Microsoft Ships KB5082417 .NET Framework Update Fixing CLR Privilege Escalation Flaw
The cumulative update addresses a critical CLR elevation-of-privilege flaw and several stability and performance defects across .NET Framework 3.5 and 4.8.1.
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Key takeaways
- KB5082417 is a cumulative update for .NET Framework 3.5 and 4.8.1 released April 14, 2026.
- It patches a critical CLR elevation-of-privilege vulnerability, CVE-2026-0847.
- It also resolves ASP.NET memory corruption under high load, WCF reliability issues, Entity Framework performance and XML serialization data-integrity problems.
- Affected platforms are Windows 11 Version 25H2 (x64/ARM64) and Windows Server 2025 (Full and Server Core).
- A restart is required, and Microsoft lists known issues affecting ASP.NET app pools, EF Code First migrations and WCF configuration warnings.
What to do now
High urgency- Test KB5082417 in non-production before fleet rollout
- Deploy via Windows Update, WSUS, SCCM or Intune and restart affected systems
- Restart ASP.NET application pools to realize memory-management improvements
- Verify installation with Get-HotFix -Id KB5082417
Microsoft has released KB5082417, a cumulative security and reliability update for .NET Framework 3.5 and 4.8.1, dated April 14, 2026. The update targets Windows 11 Version 25H2 and Windows Server 2025, and its headline fix is a critical elevation-of-privilege vulnerability in the Common Language Runtime tracked as CVE-2026-0847.
Because the update patches a CLR flaw that could allow arbitrary code execution with elevated privileges alongside ASP.NET memory-corruption and WCF reliability fixes, administrators should prioritize testing and deployment on affected .NET Framework fleets.
Microsoft's KB5082417 patches a critical .NET Framework CLR privilege-escalation flaw and several stability fixes for Windows 11 25H2 and Server 2025.
Deploy KB5082417 to patch CVE-2026-0847 and .NET Framework stability issues; test first due to known ASP.NET, EF and WCF post-install issues, and reboot to complete.
Affected & context
Microsoft released KB5082417, a cumulative security and reliability update for .NET Framework 3.5 and 4.8.1 on Windows 11 Version 25H2 and Windows Server 2025, on April 14, 2026, addressing a critical CLR elevation-of-privilege flaw (CVE-2026-0847) and several stability and performance defects.
.NET Framework underpins large numbers of line-of-business and web applications, and a CLR elevation-of-privilege flaw combined with ASP.NET memory-corruption fixes makes this a priority patch for affected fleets.
Systems running .NET Framework 3.5 and/or 4.8.1 on Windows 11 Version 25H2 (x64/ARM64) and Windows Server 2025 (Full and Server Core).
- Vendors
- Microsoft
- Products
- .NET Framework 3.5.NET Framework 4.8.1Windows 11 Version 25H2Windows Server 2025
- Geography
- Global
- CVEs
- CVE-2026-0847
What the update does
Microsoft has released KB5082417, a cumulative security and reliability update for Microsoft .NET Framework 3.5 and 4.8.1, dated April 14, 2026. The package targets Windows 11 Version 25H2 and Windows Server 2025 systems that have those .NET Framework versions installed.
According to Microsoft's documentation, the update bundles a critical security fix with several reliability and performance changes across the .NET Framework runtime.
- Elevation-of-privilege fix in the Common Language Runtime (CVE-2026-0847)
- ASP.NET memory-corruption fix under high concurrent load
- Windows Communication Foundation (WCF) reliability fixes for TCP and Named Pipe bindings
- Entity Framework performance improvements for large datasets
- XML serialization data-integrity corrections for complex object graphs
- Released April 14, 2026 for .NET Framework 3.5 and 4.8.1
- Headline fix: CLR elevation of privilege, CVE-2026-0847
Security fix and root cause
The most significant change addresses CVE-2026-0847, described as a critical elevation-of-privilege vulnerability in the Common Language Runtime that could allow an attacker to execute arbitrary code with elevated privileges. The fix adds validation checks in the CLR memory-management subsystem and strengthens security boundaries between application domains.
Per Microsoft, the underlying issues span multiple runtime subsystems: insufficient validation in CLR memory management enabled potential privilege escalation, race conditions in the ASP.NET request-processing pipeline caused memory corruption under high concurrency, and inefficient garbage-collection and query-execution patterns drove performance degradation in data-access scenarios.
Exploitation status not stated
Microsoft's documentation rates CVE-2026-0847 as critical but does not indicate active exploitation. Treat exploitation status as unknown until confirmed by an authoritative advisory.
