Header Limits <headerLimits>

Article
03/23/2022

Overview

The <headerLimits> element of the <requestFiltering> collection contains a collection of <add> elements that specify the maximum size in bytes for HTTP headers.

Note

When request filtering blocks an HTTP request because an HTTP request exceeds the header limits, IIS 7 will return an HTTP 404 error to the client and log the following substatus that identifies the reason that the request was denied:

HTTP Substatus	Description
`431`	Request Header Too Long

This substatus allows Web administrators to analyze their IIS logs and identify potential threats.

Compatibility

Version	Notes
IIS 10.0	The `<headerLimits>` element was not modified in IIS 10.0.
IIS 8.5	The `<headerLimits>` element was not modified in IIS 8.5.
IIS 8.0	The `<headerLimits>` element was not modified in IIS 8.0.
IIS 7.5	The `<headerLimits>` element was not modified in IIS 7.5.
IIS 7.0	The `<headerLimits>` element of the `<requestLimits>` collection was introduced in IIS 7.0.
IIS 6.0	The `<requestLimits>` element replaces the IIS 6.0 UrlScan [RequestLimits] features.

Setup

The default installation of IIS 7 and later includes the Request Filtering role service or feature. If the Request Filtering role service or feature is uninstalled, you can reinstall it using the following steps.

Windows Server 2012 or Windows Server 2012 R2

On the taskbar, click Server Manager.
In Server Manager, click the Manage menu, and then click Add Roles and Features.
In the Add Roles and Features wizard, click Next. Select the installation type and click Next. Select the destination server and click Next.
On the Server Roles page, expand Web Server (IIS), expand Web Server, expand Security, and then select Request Filtering. Click Next.
.
On the Select features page, click Next.
On the Confirm installation selections page, click Install.
On the Results page, click Close.

Windows 8 or Windows 8.1

On the Start screen, move the pointer all the way to the lower left corner, right-click the Start button, and then click Control Panel.
In Control Panel, click Programs and Features, and then click Turn Windows features on or off.
Expand Internet Information Services, expand World Wide Web Services, expand Security, and then select Request Filtering.
Click OK.
Click Close.

Windows Server 2008 or Windows Server 2008 R2

On the taskbar, click Start, point to Administrative Tools, and then click Server Manager.
In the Server Manager hierarchy pane, expand Roles, and then click Web Server (IIS).
In the Web Server (IIS) pane, scroll to the Role Services section, and then click Add Role Services.
On the Select Role Services page of the Add Role Services Wizard, select Request Filtering, and then click Next.
On the Confirm Installation Selections page, click Install.
On the Results page, click Close.

Windows Vista or Windows 7

On the taskbar, click Start, and then click Control Panel.
In Control Panel, click Programs and Features, and then click Turn Windows Features on or off.
Expand Internet Information Services, then World Wide Web Services, and then Security.
Select Request Filtering, and then click OK.

How To

Note for IIS 7.0 users: Some of the steps in this section may require that you install the Microsoft Administration Pack for IIS 7.0, which includes a user interface for request filtering. To install the Microsoft Administration Pack for IIS 7.0, please see the following URL:

https://www.iis.net/expand/AdministrationPack

How to add limits for HTTP headers

Open Internet Information Services (IIS) Manager:
- If you are using Windows Server 2012 or Windows Server 2012 R2:
  - On the taskbar, click Server Manager, click Tools, and then click Internet Information Services (IIS) Manager.
- If you are using Windows 8 or Windows 8.1:
  - Hold down the Windows key, press the letter X, and then click Control Panel.
  - Click Administrative Tools, and then double-click Internet Information Services (IIS) Manager.
- If you are using Windows Server 2008 or Windows Server 2008 R2:
  - On the taskbar, click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
- If you are using Windows Vista or Windows 7:
  - On the taskbar, click Start, and then click Control Panel.
  - Double-click Administrative Tools, and then double-click Internet Information Services (IIS) Manager.
In the Connections pane, go to the connection, site, application, or directory for which you want to modify your request filtering settings.
In the Home pane, double-click Request Filtering.
In the Request Filtering pane, click the Headers tab, and then click Add Header... in the Actions pane.
In the Add Header dialog box, enter the HTTP header and the maximum size that you want for the header limit, and then click OK.

For example, the "Content-type" header contains the MIME type for a request. Specifying a value of 100 would limit the length of the "Content-type" header to 100 bytes.

Configuration

Attributes

None.

Child Elements

Element	Description
`add`	Optional element. Adds a size limit for an HTTP header to the `<headerLimits>` collection.
`clear`	Optional element. Removes all references to an HTTP header from the `<headerLimits>` collection.
`remove`	Optional element. Removes a reference to an HTTP header from the `<headerLimits>` collection.

