-2021-20591 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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- 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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