CVE-2023-28220 - Layer 2 Tunneling Protocol Remote Code Execution Vulnerability Explained

The cybersecurity world is always on its toes for new exploits and vulnerabilities. One such significant risk recently emerged: CVE-2023-28220. This vulnerability targets the Layer 2 Tunneling Protocol (L2TP), threatening countless Windows systems by allowing remote code execution (RCE). In this article, we'll break down what CVE-2023-28220 means, how an attacker might exploit it, what the official sources say, some example snippets, and how you can protect yourself.

What is CVE-2023-28220?

CVE-2023-28220 is a critical security vulnerability in the Windows implementation of Layer 2 Tunneling Protocol (L2TP). L2TP is commonly used in Virtual Private Networks (VPNs) for securely tunneling traffic.

This vulnerability allows an attacker to execute arbitrary code on a target system remotely. In other words, hackers can potentially take control of a system without the user doing anything more than having an L2TP service exposed.

Microsoft Security Guidance:

CVE-2023-28220 | Windows Layer 2 Tunneling Protocol Remote Code Execution Vulnerability

NVD Description:

NIST NVD Entry for CVE-2023-28220

How Does the Vulnerability Work?

The problem exists in the way Windows' L2TP service processes specially crafted packets. An attacker who can send malicious L2TP packets to a vulnerable machine (for example, a Windows server with L2TP VPN enabled and exposed to the internet) may be able to execute arbitrary code with SYSTEM privileges.

(In Simple Terms)

If someone can reach your VPN service from the internet, they could send it messed up information and trick your system into running code of their choosing.

Technical Details

At its core, CVE-2023-28220 is a buffer overflow vulnerability. When the L2TP service receives a packet that is larger or formatted in a way it didn’t expect, it doesn’t properly check the size before copying it to a fixed-size area of memory. This classic mistake can allow the attacker to overwrite important data and execute their own code.

Here's a simplified C code snippet showing what shouldn't happen in a well-written service

// Vulnerable buffer copy (hypothetical example)
void process_l2tp_packet(char *packet, int length) {
    char buffer[256];
    // Oops! Copied straight in, no length check!
    memcpy(buffer, packet, length); 
    // ...process the packet...
}

The proper way would be

void process_l2tp_packet(char *packet, int length) {
    char buffer[256];
    if (length > sizeof(buffer)) {
        // Reject the packet or handle the error
        return;
    }
    memcpy(buffer, packet, length);
    // ...process...
}

The Windows L2TP stack made a mistake like the first example in a less direct but similar way.

How Could an Attack Look Like?

Attackers scan the internet for exposed L2TP-enabled servers. Then, they send packets purposely structured to trigger the vulnerability.

### - No authentication required: The flaw exists before user login, so the attacker doesn't need valid credentials.

Exploit Simulation

Although no public full exploit code has been released as of writing, here's a pseudocode of what an attack tool might do:

import socket

# Details of vulnerability allow overlong packet
malicious_payload = b'A' * 300 + crafted_shell_code()

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.sendto(malicious_payload, ("victim-ip.com", 1701)) # 1701 is default L2TP port

crafted_shell_code() is your exploit payload—whatever code the attacker wants to run.

Important: The actual exploit is far more complex, involving careful stack manipulation and bypassing security features, but this gives an idea of how attackers might send bad data to the service.

Pre-authentication: Attackers do not need credentials

This means attackers can use this vulnerability to take full control of affected systems, move laterally in a network, install malware, or exfiltrate sensitive data—all while bypassing normal defenses.

Who Is at Risk?

- All supported versions of Windows server and client (before April 2023) that run L2TP VPN services

1. Patch!

Microsoft released a fix in April 2023's Patch Tuesday.
- See official patch advisory

2. Firewall

Block UDP port 1701 on your firewall unless L2TP VPN is absolutely needed, and restrict access to trusted IPs.

3. Network Segmentation

Never publicly expose VPN services unless necessary. Consider placing them behind firewalls or using network-level authentication.

4. Monitor Your Systems

Keep logs of VPN attempts and monitor for any suspicious or failed login attempts.

Final Thoughts

CVE-2023-28220 is a classic example of an old-school bug with a modern, dangerous impact. If you manage any Windows-based VPN infrastructure, patch your systems immediately. For added defense, review your VPN exposure and firewall policies.

Timeline

Published on: 04/11/2023 21:15:00 UTC
Last modified on: 04/13/2023 01:14:00 UTC