How to fix Error 6507 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6507 – “Failed to Open Malformed Assembly”

Why This Error Happens

This error occurs when SQL Server’s CLR integration engine attempts to load a .NET assembly (DLL) into the database but finds that the binary is corrupted, truncated, or was compiled with an incompatible .NET runtime version. It can also surface when the assembly bytes stored in sys.assembly_files have been damaged due to a failed deployment, disk-level corruption, or an interrupted CREATE ASSEMBLY transaction. Additionally, security policies like Windows Defender Application Control (WDAC) or antivirus software may quarantine or strip portions of the file before SQL Server reads it into memory.

Step-by-Step Resolution Guide

Step 1: Verify the Source Assembly Integrity

  • Locate the original .dll file on disk (e.g., C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\) and confirm its file size matches the build output.
  • Run Get-FileHash -Path "C:\Deploy\MyAssembly.dll" -Algorithm SHA256 in PowerShell and compare it against the hash from your build server or source control.
  • If the hash doesn’t match or the file size is 0 KB, the file is corrupted and must be rebuilt or re-copied.

Step 2: Re-register the Assembly in SQL Server

  • Drop the broken assembly reference using DROP ASSEMBLY MyAssembly; (ensure no dependent objects are blocking this first).
  • Re-import the clean binary using:
  CREATE ASSEMBLY MyAssembly
  FROM 'C:\Deploy\MyAssembly.dll'
  WITH PERMISSION_SET = SAFE;
  • If deploying via Visual Studio, use Build > Deploy again to push a fresh binary rather than relying on a cached DLL.

Step 3: Check .NET Runtime Compatibility

  • Confirm the assembly was compiled targeting a CLR version supported by SQL Server 2025 (CLR 4.0/.NET Framework compatible, not a raw .NET 6+/.NET 8 assembly unless using the updated SQLCLR hosting model).
  • Open the .csproj file and verify the <TargetFramework> tag matches a supported runtime.
  • Recompile the project if an unsupported target framework (like net8.0-windows) was used instead of the required compatibility shim.

Step 4: Rule Out Security Software Interference

  • Check Windows Defender or third-party AV quarantine logs for the DLL filename around the timestamp of the error.
  • Add an exclusion path in Windows Security for your SQL Server deployment folder:
  • Go to Windows Security > Virus & threat protection > Manage settings > Exclusions.
  • Add the folder path where assemblies are staged before deployment.

Alternative or Verification Steps

  • Run SELECT * FROM sys.assembly_files; and SELECT * FROM sys.assemblies; to confirm the assembly now shows a valid, non-null content size and current timestamp.
  • If the error persists, bypass disk caching entirely by deploying the assembly directly via a VARBINARY(MAX) literal insert instead of a file path, which eliminates file-system-level corruption as a variable.