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No exploitation reported

CVE-2021-20591

No exploitation reported by the sources we read.

  • Published2021-06-11
  • WeaknessCWE-400

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

Uncontrolled 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.

Timeline

dates in UTC
  1. 2021-06-1116:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-20591?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-20591?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Mitsubishielectric r00cpu firmwareversion to verify
BranchAffectedFixed in
—all versionsno fix published
Mitsubishielectric r01cpu firmwareversion to verify
BranchAffectedFixed in
—all versionsno fix published
Mitsubishielectric r02cpu firmwareversion to verify
BranchAffectedFixed in
—all versionsno fix published

17 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • ICSA-21-147-05 CISA5 known affected

    Mitsubishi Electric MELSEC iQ-R Series

    R08/16/32/120SFCPU: All versions · R00/01/02CPU: All versions · R08/16/32/120PCPU: All versions · +2 more

Full record on NVD

Fix

  1. Mitigation from the vendor CISA ICSA-21-147-05 · 5 productsUse a firewall, a virtual private network (VPN), etc., to prevent unauthorized access when Internet access is required.
  2. Mitigation from the vendor CISA ICSA-21-147-05 · 5 productsUse within a LAN and block access from untrusted networks and hosts through firewalls.
  3. Mitigation from the vendor CISA ICSA-21-147-05 · 5 productsUse the IP filter function to restrict the connectable IP addresses. See MELSEC iQ-R Ethernet User's Manual (Application) 1.13 Security “IP filter” for more information.
  4. Mitigation from the vendor CISA ICSA-21-147-05 · 5 productsUse the MELSOFT transmission port (UDP/IP).
  5. Mitigation from the vendor CISA ICSA-21-147-05 · 5 productsIf Port 5007 of the MELSOFT transmission port (TCP/IP) is not used, set b2 to "1" in advance. See MELSEC iQ-R Ethernet User's Manual (Application) Appendix 3, Buffer Memory for more information.
  6. Mitigation from the vendor CISA ICSA-21-147-05 · 5 productsIf this vulnerability is exploited, legitimate users can recover by first disabling the port with the forced connection invalidation function and then re-enabling the port. For an example see MELSEC iQ-R Ethernet User's Manual (Application) Appendix 3, Buffer Memory.

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • NVDCVSS 3.17.5read 2026-10-23
  • NVDCVSS 2.07.8read 2026-10-23
  • NVDweakness (CWE)CWE-400read 2026-10-23
  • FIRST EPSSEPSS score1.5 %2026-10-22

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Published
2021-06-11
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-20591?

Uncontrolled 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.

Is CVE-2021-20591 actively exploited?

Active exploitation of CVE-2021-20591 has not been confirmed. Its EPSS score was 1.5% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-20591?

CVE-2021-20591 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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