CVE-2023-21695: Windows 10 PEAP RCE Vulnerability

CVE-2023-21695 Overview

CVE-2023-21695 is a Remote Code Execution (RCE) vulnerability affecting Microsoft Protected Extensible Authentication Protocol (PEAP). This network-based vulnerability allows an authenticated attacker to execute arbitrary code on vulnerable Windows systems running the Network Policy Server (NPS) role with PEAP configured. The vulnerability stems from a heap-based buffer overflow condition (CWE-122) in the PEAP authentication handling mechanism.

Critical Impact

An authenticated attacker exploiting this vulnerability could achieve remote code execution with SYSTEM privileges on vulnerable Windows servers, potentially compromising network authentication infrastructure across an organization.

Affected Products

• Microsoft Windows 10 (all versions including 1607, 1809, 20H2, 21H2, 22H2)
• Microsoft Windows 11 (21H2, 22H2)
• Microsoft Windows Server 2008 SP2 and R2 SP1
• Microsoft Windows Server 2012 and R2
• Microsoft Windows Server 2016
• Microsoft Windows Server 2019
• Microsoft Windows Server 2022

Discovery Timeline

• February 14, 2023 - CVE-2023-21695 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-21695

Vulnerability Analysis

This vulnerability resides in Microsoft's implementation of the Protected Extensible Authentication Protocol (PEAP), which is commonly used for securing wireless and wired network authentication in enterprise environments. PEAP operates by establishing a TLS tunnel between the client and the authentication server (typically a RADIUS/NPS server) before transmitting authentication credentials.

The heap-based buffer overflow (CWE-122) occurs during the processing of specially crafted PEAP packets. When a malformed authentication request is processed by a vulnerable Network Policy Server, improper bounds checking allows an attacker to write data beyond the allocated heap buffer boundaries. This memory corruption can be leveraged to achieve arbitrary code execution in the context of the NPS service, which runs with SYSTEM privileges.

While the vulnerability requires low-privilege authentication to exploit, the network attack vector and lack of required user interaction make this a significant threat to enterprise environments relying on PEAP for network access control.

Root Cause

The root cause is a heap-based buffer overflow (CWE-122) in the PEAP packet handling code within the Windows Network Policy Server component. The vulnerability occurs due to insufficient validation of length fields in PEAP authentication messages, allowing an attacker to trigger a heap overflow condition when the NPS processes malformed PEAP packets.

Attack Vector

The attack vector is network-based, requiring the attacker to have low-privilege authentication credentials on the target domain or network. The exploitation flow involves:

1. The attacker establishes a PEAP session with a vulnerable NPS server
2. During the authentication phase, the attacker sends specially crafted PEAP packets containing malformed length values
3. The NPS server processes these packets without proper bounds checking
4. The heap overflow corrupts adjacent memory structures
5. Successful exploitation results in arbitrary code execution with SYSTEM privileges on the NPS server

The vulnerability requires no user interaction and can be exploited remotely over the network, making it particularly dangerous for organizations using PEAP for authentication services.

Detection Methods for CVE-2023-21695

Indicators of Compromise

• Unexpected crashes or service restarts of the Network Policy Server (NPS) service
• Anomalous heap memory allocation patterns in rastls.dll or related PEAP components
• Suspicious authentication events from unusual source IPs targeting NPS servers
• Process injection or unusual child processes spawned by the NPS service

Detection Strategies

• Monitor Windows Event Logs for NPS service crashes (Event ID 6273, 6274) and unexpected terminations
• Implement network detection rules for malformed PEAP packets with abnormal length fields
• Deploy endpoint detection solutions capable of identifying heap overflow exploitation attempts
• Enable audit logging for RADIUS/NPS authentication events and review for anomalous patterns

Monitoring Recommendations

• Configure SIEM alerts for NPS service stability issues and authentication anomalies
• Monitor network traffic to NPS servers (UDP 1812, 1813) for unusual packet sizes or patterns
• Implement behavioral analysis on NPS servers to detect memory corruption attempts
• Enable Windows Defender Exploit Guard and monitor for heap overflow protection triggers

How to Mitigate CVE-2023-21695

Immediate Actions Required

• Apply Microsoft's February 2023 security updates to all affected Windows systems immediately
• Prioritize patching Network Policy Servers and RADIUS infrastructure
• Implement network segmentation to restrict access to NPS servers from trusted endpoints only
• Review and audit accounts with network authentication privileges

Patch Information

Microsoft has released security updates addressing CVE-2023-21695 as part of the February 2023 Patch Tuesday release. Organizations should apply the appropriate cumulative updates for their Windows versions. Detailed patch information and download links are available in the Microsoft Security Update Guide.

Workarounds

• Restrict network access to NPS servers using firewall rules to limit exposure to trusted network segments only
• Consider disabling PEAP authentication temporarily and using alternative EAP methods (such as EAP-TLS) until patches can be applied
• Implement strong authentication policies to reduce the pool of accounts that could be leveraged for exploitation
• Enable Windows Defender Credential Guard where supported to provide additional protection for authentication infrastructure

# Restrict NPS server access using Windows Firewall
# Allow RADIUS traffic only from trusted access points/controllers
netsh advfirewall firewall add rule name="Restrict NPS Access" dir=in action=allow protocol=udp localport=1812,1813 remoteip=192.168.10.0/24
netsh advfirewall firewall add rule name="Block External NPS" dir=in action=block protocol=udp localport=1812,1813