CVE-2026-55144: Windows 11 24h2 Auth Bypass Vulnerability

CVE-2026-55144 Overview

CVE-2026-55144 is a high-severity vulnerability in Microsoft Windows CryptoAPI caused by a missing required cryptographic step [CWE-325]. An authorized local attacker can exploit this weakness to perform tampering operations against data or objects protected by the affected cryptographic routines. The flaw impacts Windows 11 versions 24H2, 25H2, and 26H1, along with Windows Server 2022 and Windows Server 2025. Microsoft published the advisory on July 14, 2026, and updated the entry on July 20, 2026. Successful exploitation requires local access with existing low privileges but no user interaction, making it a relevant escalation and integrity-loss risk in multi-user or shared-endpoint environments.

Critical Impact
An authenticated local attacker can bypass a required cryptographic step in Windows CryptoAPI to tamper with protected data, undermining confidentiality and integrity guarantees on the affected system.

Affected Products

Microsoft Windows 11 (24H2, 25H2, 26H1) on x64 and ARM64
Microsoft Windows Server 2022
Microsoft Windows Server 2025

Discovery Timeline

2026-07-14 - CVE-2026-55144 published to NVD
2026-07-20 - Last updated in NVD database

Technical Details for CVE-2026-55144

Vulnerability Analysis

The vulnerability resides in Windows CryptoAPI, the operating system component that provides cryptographic services to applications and system components. CryptoAPI omits a required cryptographic step during certain operations, weakening the integrity guarantees the API is intended to enforce. An attacker who is already authenticated on the target host can leverage this gap to tamper with data or objects that rely on CryptoAPI for validation.

Because the affected code path is exposed to any local, authorized caller, the flaw is reachable from standard user contexts without special capabilities. The impact profile shows high confidentiality and integrity loss with no direct availability impact, consistent with a cryptographic bypass rather than a crash condition.

Root Cause

The root cause is classified under [CWE-325]: Missing Cryptographic Step. A required operation in the cryptographic sequence — such as a validation, verification, or transformation stage — is absent from the affected CryptoAPI code path. When that step is skipped, downstream trust decisions rely on incomplete cryptographic state, allowing an attacker to produce or accept inputs that should have been rejected.

Attack Vector

Exploitation requires local access and low privileges on the target Windows system. The attacker interacts with CryptoAPI through standard application or system interfaces that invoke the vulnerable routine. No user interaction is required, and the attack does not cross a trust boundary at the network layer. In practice, this vector fits post-compromise scenarios where an adversary already has code execution as a standard user and seeks to tamper with cryptographically protected artifacts on the endpoint.

No public proof-of-concept, exploit code, or evidence of in-the-wild exploitation has been published for CVE-2026-55144 at the time of writing. Refer to the Microsoft CVE-2026-55144 Advisory for authoritative technical details.

Detection Methods for CVE-2026-55144

Indicators of Compromise

No public indicators of compromise have been published for CVE-2026-55144. Detection should focus on behavioral signals rather than static IOCs.
Unexpected modifications to signed configuration files, certificates, or other cryptographically protected artifacts on affected Windows hosts.
Anomalous invocations of CryptoAPI functions (for example CryptVerifySignature, CryptHashData, BCryptVerifySignature) by non-standard user processes.

Detection Strategies

Inventory affected assets by querying endpoints for Windows 11 24H2/25H2/26H1 and Windows Server 2022/2025 build numbers and cross-referencing installed KB updates.
Monitor local privilege boundaries: track standard-user processes that access cryptographic providers or interact with protected system objects.
Alert on integrity failures reported by applications or services that rely on Authenticode, signed configuration, or CryptoAPI-backed validation.

Monitoring Recommendations

Enable and forward Windows Security, Application, and CAPI2 event logs to a centralized log platform for correlation.
Baseline normal CryptoAPI usage per host and alert on deviations, especially from interactive user sessions.
Track patch compliance on all affected Windows 11 and Windows Server versions to ensure the Microsoft fix is deployed fleet-wide.

How to Mitigate CVE-2026-55144

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-55144 Advisory to all affected Windows 11 and Windows Server systems.
Prioritize patching on multi-user hosts, jump servers, and systems that store or process high-value cryptographic material.
Review and restrict local logon rights so that only necessary accounts can obtain the authenticated access required for exploitation.

Patch Information

Microsoft has published fix guidance in the Microsoft CVE-2026-55144 Advisory. The advisory lists the specific cumulative updates for Windows 11 24H2, 25H2, and 26H1, as well as Windows Server 2022 and Windows Server 2025. Apply the update through Windows Update, Windows Server Update Services (WSUS), or your standard patch management workflow. Reboot affected systems to complete installation.

Workarounds

No official vendor workaround has been published; patching is the supported remediation.
Enforce least privilege and remove unnecessary local accounts to reduce the population of principals able to exploit the local attack vector.
Use application control (for example Windows Defender Application Control or AppLocker) to constrain which binaries can invoke cryptographic APIs on sensitive hosts.
Monitor and audit access to cryptographic keys, certificate stores, and signed artifacts until the patch is fully deployed.

bash

# Example: verify installed Windows build and patch state on affected hosts
# Run in an elevated PowerShell session
Get-ComputerInfo | Select-Object OsName, OsVersion, OsBuildNumber, OsHardwareAbstractionLayer
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 20