CVE-2021-20591Network attack vector

MELSOFT port denial of service via connection exhaustion

Published Jun 11, 2021 · Updated Aug 3, 2024

Uncontrolled resource consumption in all listed MELSEC iQ-R CPU modules allows remote attackers to block legitimate MELSOFT TCP connections. Improper session management retains resources when an attacker leaves a connection open, exhausting the selected MELSOFT transmission port. Unauthenticated network access is sufficient; legitimate clients lose that port, while other open ports and sequence control remain unaffected.

CVSS severity5.3
Medium
EPSS probability1.50%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Uncontrolled resource consumption in all listed MELSEC iQ-R CPU modules allows remote attackers to block legitimate MELSOFT TCP connections. Improper session management retains resources when an attacker leaves a connection open, exhausting the selected MELSOFT transmission port. Unauthenticated network access is sufficient; legitimate clients lose that port, while other open ports and sequence control remain unaffected.

The record

CVE
CVE-2021-20591
Published
Jun 11, 2021
Updated
Aug 3, 2024
Vendor
Mitsubishi Electric Corporation
Product
MELSEC iQ-R Series CPU Modules
Classifications
CWE-400, T1499.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 11, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=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

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mitsubishi Electric Corporation · MELSEC iQ-R Series CPU Modulesversion=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

Published CVSS scores

7.5NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.3Mitsubishi Electric CorporationCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
7.8NIST National Vulnerability DatabaseAV:N/AC:L/Au:N/C:N/I:N/A:C

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.002Service Exhaustion Flood

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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