Deny Query String Sequences <denyQueryStringSequences>


The <denyQueryStringSequences> element contains a collection of <add> elements that specify sequences of query string characters that IIS will deny, which helps prevent attacks on the Web server that use the query string to deliver the attack payload.

Note: You can override the query string sequences in this collection by adding query string sequences to the <alwaysAllowedQueryStrings> collection.

Note: When request filtering blocks an HTTP request because of a denied query string sequence, IIS 7 will return an HTTP 404 error to the client and log the following HTTP status with a unique substatus that identifies the reason that the request was denied:

HTTP Substatus Description
404.18 Query String Sequence Denied

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


Version Notes
IIS 7.5 The <denyQueryStringSequences> element of the <requestFiltering> element ships as a feature of IIS 7.5.
IIS 7.0 The <denyQueryStringSequences> element of the <requestFiltering> element was introduced as an update for IIS 7.0 that is available through Microsoft Knowledge Base Article 957508.
IIS 6.0 The <denyQueryStringSequences> element is roughly analogous to the [DenyQueryStringSequences] section that was added to URLScan 3.0.


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

Windows Server 2008 or Windows Server 2008 R2

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

Windows Vista or Windows 7

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

How To

How to deny a query string sequence

  1. Open 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.
  2. In the Connections pane, go to the connection, site, application, or directory for which you want to modify your request filtering settings.
  3. In the Home pane, double-click Request Filtering.
  4. In the Request Filtering pane, click the Query Strings tab, and then click Deny Query String... in the Actions pane.
  5. In the Deny Query String dialog box, enter the query string sequence that you wish to block, and then click OK.


The <denyQueryStringSequences> element of the <requestFiltering> element is configured at the site, application, or directory level.



Child Elements

Element Description
add Optional element.

Adds a query string to the collection of denied query strings.
clear Optional element.

Removes all references to query strings from the collection.
remove Optional element.

Removes a query string from the collection of denied query strings.

Configuration Sample

The following sample illustrates a combination of a <denyQueryStringSequences> element and an <alwaysAllowedQueryStrings> element that will deny any query strings if they contain either of two specific character sequences, but will always allow a specific query string that contains both of those two specific character sequences in a particular order.

            <add sequence="bad" />
            <add sequence="sequence" />
            <add queryString="bad=sequence" />

Sample Code

The following examples demonstrate how to add a query string sequence that will be denied on the Default Web Site.


appcmd.exe set config "Default Web Site" -section:system.webServer/security/requestFiltering /+"denyQueryStringSequences.[sequence='bad_querystring_sequence']" 


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"); ConfigurationElementCollection denyQueryStringSequencesCollection = requestFilteringSection.GetCollection("denyQueryStringSequences"); ConfigurationElement addElement = denyQueryStringSequencesCollection.CreateElement("add"); addElement["sequence"] = @"bad_querystring_sequence"; denyQueryStringSequencesCollection.Add(addElement); serverManager.CommitChanges(); } } }


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 denyQueryStringSequencesCollection As ConfigurationElementCollection = requestFilteringSection.GetCollection("denyQueryStringSequences") Dim addElement As ConfigurationElement = denyQueryStringSequencesCollection.CreateElement("add") addElement("sequence") = "bad_querystring_sequence" denyQueryStringSequencesCollection.Add(addElement) serverManager.CommitChanges() End Sub End Module


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 denyQueryStringSequencesCollection = requestFilteringSection.ChildElements.Item("denyQueryStringSequences").Collection;
var addElement = denyQueryStringSequencesCollection.CreateNewElement("add");
addElement.Properties.Item("sequence").Value = "bad_querystring_sequence";



Set adminManager = 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 denyQueryStringSequencesCollection = requestFilteringSection.ChildElements.Item("denyQueryStringSequences").Collection
Set addElement = denyQueryStringSequencesCollection.CreateNewElement("add")
addElement.Properties.Item("sequence").Value = "bad_querystring_sequence"

Deprecated Elements