CVE-2026-3509: CODESYS Control Runtime DoS Vulnerability

CVE-2026-3509 Overview

CVE-2026-3509 is a format string vulnerability affecting the CODESYS Control runtime system. An unauthenticated remote attacker may be able to control the format string of messages processed by the Audit Log component, potentially resulting in a denial-of-service (DoS) condition. This vulnerability is classified under CWE-134 (Use of Externally-Controlled Format String).

Critical Impact

Unauthenticated attackers can remotely crash CODESYS Control runtime systems by exploiting format string handling in the Audit Log, potentially disrupting industrial control system operations.

Affected Products

• CODESYS Control runtime system (specific versions not disclosed)

Discovery Timeline

• 2026-03-24 - CVE-2026-3509 published to NVD
• 2026-03-24 - Last updated in NVD database

Technical Details for CVE-2026-3509

Vulnerability Analysis

This vulnerability is a classic format string attack vector targeting the CODESYS Control runtime system's Audit Log functionality. Format string vulnerabilities occur when user-controlled input is passed directly to formatting functions (such as printf(), sprintf(), or similar) without proper sanitization. In this case, an attacker can craft malicious messages containing format specifiers that, when processed by the Audit Log component, cause the application to behave unexpectedly.

The network-accessible nature of this vulnerability allows remote exploitation without requiring authentication credentials. The impact is limited to availability—attackers can cause a denial-of-service condition, but confidentiality and integrity remain unaffected based on the vulnerability characteristics.

Root Cause

The root cause is the use of externally-controlled format strings (CWE-134) in the Audit Log message processing functionality. The CODESYS Control runtime fails to properly sanitize or validate input strings before passing them to formatting functions. This allows attackers to inject format specifiers such as %s, %x, %n, or others that can cause memory access violations, crashes, or undefined behavior when the format function attempts to read from or write to arbitrary memory locations.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can send specially crafted network packets containing malicious format strings to the CODESYS Control runtime system. When these messages are processed by the Audit Log component, the injected format specifiers trigger abnormal behavior in the formatting function, leading to a crash or denial-of-service condition.

The vulnerability manifests in the Audit Log message handling routines. When the runtime processes log entries, it incorporates user-controllable data into format strings without proper validation. An attacker can exploit this by including format specifiers like %s%s%s%s%s to cause the application to read from invalid memory addresses, or %n to potentially write to memory (though in this case, the primary impact is availability). See the CERTVDE Advisory VDE-2026-018 for additional technical details.

Detection Methods for CVE-2026-3509

Indicators of Compromise

• Unexpected crashes or restarts of the CODESYS Control runtime service
• Audit Log entries containing unusual format specifiers such as %s, %x, %n, or repeated % characters
• Network traffic to CODESYS services containing format string patterns in message payloads
• Abnormal memory access errors or segmentation faults in runtime logs

Detection Strategies

• Deploy network intrusion detection rules to identify packets containing format string attack patterns targeting CODESYS protocols
• Monitor CODESYS Control runtime process health and implement alerting for unexpected service terminations
• Implement application-level logging to capture malformed or suspicious Audit Log messages before processing
• Use SentinelOne Singularity XDR to detect exploitation attempts through behavioral analysis of process crashes and anomalous network activity

Monitoring Recommendations

• Configure SIEM alerts for repeated CODESYS runtime crashes or service restarts
• Establish baseline behavior for CODESYS network communications and alert on deviations
• Monitor system event logs for application faults related to format string processing
• Implement network segmentation monitoring to detect unauthorized access attempts to industrial control systems

How to Mitigate CVE-2026-3509

Immediate Actions Required

• Review the CERTVDE Advisory VDE-2026-018 for vendor-specific remediation guidance
• Implement network segmentation to isolate CODESYS Control systems from untrusted networks
• Apply firewall rules to restrict access to CODESYS runtime services to authorized IP addresses only
• Monitor for vendor patch releases and apply updates as soon as they become available

Patch Information

Consult the CERTVDE Advisory VDE-2026-018 for official patch information and remediation guidance from the vendor. Organizations should monitor CODESYS security advisories for firmware or software updates that address this format string vulnerability.

Workarounds

• Implement strict network access controls to limit connectivity to CODESYS Control runtime systems
• Deploy network-level filtering to block traffic containing common format string attack patterns
• Consider placing CODESYS systems behind a VPN or other secure remote access solution
• Enable additional logging and monitoring to detect exploitation attempts while awaiting patches

Network segmentation and access control configurations should be implemented according to your organization's security policies. Restrict access to CODESYS runtime services by configuring firewall rules to allow connections only from authorized management stations and engineering workstations.