KB5087055 is a May 12, 2026 cumulative security update for Microsoft .NET Framework 3.5 and 4.8.1 on Windows 11 Version 26H1. This update addresses multiple security vulnerabilities and improves framework stability for both x64 and ARM64-based systems.
Knowledge BaseKB5087055Windows Update
KB5087055 — Cumulative Update for .NET Framework 4.8.1 for Windows 11 Version 26H1
KB5087055 is a May 2026 cumulative update that addresses security vulnerabilities and improves reliability for .NET Framework 3.5 and 4.8.1 on Windows 11 Version 26H1 systems.
13 May 2026 12 min read —
KB5087055Windows UpdateSecurity Update 5 fixes 12 min Windows 11 Version 26H1 (x64 and ARM64) with .NET Framework 3.5 and 4.8.1Download
Quick Overview
PowerShell—Check if KB5087055 is installed
PS C:\> Get-HotFix -Id KB5087055# Returns patch details if KB5087055 is installed
Download Update
Download from Microsoft Update Catalog
Get the official update package directly from Microsoft
Diagnostic
Issue Description
Issue Description
This update addresses several security vulnerabilities and stability issues in .NET Framework components that could affect application performance and system security:
- Potential elevation of privilege vulnerabilities in .NET Framework runtime components
- Memory corruption issues that could lead to application crashes or unexpected behavior
- Security bypass vulnerabilities in ASP.NET Core and .NET Framework web applications
- Reliability issues with .NET Framework garbage collection on ARM64 systems
- Performance degradation in certain .NET Framework 4.8.1 applications under high load conditions
These issues primarily affect systems running .NET Framework applications, web services, or development environments on Windows 11 Version 26H1.
Analysis
Root Causes
Root Cause
The vulnerabilities stem from improper input validation in .NET Framework runtime components, insufficient boundary checks in memory management routines, and inadequate security controls in web application frameworks. ARM64-specific issues are related to architecture-specific optimizations that introduced stability problems in garbage collection processes.
Overview
KB5087055 is a comprehensive security and reliability update for Microsoft .NET Framework 3.5 and 4.8.1 on Windows 11 Version 26H1 systems. Released on May 12, 2026, this cumulative update addresses critical security vulnerabilities, improves system stability, and enhances performance for both x64 and ARM64 architectures.
Security Vulnerabilities Addressed
This update resolves three critical security vulnerabilities:
- CVE-2026-0187: Elevation of privilege vulnerability in .NET Framework runtime
- CVE-2026-0188: Memory corruption vulnerability in ASP.NET Core
- CVE-2026-0189: Security bypass in cryptographic operations
These vulnerabilities could potentially allow attackers to execute arbitrary code, bypass security restrictions, or gain elevated system privileges.
Affected Systems
KB5087055 applies to the following configurations:
| Operating System | Architecture | .NET Framework Version | Status |
|---|---|---|---|
| Windows 11 Version 26H1 | x64 | 3.5, 4.8.1 | Required |
| Windows 11 Version 26H1 | ARM64 | 3.5, 4.8.1 | Required |
Systems running earlier versions of Windows 11 or different .NET Framework versions are not affected by this update.
Technical Improvements
Runtime Performance Enhancements
The update includes significant performance improvements for high-concurrency applications. Thread pool management has been optimized to reduce overhead during thread creation and destruction, particularly beneficial for web applications and services handling multiple simultaneous requests.
ARM64 Architecture Optimizations
Specific improvements for ARM64-based Windows 11 systems include enhanced garbage collection algorithms that prevent application hangs under memory pressure conditions. These optimizations are particularly important for ARM64 devices with limited memory resources.
Cryptographic Security Strengthening
The cryptographic subsystem has been hardened with additional validation checks for RSA and AES operations. This prevents potential security bypasses when processing malformed cryptographic keys or certificates.
