CVE-2023-21692: Windows 10 1507 PEAP RCE Vulnerability

CVE-2023-21692 Overview

CVE-2023-21692 is a critical remote code execution vulnerability affecting Microsoft's Protected Extensible Authentication Protocol (PEAP) implementation across a wide range of Windows operating systems. This heap-based buffer overflow vulnerability allows unauthenticated remote attackers to execute arbitrary code on vulnerable systems by sending specially crafted PEAP authentication requests to a target Network Policy Server (NPS).

The vulnerability exists in the way Windows handles PEAP authentication messages, which are commonly used in enterprise environments for secure wireless network authentication, VPN connections, and 802.1X wired network access control. Successful exploitation requires no user interaction and can be triggered remotely over the network, making this vulnerability particularly dangerous for organizations running NPS servers.

Critical Impact

Unauthenticated remote attackers can achieve complete system compromise by exploiting this heap-based buffer overflow in PEAP authentication handling, potentially gaining SYSTEM-level privileges on affected Windows servers.

Affected Products

• Microsoft Windows 10 (versions 1507, 1607, 1809, 20H2, 21H2, 22H2)
• Microsoft Windows 11 (versions 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-2023-21692 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-21692

Vulnerability Analysis

This vulnerability is classified as CWE-122 (Heap-based Buffer Overflow), a memory corruption vulnerability that occurs when a program writes data beyond the allocated boundary of a heap buffer. In the context of PEAP authentication, the vulnerability is triggered when processing malformed PEAP packets, specifically during the TLS handshake phase of the authentication protocol.

The PEAP protocol encapsulates EAP (Extensible Authentication Protocol) messages within a TLS tunnel, providing an additional layer of security for authentication. However, the implementation flaw in how Windows processes certain fields within these encapsulated messages leads to a heap buffer overflow condition. An attacker can craft malicious PEAP packets that cause the vulnerable code path to write beyond the allocated buffer, potentially overwriting adjacent heap metadata or application data.

The vulnerability can be exploited without authentication, as the vulnerability is triggered during the initial PEAP negotiation phase before any credentials are validated. This pre-authentication attack surface significantly increases the risk, as any system with NPS services exposed to the network is potentially vulnerable.

Root Cause

The root cause of CVE-2023-21692 is improper bounds checking in the PEAP authentication handler when parsing variable-length fields within PEAP messages. The vulnerable code fails to properly validate the length of incoming data against the allocated buffer size, allowing an attacker-controlled length field to trigger a heap-based buffer overflow. This type of vulnerability typically occurs when parsing network protocols that use length-value encoding schemes, where the length field is trusted without adequate validation.

Attack Vector

The attack vector for this vulnerability is network-based, requiring the attacker to send specially crafted PEAP authentication packets to a vulnerable Windows system running the Network Policy Server (NPS) role. The attack scenario typically involves:

1. Identifying a target system running NPS services (commonly exposed on RADIUS ports 1812/UDP and 1813/UDP)
2. Initiating a PEAP authentication session with the target server
3. Sending malformed PEAP packets containing oversized length fields or malformed TLS data
4. Triggering the heap overflow to corrupt memory structures
5. Achieving arbitrary code execution in the context of the NPS service (typically SYSTEM privileges)

The attack does not require valid credentials, as the vulnerability is exploited during the pre-authentication phase of the PEAP handshake.

Detection Methods for CVE-2023-21692

Indicators of Compromise

• Unusual crash events or service restarts in the Network Policy Server (NPS) service (IAS service)
• Windows Event Log entries showing NPS service failures or unexpected terminations (Event ID 6273, 6274)
• Abnormal network traffic patterns on RADIUS ports (UDP 1812, 1813) with malformed PEAP packets
• Evidence of heap corruption or memory access violations in crash dumps associated with iassam.dll or rastls.dll

Detection Strategies

• Deploy network intrusion detection systems (NIDS) with signatures for malformed PEAP/RADIUS traffic patterns
• Monitor Windows Event Logs for NPS service crashes and authentication anomalies using SIEM correlation rules
• Implement packet capture and deep packet inspection on RADIUS traffic to identify malformed EAP-TLS handshakes
• Enable Windows Error Reporting to capture and analyze crash dumps from NPS service failures

Monitoring Recommendations

• Configure real-time alerting for NPS service state changes and unexpected restarts
• Implement network segmentation and monitoring to limit exposure of NPS servers to untrusted networks
• Deploy SentinelOne Singularity Platform with behavioral AI to detect exploitation attempts and post-exploitation activities
• Establish baseline traffic patterns for RADIUS authentication and alert on deviations

How to Mitigate CVE-2023-21692

Immediate Actions Required

• Apply Microsoft security updates from the February 2023 Patch Tuesday release immediately to all affected systems
• Prioritize patching NPS servers and domain controllers that handle PEAP authentication
• Restrict network access to RADIUS ports (UDP 1812, 1813) using firewall rules to only allow connections from authorized network devices
• Review and limit the exposure of NPS services to untrusted network segments

Patch Information

Microsoft released security updates addressing this vulnerability as part of the February 2023 Patch Tuesday release. Administrators should consult the Microsoft Security Response Center Advisory for specific KB articles applicable to their Windows versions. The patches address the heap-based buffer overflow by implementing proper bounds checking in the PEAP authentication handler.

Workarounds

• If PEAP authentication is not required, consider disabling the NPS service or removing the Network Policy and Access Services role from affected servers
• Implement network-level restrictions using firewalls to limit access to RADIUS ports from trusted IP ranges only
• Consider deploying alternative authentication mechanisms such as EAP-TLS with certificate-based authentication while awaiting patch deployment
• Enable Windows Defender Exploit Guard with Attack Surface Reduction (ASR) rules to provide additional protection against memory corruption attacks

# Verify NPS service status and restrict access
# Check if NPS service is running
sc query IAS

# Stop NPS service if not needed (temporary mitigation)
net stop IAS

# Firewall rule to restrict RADIUS access to specific subnets
netsh advfirewall firewall add rule name="Restrict RADIUS Access" dir=in action=allow protocol=UDP localport=1812,1813 remoteip=10.0.0.0/8

# Block all other RADIUS traffic
netsh advfirewall firewall add rule name="Block RADIUS External" dir=in action=block protocol=UDP localport=1812,1813