A 401 status code in IIS logs indicates that a user attempted to access a protected resource without providing valid credentials. This response means the request lacks proper authentication, not necessarily due to lack of permission, but because identity hasn't been verified. Common causes include missing or incorrect authentication headers, misconfigured authentication methods, expired tokens or cookies, and incorrect file/folder permissions. To troubleshoot, check IIS logs for time of request, IP address, URI, and substatus codes such as 401.1 (no credentials), 401.2 (wrong method), or 401.3 (ACL issue). Common fixes include verifying authentication settings in IIS Manager, ensuring correct methods are enabled, checking application pool identity access, testing with various clients, and inspecting headers via Fiddler or browser tools. Frequent 401s from bots can be mitigated by blocking IPs or adding rate limits. Overall, 401s are typically normal, unless there's a sudden spike requiring further investigation.

A 401 status code in IIS logs means that a user tried to access a protected resource on the web server but didn’t provide valid credentials. In short, it's an "unauthorized" response — not because someone doesn’t have permission, but because they haven’t proven who they are yet.

This is pretty common in environments where authentication is required, like internal company websites or APIs. Let’s break down what this usually means and how you can go about troubleshooting or handling it.

What Causes a 401 Error in IIS?

There are several reasons why a 401 shows up in IIS logs:

• Missing or incorrect authentication headers: The client didn’t send the right credentials (like a token or username/password).
• Authentication method misconfiguration: For example, if the site expects Windows Authentication but it’s turned off.
• Token or cookie expiration: If using token-based auth (like OAuth), an expired token will trigger a 401.
• Incorrect permissions on files/folders: Even if authenticated, missing NTFS or IIS-level permissions can cause this.

In practice, you might see this when a browser pops up a login box again after submitting bad credentials — that’s a 401 in action.

How to Check IIS Logs for More Details

The raw 401 in the log is just the start. You’ll want to dig into the IIS logs to get more context. Here’s what to look for:

• Time of request – Helps correlate with user reports.
• IP address or user agent – Can tell you if it’s a bot, internal user, etc.
• URI requested – Which page or API endpoint was accessed?
• Substatus code – This gives more detail. For example:
  • 401.1 – No credentials provided
  • 401.2 – Failed authentication due to wrong method
  • 401.3 – ACL (access control list) issue

You can find these logs typically under C:\inetpub\logs\LogFiles. Use tools like Excel or LogParser to filter by status code and narrow down patterns.

Common Fixes and Best Practices

Here are some steps you can take depending on the situation:

• Check authentication settings in IIS Manager
  • Make sure the correct method (Windows Auth, Basic Auth, etc.) is enabled
  • Disable unused methods to reduce confusion
• Verify application pool identity
  • It should have access to any backend resources (e.g., databases)
• Test with different clients
  • Try accessing the URL from a browser, Postman, or curl to see if behavior differs
• Use Fiddler or browser dev tools
  • Look at request/response headers to check for missing tokens or failed challenges

If you're seeing lots of 401s from bots or crawlers, consider blocking suspicious IPs or adding rate limiting.

So, when you see a 401 status code in your IIS logs, don't panic. It’s usually a sign that authentication is working as expected — just someone trying (and failing) to get in. With a bit of digging into substatuses and logs, you can usually figure out whether it's a real issue or just background noise.

Mostly, it's normal traffic — unless it spikes suddenly, which might be worth a closer look.

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