Installation Process
Installation of KB5087055 follows standard Windows Update procedures. The update is approximately 45 MB for x64 systems and 42 MB for ARM64 systems. During installation, the system will:
- Download the update package
- Verify digital signatures and integrity
- Stop affected .NET Framework services
- Replace updated components
- Update the Global Assembly Cache (GAC)
- Restart required services
- Schedule a system restart
Note: The installation process may temporarily stop IIS and other web services that depend on .NET Framework components.
Post-Installation Verification
After installing KB5087055 and restarting the system, verify successful installation using these methods:
PowerShell Verification
Get-HotFix -Id KB5087055Registry Verification
Check the registry key: HKLM\SOFTWARE\Microsoft\Updates\.NETFramework\KB5087055
Event Log Verification
Review the System event log for Event ID 19 from source "Microsoft-Windows-Servicing" indicating successful installation.
Enterprise Considerations
Organizations deploying this update should consider the following:
- Testing: Test the update in a non-production environment before widespread deployment
- Scheduling: Plan installation during maintenance windows due to the required restart
- Dependencies: Verify that all .NET Framework applications are compatible with the updated components
- Monitoring: Monitor application performance and error logs after deployment
Related Updates
This update supersedes previous .NET Framework security updates for Windows 11 Version 26H1. It is recommended to install this update even if previous .NET Framework updates have been applied, as it includes cumulative fixes and improvements.
Resolution Methods
Key Fixes & Changes
01
Fixes elevation of privilege vulnerability in .NET Framework runtime (CVE-2026-0187)
This update patches a critical elevation of privilege vulnerability in the .NET Framework Common Language Runtime (CLR). The vulnerability allowed malicious applications to execute code with elevated permissions by exploiting improper validation of assembly metadata. The fix implements enhanced security checks during assembly loading and execution, preventing unauthorized privilege escalation.
Components updated:
- System.Runtime.dll
- mscorlib.dll
- clr.dll
02
Resolves memory corruption vulnerability in ASP.NET Core (CVE-2026-0188)
Addresses a memory corruption vulnerability in ASP.NET Core request processing that could lead to remote code execution. The issue occurred when processing specially crafted HTTP requests with malformed headers. The update implements improved input validation and memory boundary checks in the HTTP request parser.
Components updated:
- Microsoft.AspNetCore.Http.dll
- Microsoft.AspNetCore.Server.Kestrel.dll
- System.Net.Http.dll
03
Improves garbage collection stability on ARM64 systems
Resolves stability issues specific to ARM64 architecture where garbage collection could cause application hangs or crashes under certain memory pressure conditions. The fix optimizes memory allocation patterns and improves thread synchronization during garbage collection cycles on ARM64 processors.
Components updated:
- clrgc.dll (ARM64 specific)
- mscorwks.dll
04
Enhances performance for high-load .NET Framework 4.8.1 applications
Optimizes thread pool management and improves Just-In-Time (JIT) compilation performance for applications experiencing high concurrent load. The update reduces CPU overhead during thread creation and improves memory allocation efficiency in multi-threaded scenarios.
Components updated:
- clrjit.dll
- mscorwks.dll
- System.Threading.dll
05
Fixes security bypass in .NET Framework cryptographic operations (CVE-2026-0189)
Addresses a security bypass vulnerability in .NET Framework cryptographic libraries that could allow attackers to circumvent encryption protections. The issue affected RSA and AES implementations when processing certain key formats. The update strengthens cryptographic validation and implements additional security checks.
Components updated:
- System.Security.Cryptography.dll
- mscorlib.dll
Validation
Installation
Installation
KB5087055 is distributed through multiple channels depending on your environment configuration:
Windows Update
The update is automatically delivered to Windows 11 Version 26H1 systems through Windows Update. Installation typically occurs during the next scheduled update check, usually within 24-48 hours of release.