Affected systems
The update applies to systems with .NET Framework 3.5 and/or 4.8.1 installed on the following platforms:
- Windows 11 Version 25H2 (x64) — standard installation
- Windows 11 Version 25H2 (ARM64) — standard installation
- Windows Server 2025 (x64) — Full installation
- Windows Server 2025 (x64) — Server Core
The installer automatically detects installed .NET Framework versions and applies the appropriate components. A system restart is required to complete installation.
Deployment and verification
KB5082417 ships through Windows Update starting April 14, 2026, and is available for offline and enterprise deployment through the Microsoft Update Catalog, WSUS, Microsoft Configuration Manager (SCCM) and Microsoft Intune. Package sizes range from roughly 38 MB (ARM64) to 52 MB (Windows Server 2025), and Microsoft says installation typically takes 5–15 minutes.
Administrators can confirm the update is present with PowerShell:
Get-HotFix -Id KB5082417Known issues after installing
Microsoft lists several post-installation issues that administrators should factor into deployment planning:
- ASP.NET applications may not benefit from memory-management improvements until their application pools are restarted.
- Entity Framework Code First migrations may see temporary database connectivity issues right after installation, particularly for apps that perform automatic schema updates at startup.
- WCF services with custom binding configurations may emit additional validation warnings in event logs; Microsoft says these do not affect functionality but flag deprecated configuration patterns.
Test before production
Microsoft recommends testing the update in non-production environments first, especially for applications with complex .NET Framework configurations or custom security implementations.
Timeline
KB5082417 released
Microsoft publishes the cumulative .NET Framework 3.5 and 4.8.1 update via Windows Update and the Microsoft Update Catalog.
Source: Microsoft update documentation
Confidence: High
Impact
KB5082417 closes a critical CLR privilege-escalation flaw and resolves ASP.NET, WCF, Entity Framework and XML serialization defects on affected .NET Framework fleets.
Business impact
Unpatched systems remain exposed to a critical privilege-escalation flaw and to stability/performance issues in web and line-of-business applications.
Technical impact
Fixes CLR memory-management validation, ASP.NET request-pipeline race conditions, WCF connection handling, Entity Framework query performance and XML serialization of complex object graphs.
Security impact
Addresses CVE-2026-0847, a critical CLR elevation-of-privilege vulnerability that could enable arbitrary code execution with elevated privileges.
Affected audience: IT admins, MSPs, .NET application developers
Action required.
Technical details
- CVEs
- CVE-2026-0847
- Affected versions
- .NET Framework 3.5, .NET Framework 4.8.1
Detection methods
- Confirm patch presence with Get-HotFix -Id KB5082417
Mitigations
- Apply KB5082417 and restart affected systems
Response
Customer guidance
Microsoft advises testing the update in non-production environments before deploying to production, restarting after installation, and restarting ASP.NET application pools to realize memory-management improvements.
Response status: Patched
Patch available: Yes
Workaround available: No
FAQ
What does KB5082417 fix?
It is a cumulative security and reliability update for .NET Framework 3.5 and 4.8.1 that patches a critical CLR elevation-of-privilege vulnerability (CVE-2026-0847) and resolves ASP.NET memory corruption, WCF reliability, Entity Framework performance and XML serialization data-integrity issues.
Which systems does KB5082417 apply to?
Systems with .NET Framework 3.5 and/or 4.8.1 installed on Windows 11 Version 25H2 (x64/ARM64) and Windows Server 2025 (Full installation and Server Core).
How do I check if KB5082417 is installed?
Run Get-HotFix -Id KB5082417 in PowerShell; it returns patch details if the update is present.
Is a restart required?
Yes. Microsoft states a system restart is required after installation to complete the update and activate the security enhancements.
Are there known issues after installing?
Yes. ASP.NET apps may need app-pool restarts to benefit from memory improvements, Entity Framework Code First migrations may hit temporary connectivity issues, and WCF services with custom bindings may log additional validation warnings.
The bottom line
Microsoft's KB5082417 delivers a critical CLR privilege-escalation fix (CVE-2026-0847) plus ASP.NET, WCF, Entity Framework and XML serialization fixes for .NET Framework 3.5 and 4.8.1 on Windows 11 25H2 and Windows Server 2025.
Prioritize this update because of the critical CLR elevation-of-privilege fix, but test first given the listed known issues.
What happens next
Administrators should stage and test the update, then roll it out via their preferred channel and restart affected systems, watching for the documented ASP.NET, EF and WCF post-install behaviors.
What to do
Test KB5082417 in non-production, deploy fleetwide, and verify with Get-HotFix.
Sources
Microsoft · Apr 14, 2026 · Primary source
Claims supported
- KB5082417 is a cumulative security and reliability update for .NET Framework 3.5 and 4.8.1 released April 14, 2026.
- The update fixes CVE-2026-0847, a critical CLR elevation-of-privilege vulnerability.
- Affected platforms are Windows 11 Version 25H2 and Windows Server 2025.