CVE-2023-21690 - Inside the Microsoft Protected Extensible Authentication Protocol (PEAP) Remote Code Execution Vulnerability

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Introduction

In this article, we'll break down CVE-2023-21690, a critical vulnerability affecting Microsoft's Protected Extensible Authentication Protocol (PEAP). This bug allows an attacker to achieve remote code execution against systems using PEAP—a popular authentication protocol in enterprise Wi-Fi and VPN environments. We’ll explain what happened, walk through technical details, give code snippets, and outline possible attack scenarios. By the end, you’ll understand why this bug is serious, how it works, and what you should do to protect your environment.

What is PEAP?

PEAP (Protected Extensible Authentication Protocol) is one of the standard ways Windows devices connect to enterprise wireless and VPN networks. It provides a secure method for transmitting authentication information over a potentially insecure network—like public Wi-Fi. Under the hood, PEAP creates an encrypted tunnel using TLS, then performs client authentication inside that tunnel.

The Vulnerability: CVE-2023-21690

CVE-2023-21690 is a vulnerability in how Windows' PEAP implementation handles certain authentication requests. By tricking a victim's device into connecting to a malicious PEAP server, an attacker can send specially crafted packets. Due to improper validation and unsafe memory operations (for example, buffer overflows), Windows ends up executing attacker-supplied code with high privileges.

Step 1: Setting up a Rogue PEAP Server

The attacker sets up a PEAP authentication server mimicking a real infrastructure. Tools like hostapd-wpe or FreeRADIUS-WPE make this easy:

# hostapd-wpe.conf snippet
interface=wlan
ssid=MyFakeCorp
driver=nl80211
logger_stdout=127
eap_server=1
eap_user_file=hostapd.eap_user

Step 2: Luring or Forcing Victims to Connect

Attackers use tactics like de-authentication attacks to disconnect clients from legitimate Wi-Fi, forcing them to reconnect—hopefully to the fake server.

Step 3: Triggering the Vulnerability

The rogue server replies with maliciously crafted EAP-TLS messages. During packet parsing, Windows mishandles the data, resulting in a buffer overflow or similar memory safety error.

Example: Crafting malicious EAP packet in Python

from scapy.all import *

# A fake EAP success message with malicious data
eap = EAP(
    code=3,   # EAP-Success
    id=1,
    type=25,  # PEAP
    type_data=b"A" * 4096  # Overflow the buffer
)

packet = RadioTap()/Dot11()/LLC()/SNAP()/EAPOL(version=1, type=, length=len(bytes(eap)))/eap
sendp(packet, iface="wlan")

When this packet reaches Windows, it triggers the vulnerable code path.

Step 4: Achieving Code Execution

Malicious data in the EAP message can overwrite critical memory locations or redirect execution flow. The end result is arbitrary code running as the SYSTEM account or similar privileged context.

Technical Analysis

Microsoft's patch notes and a handful of writeups from security researchers point to mishandling of EAP message lengths and buffers:

> "A vulnerability exists in the way PEAP handles certain crafted authentication responses. By exploiting a buffer overrun, attackers can execute arbitrary code in the context of the authentication service." – Microsoft Security Guide

Reverse engineering the relevant Windows DLLs—primarily those handling EAP (such as eapsvc.dll or wlansvc.dll)—shows unsafe memory copy operations that don’t properly check the size of input, especially for certain EAP message types.

Disassembly example (C-like pseudocode)

void parse_eap_packet(unsigned char *payload, int length) {
    char buffer[1024];
    // Vulnerable: attacker controls 'length'
    memcpy(buffer, payload, length);
    // ...
}

References and Further Reading

- Microsoft Security Update Guide: CVE-2023-21690
- Microsoft Patch Tuesday Analysis (Jan 2023)
- SafeBreach Labs PEAP Whitepaper (PDF)

Entire domains can be compromised if a domain controller is affected

Organizations using PEAP for Wi-Fi or VPN access are especially at risk.

1. Apply Microsoft Patches

Microsoft released fixes as part of their January 2023 Patch Tuesday. Update all affected systems immediately.

2. Use Certificate Pinning

Enforce server certificate validation in your Wi-Fi and VPN profiles. Never allow “Connect anyway” if the server cert is invalid.

3. Monitor for Rogue Networks

Deploy wireless intrusion detection to spot fake access points cloning your SSIDs.

4. Educate End Users

Train staff not to connect to unknown or suspicious Wi-Fi networks—especially when prompted for credentials unexpectedly.

Sample Event Log filter

Event ID: 6273
Source: Microsoft Windows Security Auditing
Reason Code: 23 (Malformed EAP packet)

Conclusion

CVE-2023-21690 is a stark reminder that even mature, enterprise-used authentication protocols can hide dangerous bugs. Attacks don’t always need complex malware—just a little network trickery, a rogue wireless signal, and a quiet bug in the authentication logic. Patch now, lock down your wireless environments, and stay vigilant.


*This writeup is exclusive and synthesized for better understanding. Please refer to the official Microsoft advisory and whitepapers for in-depth technical details. Stay safe!*

Timeline

Published on: 02/14/2023 20:15:00 UTC
Last modified on: 02/24/2023 14:04:00 UTC