CVE-2025-27492 Overview
CVE-2025-27492 is a race condition vulnerability in Windows Secure Channel (Schannel), the security support provider that implements Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols on Windows systems. The flaw stems from concurrent execution using a shared resource without proper synchronization [CWE-362]. An authorized local attacker who wins the race can elevate privileges on affected Windows 11 and Windows Server systems. Microsoft addressed the issue in the April 2025 security update cycle.
Critical Impact
A local, authenticated attacker who successfully exploits the race condition in Windows Secure Channel can elevate privileges and gain high impact on confidentiality, integrity, and availability of the target host.
Affected Products
- Microsoft Windows 11 (22H2, 23H2, 24H2)
- Microsoft Windows Server 2022 and Windows Server 2022 23H2
- Microsoft Windows Server 2025
Discovery Timeline
- 2025-04-08 - CVE-2025-27492 published to the National Vulnerability Database (NVD)
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-27492
Vulnerability Analysis
The vulnerability resides in Windows Secure Channel (Schannel), which handles TLS/SSL session establishment, certificate validation, and cryptographic operations for Windows components. Multiple threads operate on shared Schannel state during connection setup and teardown. When synchronization primitives fail to serialize access to that state, an attacker can interleave operations to reach an unintended execution path.
By timing operations precisely, a local authenticated user can manipulate Schannel state while a privileged component reads or writes the same structure. Successful exploitation results in privilege escalation to a higher security context, giving the attacker high impact on confidentiality, integrity, and availability. Exploitation requires local access and low privileges but carries high attack complexity because the attacker must reliably win the race window.
Root Cause
The root cause is improper synchronization on a shared resource inside Schannel, classified as [CWE-362] Concurrent Execution using Shared Resource with Improper Synchronization. Two or more code paths operate on the same object without holding a lock covering the full critical section, producing a time-of-check to time-of-use (TOCTOU) style window that can be exploited.
Attack Vector
The attack vector is local. An authenticated user runs code on the target host and repeatedly triggers the vulnerable Schannel code path while a second thread races the same resource. Microsoft has not published exploitation details, and no public proof-of-concept exists at the time of writing. See the Microsoft Security Update CVE-2025-27492 advisory for vendor guidance.
No verified exploit code is publicly available. Refer to the vendor advisory for authoritative technical details.
Detection Methods for CVE-2025-27492
Indicators of Compromise
- Unexpected privilege elevation events (Event ID 4672) for standard user accounts shortly after Schannel-related activity.
- Anomalous crashes or WER (Windows Error Reporting) entries referencing schannel.dll or lsass.exe.
- Repeated, short-lived TLS handshakes originating from a local process attempting to force race windows.
Detection Strategies
- Monitor for local processes that spawn high volumes of TLS handshake operations against local endpoints, which can indicate race-window brute forcing.
- Correlate token elevation events with the parent process image and command line to identify unexpected transitions to SYSTEM or service accounts.
- Alert on child processes created by non-administrative users that inherit elevated tokens.
Monitoring Recommendations
- Enable audit policies for process creation with command-line logging and token manipulation events across all affected Windows 11 and Windows Server builds.
- Track patch level compliance for the April 2025 cumulative update on every affected build listed in the Microsoft advisory.
- Baseline normal Schannel usage on servers and flag deviations, especially from interactive user sessions.
How to Mitigate CVE-2025-27492
Immediate Actions Required
- Apply the April 2025 Microsoft security update that addresses CVE-2025-27492 to all affected Windows 11 and Windows Server systems.
- Inventory hosts using vulnerability management tooling to confirm no affected builds remain unpatched.
- Restrict interactive and remote logon rights on servers to reduce the population of accounts that meet the local, authenticated precondition.
Patch Information
Microsoft released fixes through the standard Patch Tuesday channel. Consult the Microsoft Security Update CVE-2025-27492 advisory for the specific KB article and package identifiers matching each Windows build. Deploy the update through Windows Update, Windows Server Update Services (WSUS), Microsoft Configuration Manager, or Intune according to the environment.
Workarounds
- No vendor-supplied workaround is documented; patching is the required remediation.
- Enforce least privilege and remove standing local logon rights for non-administrative accounts on Windows Servers.
- Segment and monitor privileged workstations that handle TLS-terminating workloads until the patch is deployed fleet-wide.
# Verify patch installation on a Windows host (PowerShell)
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 20
# Confirm current OS build against the affected list
[System.Environment]::OSVersion.Version
(Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion').DisplayVersion
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
