CVE-2024-26200 - Dissecting the Latest RRAS Remote Code Execution Vulnerability

Windows servers play a crucial role for many businesses, and their security is vital. In early 2024, Microsoft disclosed CVE-2024-26200, a critical vulnerability impacting the Windows Routing and Remote Access Service (RRAS). If left unpatched, this flaw can let a remote attacker run malicious code with system privileges—basically, take control of the target machine.

In this post, I’ll break down what this vulnerability is, how it happens, who’s at risk, and—especially for the blue teams—a general idea of how attacks could work, all in clear language with code samples and original links for further investigation.

What Is RRAS?

RRAS is a Windows Server component allowing routing services, remote access through VPNs, and NAT functionalities. It's often enabled on servers supporting remote workforces and branch offices.

Official References

- Microsoft Security Update Guide – CVE-2024-26200
- NIST National Vulnerability Database – CVE-2024-26200

Technical Details

While Microsoft keeps certain details confidential (to prevent mass exploitation), security researchers and patch diffing have revealed enough for defenders and lab experiments.

The vulnerability resides in how RRAS handles specially crafted remote network packets. Attackers can send these packets to a listening RRAS service—often through protocols like PPTP, L2TP, or SSTP—and trigger a memory corruption or pointer manipulation, paving the way to execute arbitrary code.

High-Level Attack Flow

1. Discover Target: Find servers exposing RRAS (usually port 1723 for PPTP, port 1701 for L2TP, etc.).

Imagine the vulnerable code in RRAS looks like this (VERY simplified)

void HandleRRASPacket(char* packet, int len) {
    char buffer[256];
    // Bug: No check on packet size
    memcpy(buffer, packet, len);  // Potential buffer overflow!
    // ... process packet
}

An attacker could create an “evil” packet longer than 256 bytes and send it to the RRAS server

import socket

target_ip = "192.168.1.100"
rras_port = 1723  # Typical for PPTP

# Malicious payload: 300 'A's, causing buffer overflow
payload = b'A' * 300

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((target_ip, rras_port))
s.send(payload)
s.close()

*Note: This is a simplified illustration and will NOT directly work against real RRAS, but demonstrates the logic.*

Modern Windows protections (DEP, ASLR, stack cookies) make simple buffer overflows harder.

- Still, a skilled attacker could develop a working exploit with enough knowledge of the system and patch-diffing.
- Previous proof-of-concepts (PoCs) for similar RRAS vulnerabilities have targeted *use-after-free* or *heap corruption* bugs exploitable via remote packets.

You can quickly find out if RRAS is enabled on a Windows Server using PowerShell

Get-Service -Name RemoteAccess

A status of Running means your server is potentially at risk. Also, check firewall rules for open VPN-related ports.

The only secure solution is to apply Microsoft's official patches

- Find your server’s update here.

Final Thoughts

CVE-2024-26200 is a reminder that exposed VPN and routing services on Windows need strict controls and constant patching. Attackers are quick to develop public exploits once details emerge.

#### Further Reading/Sources

- Microsoft RRAS Documentation
- Kevin Beaumont’s analysis on Twitter
- CVE-2024-26200 at MSRC


Stay vigilant. Patch early, patch often, and monitor your endpoints. Questions or comments? Sound off below!

Timeline

Published on: 04/09/2024 17:15:37 UTC
Last modified on: 04/10/2024 13:24:00 UTC