Troubleshooting IIS HTTP Error 503 Due to Missing or Corrupted DLL Files

Troubleshooting IIS HTTP Error 503 Due to Missing or Corrupted DLL Files

Overview

Service Unavailable – HTTP Error 503 is a common IIS error that occurs when an Application Pool stops automatically. One of the most frequent root causes is missing or corrupted IIS module DLL files. When IIS fails to load a required DLL, it terminates the worker process, resulting in a 503 error.

This document explains how to systematically troubleshoot and resolve IIS 503 errors caused by corrupted or missing DLL files, using two real-world case studies:

• caches.dll (Output Caching Module)

• defreq.dll (Request Filtering / Default Requests Module)

⚠️ Important: The same methodology applies to any other IIS module DLL that becomes corrupted or goes missing.

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How to Identify the Root Cause

Always start by checking the Event Viewer:

1. Open Event Viewer

2. Navigate to Windows Logs → Application

3. Look for IIS-related errors

Typical error message:

The Module DLL C:\Windows\System32\inetsrv\<dll_name>.dll failed to load. The data is the error.

This confirms IIS is attempting to load a module whose DLL is missing, corrupted, or inaccessible.

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Case Study 1: caches.dll Corruption

Error Observed

The Module DLL C:\Windows\System32\inetsrv\caches.dll failed to load. The data is the error.

This DLL belongs to the Output Caching Module in IIS.

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Step 1 — Verify if caches.dll Exists

1. Open File Explorer

2. Navigate to:

   C:\Windows\System32\inetsrv\

3. Confirm whether caches.dll is present

• ❌ If missing → IIS cannot start the module

• ✅ If present → Continue with permission and repair checks

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Step 2 — Check File Permissions

Even if the file exists, IIS may fail to load it due to incorrect permissions.

1. Right-click caches.dll → Properties

2. Open the Security tab

3. Ensure the following accounts have Read & Execute permission:

• IIS_IUSRS

• IUSR

• NETWORK SERVICE

• Administrators

Apply changes if required.

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Step 3 — Re-register IIS Modules

Open Command Prompt (Run as Administrator) and execute:

dism /online /enable-feature /featurename:IIS-WebServerRole /all
dism /online /enable-feature /featurename:IIS-WebServerManagementTools /all

Then restart IIS:

iisreset

This re-enables and re-registers IIS core components.

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Step 4 — Repair Corrupted System Files

If the DLL exists but is corrupted, repair it using system tools.

Run (Admin Command Prompt):

sfc /scannow

After completion, run:

DISM /Online /Cleanup-Image /RestoreHealth

Restart IIS once completed.

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Case Study 2: defreq.dll Corruption

Error Observed

The Module DLL C:\Windows\System32\inetsrv\defreq.dll failed to load. The data is the error.

This DLL belongs to Request Filtering / Default Requests Module.

First, follow all steps from Case Study 1. If the issue persists, proceed with the advanced steps below.

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Step 1 — Open IIS Configuration File

Open the IIS server-level configuration file:

C:\Windows\System32\inetsrv\config\applicationhost.config

Use Notepad (Run as Administrator).

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Step 2 — Locate Request Filtering Module References

Press Ctrl + F and search for:

• defreq.dll

• RequestFilteringModule

You may find entries like:

<add name="RequestFilteringModule" image="%windir%\System32\inetsrv\defreq.dll" />

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Step 3 — Comment Out the Broken Module

Use XML comment syntax to disable the module:

<!-- <add name="RequestFilteringModule" image="%windir%\System32\inetsrv\defreq.dll" /> -->

If the module appears multiple times (for example under globalModules, modules, or handlers), comment out every occurrence.

Example:

<!--
<add name="RequestFilteringModule" image="%windir%\System32\inetsrv\defreq.dll" />
-->

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Step 4 — Comment All Related References

Ensure every reference to the following is commented out:

• defreq.dll

• DefaultRequestsModule

• RequestFilteringModule

• RequestFiltering

Save the file.

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Step 5 — Restart IIS

Run:

iisreset

IIS should now start successfully without throwing HTTP 503 or 500 errors.

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Important Notes

• Commenting out a module is a temporary workaround that allows IIS to run

• Security features related to the disabled module may not function until the DLL is restored

• This approach is safe for troubleshooting and service restoration

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Applies to Other DLL Corruption Issues

The same troubleshooting process applies if any other IIS module DLL becomes corrupted or missing, such as:

• rewrite.dll

• aspnetcore.dll

• static.dll

• caches.dll

• defreq.dll

Key principle:

If IIS cannot load a module DLL, identify it via Event Viewer, repair or disable the reference, and restart IIS.

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Summary Checklist

✔️ Check Event Viewer for the failing DLL name ✔️ Verify DLL exists in inetsrv directory ✔️ Check file permissions ✔️ Run sfc /scannow and DISM /RestoreHealth ✔️ Disable broken module references in applicationhost.config if needed ✔️ Restart IIS

Following this guide will resolve most HTTP 503 Service Unavailable errors caused by corrupted or missing IIS DLL files.