CVE-2025-41739: CODESYS Control Runtime DoS Vulnerability

CVE-2025-41739 Overview

CVE-2025-41739 is an Out-of-Bounds Read vulnerability affecting the CODESYS Control runtime system on Linux and QNX platforms. An unauthenticated remote attacker who successfully exploits a race condition can trigger this flaw in the communication servers via crafted socket communication, potentially resulting in a denial of service condition.

This vulnerability is classified as CWE-125 (Out-of-bounds Read) and carries a CVSS v3.1 score of 5.9 (Medium severity). The attack requires network access and exploitation of a race condition, which increases the complexity but does not require authentication or user interaction.

Critical Impact

Successful exploitation allows remote attackers to cause denial of service on industrial control systems running CODESYS Control runtime, potentially disrupting critical operational technology (OT) environments.

Affected Products

• CODESYS Control runtime system on Linux
• CODESYS Control runtime system on QNX
• Communication servers within CODESYS Control runtime

Discovery Timeline

• 2025-12-01 - CVE-2025-41739 published to NVD
• 2025-12-01 - Last updated in NVD database

Technical Details for CVE-2025-41739

Vulnerability Analysis

The vulnerability resides in the communication servers of the CODESYS Control runtime system. When processing socket communications, the runtime fails to properly validate buffer boundaries under certain race conditions, leading to an out-of-bounds read operation.

The CVSS vector CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H indicates:

• Attack Vector (AV:N): Network-accessible, allowing remote exploitation
• Attack Complexity (AC:H): High complexity due to race condition requirement
• Privileges Required (PR:N): No authentication needed
• User Interaction (UI:N): No user interaction required
• Scope (S:U): Impact limited to the vulnerable component
• Confidentiality (C:N): No confidentiality impact
• Integrity (I:N): No integrity impact
• Availability (A:H): High availability impact (denial of service)

The EPSS (Exploit Prediction Scoring System) probability is 0.107% with a percentile of 29.59, suggesting relatively low likelihood of exploitation in the wild at this time.

Root Cause

The root cause is improper bounds checking in the communication server's socket handling routines. When processing incoming network data, a race condition exists that can cause the runtime to read memory beyond allocated buffer boundaries. This is a classic CWE-125 vulnerability where memory access operations do not properly validate array indices or pointer arithmetic against buffer limits.

Attack Vector

The attack requires an unauthenticated remote attacker to:

1. Establish a network connection to the CODESYS Control runtime communication server
2. Send specially crafted socket communications designed to trigger the race condition
3. Successfully win the race condition to cause the out-of-bounds read
4. The resulting memory access violation causes the service to crash or become unresponsive

The vulnerability is exploited through crafted socket communications targeting the communication servers. The attacker must successfully trigger a race condition in the socket processing logic to cause the out-of-bounds memory read. Due to the timing-dependent nature of race conditions, exploitation may require multiple attempts. See the security advisory at https://certvde.com/de/advisories/VDE-2025-099 for additional technical details.

Detection Methods for CVE-2025-41739

Indicators of Compromise

• Unexpected crashes or restarts of CODESYS Control runtime services
• Abnormal network traffic patterns to CODESYS communication server ports
• High volume of malformed socket connection attempts from single sources
• Memory access violations in system logs related to CODESYS processes
• Service availability issues correlating with network activity spikes

Detection Strategies

Network-based detection should focus on monitoring traffic to CODESYS Control runtime communication ports for anomalous patterns. Implement intrusion detection rules to identify:

• Rapid connection attempts that may indicate race condition exploitation attempts
• Malformed or unusual socket communication payloads
• Connection patterns inconsistent with normal SCADA/ICS traffic profiles

Host-based detection should monitor CODESYS runtime processes for:

• Segmentation faults or access violations
• Unexpected process terminations
• Memory corruption indicators in application logs

SentinelOne Singularity platform can detect anomalous process behavior associated with out-of-bounds read exploitation attempts, providing real-time visibility into runtime integrity and crash events.

Monitoring Recommendations

Organizations should implement continuous monitoring of CODESYS Control runtime systems with focus on:

1. Process Monitoring: Track CODESYS runtime process health and restart frequency
2. Network Monitoring: Log and analyze all connections to CODESYS communication servers
3. System Logs: Centralize and monitor system logs for memory access violations
4. Performance Baselines: Establish normal operation baselines to identify anomalies
5. Availability Monitoring: Implement alerting for service availability degradation

How to Mitigate CVE-2025-41739

Immediate Actions Required

• Review the CERT@VDE advisory VDE-2025-099 for vendor-specific guidance
• Implement network segmentation to isolate CODESYS Control runtime systems
• Restrict network access to communication server ports using firewalls
• Enable logging and monitoring on affected systems
• Prepare incident response procedures for potential denial of service events

Patch Information

Organizations should consult the official CERT@VDE advisory at https://certvde.com/de/advisories/VDE-2025-099 for patch availability and installation guidance from CODESYS. Apply vendor-provided security updates as soon as they become available, following proper change management procedures for OT environments.

Workarounds

If immediate patching is not possible, implement the following compensating controls:

Network segmentation is the primary defensive measure for industrial control systems. Ensure CODESYS Control runtime systems are isolated within dedicated network segments with strict access controls.

# Example firewall rule to restrict access to CODESYS ports (adjust port numbers per deployment)
# Limit access to known management IPs only
iptables -A INPUT -p tcp --dport <CODESYS_PORT> -s <TRUSTED_MGMT_NETWORK> -j ACCEPT
iptables -A INPUT -p tcp --dport <CODESYS_PORT> -j DROP

Additional workarounds include:

• Deploy application-layer firewalls or ICS-aware security appliances
• Implement VPN requirements for remote access to OT networks
• Disable unnecessary communication server features if operationally feasible
• Increase monitoring and alerting thresholds for availability events