HTTP 400 Bad Request (Request Header too long) responses to HTTP requests

This article helps you resolve the HTTP 400 error that occurs when an Hypertext Transfer Protocol (HTTP) request that needs Kerberos authentication is sent from a browser to a website that's hosted on Microsoft Internet Information Services (IIS) and is configured to use Kerberos authentication.

Original product version:   Windows Server 2016
Original KB number:   2020943


An HTTP request that needs Kerberos authentication is sent from a browser to a website that's hosted on IIS. The website is configured to use Kerberos authentication. However, instead of receiving the expected webpage, you receive an error message that resembles the following:

HTTP 400 - Bad Request (Request header too long)

This response could be generated by any HTTP request that includes Windows Remote Management (WinRM).


This issue may occur if the user is a member of many Active Directory user groups.

The HTTP request to the server contains the Kerberos token in the WWW-Authenticate header. The header size increases together with the number of user groups. If the HTTP header or packet size increases past the limits that are configured on the server, the server may reject the request and send an error message as the response.

Workaround 1: Decrease the number of Active Directory groups

Decrease the number of Active Directory groups that the user is a member of.

Workaround 2: Set MaxFieldLength and MaxRequestBytes registry entries

Increase the settings for the MaxFieldLength and the MaxRequestBytes registry entries on the server so that the user's request headers don't exceed these values. To determine the appropriate settings, use the following calculations:

  1. Calculate the size of the user's Kerberos token by using the formula that's described in Problems with Kerberos authentication when a user belongs to many groups.

  2. Set the value of MaxFieldLength and MaxRequestBytes on the server to 4/3 * T bytes, where T is the user's token size in bytes. HTTP encodes the Kerberos token by using base64 encoding.


    This replaces every three bytes in the token with four base64-encoded bytes. Changes that are made to the registry do not take effect until you restart the HTTP service. Additionally, you may have to restart any related services, such as IIS services.

Depending on your application environment, you might also be able to work around this problem by configuring the website to use Windows NT LAN Manager (NTLM) instead of Kerberos. However, some application environments require Kerberos authentication to be used for delegation. We consider Kerberos authentication to be more secure than NTLM. Therefore, we recommend that you do not disable Kerberos authentication before you consider the security and delegation ramifications of doing this.

More information

By default, there is no MaxFieldLength registry entry. This entry specifies the maximum size limit of each HTTP request header. The MaxRequestBytes registry entry specifies the upper limit for the total size of the Request line and the headers. Typically, this registry entry is configured together with the MaxRequestBytes registry entry. If the MaxRequestBytes value is lower than the MaxFieldLength value, the MaxFieldLength value is adjusted. In large Active Directory environments, users may experience logon failures if the values for both these entries are not set to a sufficiently high value.

For IIS 6.0 and later, the MaxFieldLength and MaxRequestBytes registry keys are located at the following sub key:

Set the key values as shown in the following table:

Name Value Type Value Data
 MaxFieldLength  DWORD  (4/3 * T bytes) + 200
 MaxRequestBytes  DWORD  (4/3 * T bytes) + 200

You can also set the registry keys to their maximum values, as shown in the next table. You should consider all potential security ramifications if he makes any changes to the registry settings.

Name Value Type Value Data
 MaxFieldLength  DWORD  65536 (Dec) or 10000 (Hex)
 MaxRequestBytes  DWORD  16777216 (Dec) or 100000 (Hex)


Changing these registry keys should be considered to be extremely dangerous. These keys allow larger HTTP packets to be sent to IIS. This, in turn, may cause Http.sys to use more memory. Therefore, such changes can increase the computer's vulnerability to malicious attacks.

If MaxFieldLength is set to its maximum value of 64 KB, the MaxTokenSize registry value should be set to 3/4 * 64 = 48 KB. For more information about the MaxTokenSize setting, see Problems with Kerberos authentication when a user belongs to many groups.