Troubleshoot 401 – Unauthorized: Access is denied due to invalid credentials in IIS

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Surender Kumar

Surender Kumar has more than twelve years of experience in server and network administration. His fields of interest are Windows Servers, Active Directory, PowerShell, web servers, networking, Linux, virtualization, and penetration testing. He loves writing for his blog.

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401 – Unauthorized

401 Unauthorized Access is denied due to invalid credentials

The 401 – Unauthorized: Access is denied due to invalid credentials error is a common access-related error that you may experience with a website hosted on IIS.

How IIS authentication works

The error itself indicates that it is caused by a failure to authorize access. Someone who is a beginner in IIS could find the error description "you do not have permission to view this directory or page using the credentials that you supplied" slightly confusing. If you think from an end user's perspective, you might be wondering when you supplied any credentials while accessing the website. Well, this happened automatically on the server side. By default, every website hosted on IIS has anonymous authentication enabled.

If you open the IIS Manager, select your website, and then double-click Authentication under the IIS section in the Features view, you will see the various authentication modes (such as basic authentication, forms authentication, anonymous authentication, etc.) that are supported by IIS.

Viewing various authentication modes supported by a website in IIS

Each website has to have at least one authentication mode enabled and, by default, Anonymous Authentication mode enabled. Authentication is a mechanism that is used to verify the visitor's identity to your website. See the following screenshot for reference:

Default authentication modes supported by a website in IIS

Anonymous authentication allows visitors to access the public content of your website anonymously without having to supply any credentials. If you don't want to use it, select the authentication mode, and then click Disable in the Actions pane on the right. To view or modify which credentials are used by anonymous authentication, click the Edit link on the right. You will see two options, as shown in the following screenshot:

View or modify the credentials used by anonymous authentication in IIS

By default, each website is set to use the IUSR user for anonymous user identity, which is a built-in account starting with IIS version 7. If you are using a custom username as the application pool identity, make sure you select the application pool identity option here. This way, you don't have to worry about updating the user's password in the website configuration over and over when the user's password is changed.

Cause of error

The primary cause of the 401 – Unauthorized: Access is denied due to invalid credentials error is that you are using a custom username as the application pool identity. In simple terms, when you set the application pool to use a custom application pool identity (a custom username and password) rather than the default identity (which is ApplicationPoolIdentity), the Anonymous Authentication mode of the website continues using the IUSR user for authentication, which results in this error.

Debugging the error

As discussed in previous posts, the error page we saw above is a custom error page and does not reveal much helpful information for administrators. If you enable the detailed errors, you will see an error page with much detailed information, as shown in the following screenshot:

Error message 401.3 – You do not have permission to view this directory or page using the credentials you supplied (access denied due to Access Control Lists)

The detailed error gave you the HTTP 401.3 status code, which will guide you in checking the access control permissions on the file system. Make sure that the user who is set as the application pool identity has the read and execute permissions on the website's root directory. If the error persists, the most likely cause is incorrect anonymous user identity. The following screenshot shows the problematic scenario:

Mismatched application pool identity and anonymous authentication identity user in IIS

The screenshot shows a mismatched application pool identity and anonymous authentication identity on the website, which is causing the error. For a website to work properly, both should be the same.

Resolving the error

Now that you know the cause of the HTTP 401.3 status code, do the following to fix the error:

If you are using a custom username as the application pool identity, make sure that the user has read and execute permissions on the website's root directory.

Ensuring that the custom application pool user has read and execute permissions on the root directory

If you're using the default ApplicationPoolIdentity for your application pool, make sure that the built-in IUSR user or IIS_IUSRS group has exactly the same permissions as shown in the screenshot below.

Ensuring that the default application pool identity has read and execute permissions on the root directory

If the error persists, edit the Anonymous Authentication setting of your website, as explained in the How IIS authentication works section. Be sure it is set to use the Application pool identity. See the following screenshot for reference:

Modifying the anonymous authentication identity to match the application pool identity in IIS

Common 401 substatus codes

The following table covers the most common HTTP 401 substatus codes, along with their possible causes and troubleshooting advice:

Conclusion

I hope you're enjoying this series of resolving common HTTP errors. In the next post, we will discuss how to fix the HTTP Error 403.14 – Forbidden error.