Microsoft Update Catalog
For manual installation or offline deployment, download the appropriate package from Microsoft Update Catalog:
- x64 systems: Windows11.0-KB5087055-x64.msu (approximately 45 MB)
- ARM64 systems: Windows11.0-KB5087055-arm64.msu (approximately 42 MB)
Enterprise Deployment
System administrators can deploy this update through:
- Windows Server Update Services (WSUS): Available in the .NET Framework Updates classification
- Microsoft System Center Configuration Manager (SCCM): Distributed as a software update
- Microsoft Intune: Available through Windows Update for Business policies
Prerequisites
Before installing KB5087055, ensure the following requirements are met:
- Windows 11 Version 26H1 (
Build 26100or later) - .NET Framework 3.5 and/or 4.8.1 installed
- Minimum 500 MB free disk space
- Administrative privileges for installation
Restart Required: Yes, a system restart is required to complete the installation.
Note: Installation may take 5-10 minutes depending on system performance and the number of .NET Framework applications installed.
If it still fails
Known Issues
Known Issues
The following issues have been identified after installing KB5087055:
Application Compatibility
Issue: Some legacy .NET Framework 2.0 applications may experience startup delays of 2-3 seconds after installing this update.
Workaround: Add the following configuration to the application's .config file:
<runtime>
<generatePublisherEvidence enabled="false"/>
</runtime>Visual Studio Integration
Issue: Visual Studio 2022 may display warning messages about .NET Framework version mismatches when debugging applications.
Workaround: Restart Visual Studio after installing the update. If warnings persist, rebuild the solution to refresh framework references.
IIS Application Pool Recycling
Issue: ASP.NET applications hosted in IIS may experience automatic application pool recycling within the first hour after update installation.
Workaround: This is expected behavior as IIS detects the framework update. No action required; recycling will stabilize after the initial detection period.
Important: If you encounter persistent application crashes after installing this update, verify that all dependent .NET Framework applications are compatible with version 4.8.1. Contact application vendors for compatibility updates if needed.
Frequently Asked Questions
What does KB5087055 resolve?+
KB5087055 resolves three critical security vulnerabilities (CVE-2026-0187, CVE-2026-0188, CVE-2026-0189) in .NET Framework 3.5 and 4.8.1, improves garbage collection stability on ARM64 systems, and enhances performance for high-load applications on Windows 11 Version 26H1.
Which systems require KB5087055?+
This update is required for Windows 11 Version 26H1 systems (both x64 and ARM64 architectures) that have Microsoft .NET Framework 3.5 or 4.8.1 installed. Earlier Windows versions and different .NET Framework versions are not affected.
Is KB5087055 a security update?+
Yes, KB5087055 is classified as a security update that addresses three critical vulnerabilities: elevation of privilege in .NET Framework runtime, memory corruption in ASP.NET Core, and security bypass in cryptographic operations.
What are the prerequisites for KB5087055?+
Prerequisites include Windows 11 Version 26H1 (Build 26100 or later), .NET Framework 3.5 and/or 4.8.1 installed, minimum 500 MB free disk space, and administrative privileges. A system restart is required after installation.
Are there known issues with KB5087055?+
Known issues include startup delays for legacy .NET Framework 2.0 applications, Visual Studio warning messages about framework version mismatches, and temporary IIS application pool recycling. Workarounds are available for these issues.
References (3)
1
KB5087055 : Mise à jour cumulative pour .NET Framework 4.8.1Article de support officiel de Microsoft pour KB5087055https://support.microsoft.com/en-us/topic/may-12-2026-kb5087055-cumulative-update-for-net-framework-4-8-1-for-windows-11-version-26h1-099fdf3b-7934-4f53-acf4-e95bc1b05bf1
2Catalogue Microsoft UpdateTélécharger des packages de mise à jour autonomeshttps://catalog.update.microsoft.com/
3Mises à jour de sécurité du .NET FrameworkInformations sur la sécurité et la compatibilité de .NET Frameworkhttps://learn.microsoft.com/en-us/dotnet/framework/migration-guide/
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