CVE-2021-34593Public exploitNetwork attack vector

Service interruption via unclosed invalid-size connections

Published Oct 26, 2021 · Updated Sep 17, 2024

Denial of service in CODESYS V2 PLCWinNT and Runtime Toolkit before 2.4.7.56 allows remote attackers to block PLC connections. The TCP communication server trusts a signed packet-size field, attempts a 4 GB allocation for 0xffffffff, and returns after allocation failure without closing the client socket. Nineteen unauthenticated requests can exhaust the connection limit, blocking further clients; other malformed requests can stop PLC programs or leak memory.

CVSS severity7.5
High
EPSS probability2.74%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in CODESYS V2 PLCWinNT and Runtime Toolkit before 2.4.7.56 allows remote attackers to block PLC connections. The TCP communication server trusts a signed packet-size field, attempts a 4 GB allocation for 0xffffffff, and returns after allocation failure without closing the client socket. Nineteen unauthenticated requests can exhaust the connection limit, blocking further clients; other malformed requests can stop PLC programs or leak memory.

The record

CVE
CVE-2021-34593
Published
Oct 26, 2021
Updated
Sep 17, 2024
Vendor
CODESYS GmbH
Product
CODESYS Development System V2.3
Classifications
CWE-755, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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=PLCWinNT <V2.4.7.56
  2. Affected versionversion=Runtime Toolkit 32 bit full <V2.4.7.56

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

CODESYS GmbH · CODESYS Development System V2.3version=PLCWinNT <V2.4.7.56; version=Runtime Toolkit 32 bit full <V2.4.7.56

Published CVSS scores

5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5CERT@VDECVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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 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 exploitPublished

Weakness, pattern, technique

CWE-755Improper Handling of Exceptional Conditions
T1499.004Application or System Exploitation

Public exploit references

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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