CODESYS V2 Denial of Service


CODESYS V2 Denial of Service


CODESYS Runtime Toolkit 32-bit, CODESYS PLCWinNT

Vulnerable Version


Fixed Version


CVE Number







Gerhard Hechenberger, Steffen Robertz (Office Vienna) | SEC Consult Vulnerability Lab

These vulnerabilities were discovered during the research cooperation initiative "OT Cyber Security Lab" between Verbund AG and SEC Consult Group.


The CODESYS runtime versions before V2.4.7.56 are vulnerable for a denial of service (DoS) condition of the CODESYS development system connection. By exploiting this vulnerability, an attacker can prevent the remote administration of affected PLCs.

Vendor description

"CODESYS is the leading manufacturer-independent IEC 61131-3 automation
software for engineering control systems."


Business recommendation

The vendor provides patches. The vendors of products using the affected
software should provide new firmware versions immediately. Users of these
products should update their devices to those fixed firmware versions.

Vulnerability overview/description

The CODESYS Control runtime system is the core of many PLCs. The runtime is 
accepting TCP connections on a pre-configured port to connect to the 
development system. By sending requests that define an invalid packet size,
a memory allocation error can be triggered. This leads to a denial of service
condition of the remote connectivity of the CODESYS service, which prevents
clients from connecting to the affected PLC.

CODESYS released a dedicated security note, which corresponds to this advisory:

Proof of concept

A detailed proof of concept will be made public after the affected product
vendors had time to provide new firmware versions.

Vulnerable / tested versions

Vendor contact timeline

2021-05-25 Contacting 3rd party vendor of a product using the CODESYS runtime about this issue.
2021-08-11 Vendor states that this issue was already fixed in a recent CODESYS release.
2021-09-01 The vendor confirms and ensures the issue is investigated in collaboration with CODESYS.
2021-08-18 A check on the product's most recent public firmware release shows that the vulnerability still exists. The vendor is notified again about this outcome.
2021-10-15 CODESYS informs about the assigned CVE-2021-34593 and the planned publishing date.
2021-10-28 Coordinated release.


Immediately update to the patched version of CODESYS.


To mitigate this issue, access to the CODESYS service port of the affected
devices should be limited as far as possible. In the long run, the updated
firmware of the product vendor containing a patched CODESYS service must be

Advisory URL

SEC Consult Vulnerability Lab

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EOF Gerhard Hechenberger, Steffen Robertz / @2021