CVE-2025-26649 Overview
CVE-2025-26649 is a race condition vulnerability in the Windows Secure Channel (Schannel) security support provider. The flaw stems from concurrent execution using a shared resource with improper synchronization [CWE-362]. An authorized local attacker can exploit the timing window to elevate privileges on affected Windows systems. Microsoft published the advisory on April 8, 2025, and the vulnerability affects Windows 11 (22H2, 23H2, 24H2) along with Windows Server 2022, Server 2022 23H2, and Server 2025. Successful exploitation results in a complete compromise of confidentiality, integrity, and availability on the targeted host.
Critical Impact
A low-privileged local user who wins the race condition against Schannel can escalate to elevated privileges and gain full control over the affected Windows host.
Affected Products
- Microsoft Windows 11 (22H2, 23H2, 24H2)
- Microsoft Windows Server 2022 and Server 2022 23H2
- Microsoft Windows Server 2025
Discovery Timeline
- 2025-04-08 - Microsoft releases security advisory and patch for CVE-2025-26649
- 2025-04-08 - CVE-2025-26649 published to the National Vulnerability Database
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-26649
Vulnerability Analysis
The vulnerability resides in Windows Secure Channel, the security support provider responsible for implementing SSL, TLS, and DTLS on Windows. Schannel operates in a multi-threaded context and manages shared cryptographic state, session structures, and credential objects across concurrent operations. Improper synchronization on one of these shared resources creates a race window that an attacker can leverage to manipulate object state between a check and its subsequent use.
Because Schannel runs with elevated trust inside the Local Security Authority Subsystem Service (lsass.exe) context, corrupting or reusing objects during the race window allows an attacker to influence security-sensitive decisions or memory that a privileged component later consumes. The result is a local privilege escalation on the affected host.
Root Cause
The root cause is a concurrent access defect classified under [CWE-362]. A shared Schannel resource is accessed by multiple threads without adequate locking or atomicity guarantees. The narrow timing window explains the high attack complexity reflected in the vector string, but the outcome once the race is won is full compromise of confidentiality, integrity, and availability.
Attack Vector
Exploitation requires local access and low privileges on the target system. An authorized user runs code that repeatedly triggers Schannel operations while a second thread races to modify or free the shared object at a precise moment. When the race is won, the attacker gains code execution or state control in a higher-privileged context. No user interaction is required, and no network access is needed to trigger the flaw.
No verified proof-of-concept code is publicly available for CVE-2025-26649. See the Microsoft CVE-2025-26649 Advisory for vendor technical details.
Detection Methods for CVE-2025-26649
Indicators of Compromise
- Unexpected crashes or exception events involving lsass.exe or Schannel components (schannel.dll) on affected hosts.
- Repeated, high-frequency calls into Schannel APIs from a single non-privileged process, consistent with race-window probing.
- New processes or scheduled tasks spawned with SYSTEM privileges shortly after anomalous Schannel activity.
Detection Strategies
- Monitor for local processes performing tight loops of TLS or SSPI operations without corresponding network activity, a pattern consistent with race exploitation.
- Correlate Windows Error Reporting entries for lsass.exe faults with subsequent privilege changes on the same host.
- Alert on token manipulation or privilege assignment events (Security event IDs 4672, 4673, 4703) that follow unusual Schannel usage by low-privilege users.
Monitoring Recommendations
- Enable Sysmon process creation and image load logging to capture non-standard modules loading alongside Schannel.
- Aggregate endpoint telemetry into a centralized data lake and hunt for the behavioral pattern of race-window probing followed by privilege gain.
- Track patch compliance for the April 2025 Windows security update across all Windows 11 and Windows Server 2022/2025 endpoints.
How to Mitigate CVE-2025-26649
Immediate Actions Required
- Apply the April 2025 Microsoft security update addressing CVE-2025-26649 to all affected Windows 11 and Windows Server systems.
- Prioritize patching for multi-user systems, terminal servers, and jump hosts where multiple low-privileged users have interactive access.
- Audit local accounts and remove unnecessary interactive logon rights to reduce the population of users who could stage a local attack.
Patch Information
Microsoft released fixes on April 8, 2025 through the standard Windows Update channel. Refer to the Microsoft CVE-2025-26649 Advisory for the specific KB article and build numbers that correspond to each affected Windows 11 and Windows Server release.
Workarounds
- No official workaround is published by Microsoft; installing the security update is the supported remediation.
- Restrict local logon rights and enforce least privilege to reduce the number of accounts capable of triggering the race locally.
- Apply application control policies (WDAC or AppLocker) to block execution of unapproved binaries by standard users on sensitive hosts.
# Verify the April 2025 cumulative update is installed on a Windows host
wmic qfe list brief /format:table
# PowerShell equivalent to list installed hotfixes and check patch status
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 20
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
