CVE-2026-21525: Windows 10 1607 DOS Vulnerability

CVE-2026-21525 Overview

CVE-2026-21525 is a null pointer dereference vulnerability in the Windows Remote Access Connection Manager (RASMAN) service. This flaw allows an unauthorized local attacker to cause a denial of service condition by triggering a null pointer dereference, resulting in system instability or service crashes that can disrupt critical remote access functionality.

Critical Impact

This vulnerability is currently being exploited in the wild and has been added to the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate attention from organizations running affected Windows systems.

Affected Products

• Microsoft Windows 10 (versions 1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (versions 23H2, 24H2, 25H2)
• Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

• February 10, 2026 - CVE-2026-21525 published to NVD
• February 11, 2026 - Last updated in NVD database

Technical Details for CVE-2026-21525

Vulnerability Analysis

This vulnerability exists within the Windows Remote Access Connection Manager (RASMAN) service, which is responsible for managing dial-up and VPN connections on Windows systems. The flaw stems from improper handling of certain operations that can lead to a null pointer being dereferenced during service execution.

When exploited, the vulnerability causes the RASMAN service to crash, effectively denying remote access capabilities to legitimate users. In enterprise environments where VPN connectivity is essential for remote workers, this can have significant operational impact. The vulnerability is classified as CWE-476 (Null Pointer Dereference), a common memory corruption issue that occurs when a program attempts to use a pointer that has a null value.

The local attack vector means an attacker needs existing access to the target system to exploit this vulnerability. However, no privileges are required to trigger the condition, and no user interaction is necessary, making it relatively straightforward to exploit once local access is obtained.

Root Cause

The vulnerability is caused by insufficient validation of pointer values within the Remote Access Connection Manager service. When specific conditions are met during connection management operations, the service fails to properly verify that a pointer references valid memory before attempting to dereference it. This lack of null pointer checking allows an attacker to craft inputs or trigger conditions that result in the service attempting to access memory at address zero, causing an immediate crash.

Attack Vector

The attack requires local access to the target system. An unauthorized attacker can exploit this vulnerability without requiring any special privileges or user interaction. The exploitation process involves interacting with the RASMAN service in a manner that triggers the null pointer condition. Since this vulnerability is listed in the CISA KEV catalog, active exploitation techniques are known to threat actors.

The attack can be executed by sending specially crafted requests or inputs to the Remote Access Connection Manager service that trigger code paths where the null pointer dereference occurs. The service crash disrupts VPN and dial-up connectivity for all users on the affected system.

Detection Methods for CVE-2026-21525

Indicators of Compromise

• Unexpected crashes or restarts of the Remote Access Connection Manager (RasMan) service
• Windows Event Log entries indicating service failures related to rasman.dll or RASMAN service
• Multiple denial of service events targeting remote access infrastructure
• System event logs showing null pointer or access violation exceptions in RASMAN-related processes

Detection Strategies

• Monitor Windows Event Logs for Application Error events (Event ID 1000) involving svchost.exe hosting the RASMAN service
• Deploy endpoint detection rules to identify repeated service crashes of the Remote Access Connection Manager
• Implement SentinelOne Singularity platform for real-time behavioral detection of exploitation attempts
• Configure SIEM alerts for patterns of RASMAN service failures across multiple endpoints

Monitoring Recommendations

• Enable detailed logging for the Remote Access Connection Manager service
• Configure Windows Event Forwarding to centralize security events from all affected systems
• Establish baseline metrics for RASMAN service availability to detect anomalous crash patterns
• Monitor for suspicious local user activity preceding service failures

How to Mitigate CVE-2026-21525

Immediate Actions Required

• Apply Microsoft security updates immediately as this vulnerability is being actively exploited
• Review the Microsoft Security Update Guide for CVE-2026-21525 for specific patch information
• Prioritize patching systems that provide critical remote access services
• Implement network segmentation to limit lateral movement opportunities for attackers who have gained local access

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should obtain and apply the appropriate patches from the Microsoft Security Response Center (MSRC) Update Guide. Due to the active exploitation status noted in the CISA Known Exploited Vulnerabilities Catalog, federal agencies and critical infrastructure organizations are required to remediate this vulnerability according to CISA timelines.

Workarounds

• If the Remote Access Connection Manager service is not required, consider disabling it temporarily until patches can be applied
• Restrict local access to critical systems to reduce the attack surface
• Implement application whitelisting to prevent unauthorized applications from interacting with system services
• Deploy SentinelOne endpoint protection for runtime defense against exploitation attempts

# Disable Remote Access Connection Manager service if not required (PowerShell)
Stop-Service -Name "RasMan" -Force
Set-Service -Name "RasMan" -StartupType Disabled

# Verify service status
Get-Service -Name "RasMan" | Select-Object Name, Status, StartType