CVE-2021-22659Network attack vector

Unauthenticated service interruption via Modbus buffer overflow

Published Mar 25, 2021 · Updated Jun 3, 2026

Buffer overflow in Modbus TCP handling in Rockwell Automation MicroLogix 1400 21.6 and earlier allows remote attackers to interrupt service. The Modbus packet-processing path copies attacker-controlled data without checking its size, allowing random register reads or writes before the controller faults. Modbus TCP must be enabled; exploitation makes the FAULT LED flash red, can break communications, and requires manual fault clearing.

CVSS severity8.1
High
EPSS probability1.72%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Buffer overflow in Modbus TCP handling in Rockwell Automation MicroLogix 1400 21.6 and earlier allows remote attackers to interrupt service. The Modbus packet-processing path copies attacker-controlled data without checking its size, allowing random register reads or writes before the controller faults. Modbus TCP must be enabled; exploitation makes the FAULT LED flash red, can break communications, and requires manual fault clearing.

The record

CVE
CVE-2021-22659
Published
Mar 25, 2021
Updated
Jun 3, 2026
Vendor
Rockwell Automation
Product
MicroLogix 1400
Classifications
CWE-120, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Jun 3, 2026Record 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=MicroLogix 1400, All series Version 21.6 and below

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Rockwell Automation · MicroLogix 1400version=MicroLogix 1400, All series Version 21.6 and below

Published CVSS scores

7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
8.1Rockwell AutomationCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:H
8.6NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H

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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
T1499.004Endpoint Denial of Service: Application or System Exploitation

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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