Configuration Sample

The following example Web.config file will configure IIS to deny access for HTTP requests where the length of the "Content-type" header is greater than 100 bytes.

<configuration>
   <system.webServer>
      <security>
         <requestFiltering>
            <requestLimits>
               <headerLimits>
                  <add header="Content-type" sizeLimit="100" />
               </headerLimits>
            </requestLimits>
         </requestFiltering>
      </security>
   </system.webServer>
</configuration>

Sample Code

The following code samples will configure IIS to deny access for HTTP requests where the length of the "Content-type" header is greater than 100 bytes.

AppCmd.exe

appcmd.exe set config "Default Web Site" -section:system.webServer/security/requestFiltering /+"requestLimits.headerLimits.[header='Content-type',sizeLimit='100']"

PowerShell

$requestLimits = Get-IISConfigSection -CommitPath 'Default Web Site' -SectionPath 'system.webServer/security/requestFiltering' | Get-IISConfigElement -ChildElementName 'requestLimits'
$headerLimits = Get-IISConfigCollection -ConfigElement $requestLimits -CollectionName 'headerLimits'
New-IISConfigCollectionElement -ConfigCollection $headerLimits -ConfigAttribute @{ 'header'='Content-Type'; 'sizeLimit'=100 }

C#

using System;
using System.Text;
using Microsoft.Web.Administration;

internal static class Sample
{
   private static void Main()
   {
      using (ServerManager serverManager = new ServerManager())
      {
         Configuration config = serverManager.GetWebConfiguration("Default Web Site");
         ConfigurationSection requestFilteringSection = config.GetSection("system.webServer/security/requestFiltering");
         ConfigurationElement requestLimitsElement = requestFilteringSection.GetChildElement("requestLimits");
         ConfigurationElementCollection headerLimitsCollection = requestLimitsElement.GetCollection("headerLimits");

         ConfigurationElement addElement = headerLimitsCollection.CreateElement("add");
         addElement["header"] = @"Content-type";
         addElement["sizeLimit"] = 100;
         headerLimitsCollection.Add(addElement);

         serverManager.CommitChanges();
      }
   }
}

VB.NET

Imports System
Imports System.Text
Imports Microsoft.Web.Administration

Module Sample

   Sub Main()
      Dim serverManager As ServerManager = New ServerManager
      Dim config As Configuration = serverManager.GetWebConfiguration("Default Web Site")
      Dim requestFilteringSection As ConfigurationSection = config.GetSection("system.webServer/security/requestFiltering")
      Dim requestLimitsElement As ConfigurationElement = requestFilteringSection.GetChildElement("requestLimits")
      Dim headerLimitsCollection As ConfigurationElementCollection = requestLimitsElement.GetCollection("headerLimits")

      Dim addElement As ConfigurationElement = headerLimitsCollection.CreateElement("add")
      addElement("header") = "Content-type"
      addElement("sizeLimit") = 100
      headerLimitsCollection.Add(addElement)

      serverManager.CommitChanges()
   End Sub

End Module

JavaScript

var adminManager = new ActiveXObject('Microsoft.ApplicationHost.WritableAdminManager');
adminManager.CommitPath = "MACHINE/WEBROOT/APPHOST/Default Web Site";
var requestFilteringSection = adminManager.GetAdminSection("system.webServer/security/requestFiltering", "MACHINE/WEBROOT/APPHOST/Default Web Site");

var requestLimitsElement = requestFilteringSection.ChildElements.Item("requestLimits");
var headerLimitsCollection = requestLimitsElement.ChildElements.Item("headerLimits").Collection;

var addElement = headerLimitsCollection.CreateNewElement("add");
addElement.Properties.Item("header").Value = "Content-type";
addElement.Properties.Item("sizeLimit").Value = 100;
headerLimitsCollection.AddElement(addElement);

adminManager.CommitChanges();

VBScript

Set adminManager = WScript.CreateObject("Microsoft.ApplicationHost.WritableAdminManager")
adminManager.CommitPath = "MACHINE/WEBROOT/APPHOST/Default Web Site"
Set requestFilteringSection = adminManager.GetAdminSection("system.webServer/security/requestFiltering", "MACHINE/WEBROOT/APPHOST/Default Web Site")
Set requestLimitsElement = requestFilteringSection.ChildElements.Item("requestLimits")
Set headerLimitsCollection = requestLimitsElement.ChildElements.Item("headerLimits").Collection

Set addElement = headerLimitsCollection.CreateNewElement("add")
addElement.Properties.Item("header").Value = "Content-type"
addElement.Properties.Item("sizeLimit").Value = 100
headerLimitsCollection.AddElement(addElement)

adminManager.CommitChanges()

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