FTP Request Filtering <requestFiltering>

Overview

FTP Request Filtering is a built-in security feature that was introduced in FTP 7, and is roughly analogous to the functionality that was available through request filtering features that are available for HTTP sites. All of the settings for the request filtering feature are located within the <requestFiltering> element, which contains several child elements for each of the following feature areas:

  • <denyUrlSequences> - This element can contain a collection of URL sequence patterns that IIS 7 will deny; for example: you can deny parts of URL sequences that an attacker might try to exploit.
  • <fileExtensions> - This element can contain a collection of file name extensions that IIS 7 will either deny or allow; for example: you can block all requests for *.EXE or *.COM files.
  • <hiddenSegments> - This element can contain a collection of URLs that cannot be browsed; for example: you can deny requests for the ASP.NET App_Code folder.
  • <requestLimits> - This element contains the settings for URL, content, and command lengths.

On significant difference between FTP request filtering and HTTP request filtering is that HTTP request filtering includes an additional <verbs> element, which may contain a collection of HTTP verbs that IIS will either deny or allow. FTP provides this same type of filtering through the <commandFiltering> element that is configured at the site level.

Note: When request filtering blocks an FTP request because an FTP request violates the request filtering rules, FTP 7 will return an FTP error to the client and log one of the following FTP substatuses that identifies the reason that the request was denied:

FTP Substatus Description
8 Hidden segment was detected in the path based on request filtering rules.
9 Denied URL sequence detected in the path based on request filtering rules.
10 High bit characters detected in the path based on request filtering rules.
11 File name extension was denied based on request filtering rules
12 Path is too long based on request filtering rules.
51 Command filtering rules denied the access.

Notification of the conditions that are indicated by these substatuses allows Web administrators to analyze their IIS logs and identify potential threats.

Compatibility

Version Notes
IIS 7.5 The <requestFiltering> element of the <system.ftpServer/security> element ships as a feature of IIS 7.5.
IIS 7.0 The <requestFiltering> element of the <system.ftpServer/security> element was introduced in FTP 7.0, which was a separate download for IIS 7.0.
IIS 6.0 The FTP service in IIS 6.0 did not support request filtering.

Note: The FTP 7.0 and FTP 7.5 services shipped out-of-band for IIS 7.0, which required downloading and installing the modules from the following URL:

http://www.iis.net/expand/FTP

With Windows 7 and Windows Server 2008 R2, the FTP 7.5 service ships as a feature for IIS 7.5, so downloading the FTP service is no longer necessary.

Setup

To support FTP publishing for your Web server, you must install the FTP service. To do so, use the following steps.

IIS 7.5 for 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, expand FTP Server.
  5. Select FTP Service.

    Note: To support ASP.Membership authentication or IIS Manager authentication for the FTP service, you will also need to select FTP Extensibility.
  6. Click Next.
  7. On the Confirm Installation Selections page, click Install.
  8. On the Results page, click Close.

IIS 7.5 for 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, and then FTP Server.
  4. Select FTP Service.

    Note: To support ASP.Membership authentication or IIS Manager authentication for the FTP service, you will also need to select FTP Extensibility.
  5. Click OK.

IIS 7.0 for Windows Server 2008 and Windows Vista

  1. Download the installation package from the following URL:
  2. Follow the instructions in the following walkthrough to install the FTP service:

How To

Note: FTP Request Filtering did not have a user interface in the FTP 7.0 release; the FTP Request Filtering UI was added in the FTP 7.5 release.

How to deny FTP access to a specific file name extension

  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 site or directory for which you want to modify your request filtering settings.
  3. In the Home pane, double-click FTP Request Filtering.
  4. In the FTP Request Filtering pane, click the File Name Extensions tab.
  5. Click Deny File Name Extension... in the Actions pane.
  6. In the Deny File Name Extension dialog box, enter the file name extension that you wish to block. For example, to prevent access to files with a file name extension of .inc, you would enter "inc" in the dialog box.
  7. Click OK.

How to edit the request filtering feature settings and request limits

  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 site or directory for which you want to modify your request filtering settings.
  3. In the Home pane, double-click FTP Request Filtering.
  4. Click Edit Feature Settings... in the Actions pane.
  5. Specify your options. For example, you could make the following changes:
    • Change the maximum URL length to 2KB by specifying 2048.
    • Change the maximum command length to 1KB by specifying 1024.
    • Note: It is recommended that you do not clear the Allow unlisted commands check box.
  6. Click OK.

How to add an FTP hidden segment

  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 site or directory for which you want to modify your request filtering settings.
  3. In the Home pane, double-click FTP Request Filtering.
  4. In the FTP Request Filtering pane, click the Hidden Segments tab.
  5. Click Add Hidden Segment... in the Actions pane.
  6. In the Add Hidden Segment dialog box, enter the path that you want to hide.
  7. Click OK.

How to deny an FTP URL 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 site or directory for which you want to modify your request filtering settings.
  3. In the Home pane, double-click FTP Request Filtering.
  4. In the FTP Request Filtering pane, click the Denied URL Sequences tab.
  5. Click Add URL Sequence... in the Actions pane.
  6. In the Add Deny Sequence dialog box, enter the URL sequence that you wish to block.
  7. Click OK.

Configuration

The <requestFiltering> element is configured at the global, site or URL level.

Attributes

Attribute Description
allowHighBitCharacters Optional Boolean attribute.

Specifies whether to allow non-ASCII characters in URLs.

The default value is true.

Child Elements

Element Description
fileExtensions Optional element.

Specifies which file name extensions are allowed or denied to limit types of requests sent to the FTP server.
requestLimits Optional element.

Specifies limits on requests processed by the FTP server.
hiddenSegments Optional element.

Specifies that certain segments of URLs can be made inaccessible to clients.
denyUrlSequences Optional element.

Specifies sequences that should be denied to help prevent URL-based attacks on the FTP server.

Configuration Sample

The following sample illustrates several security-related configuration settings in the <system.ftpServer> element for an FTP site. More specifically, the <location> settings in this example demonstrate how to:

  • Specify an FTP authorization rule for read and write access for the administrators group.
  • Specify FTP request filtering options that deny *.exe, *.bat, and *.cmd files.
  • Specify FTP request limits for a maximum content length of 1000000 bytes and a maximum URL length of 1024 bytes.
  • Block FTP access to the _vti_bin virtual directory, which is used with the FrontPage Server Extensions.
  • Specify FTP IP filtering options that allow access from 127.0.0.1 and deny access from the 169.254.0.0/255.255.0.0 range of IP addresses.
<location path="ftp.example.com">
  <system.ftpServer>
  <security>
    <authorization>
      <add accessType="Allow" roles="administrators" permissions="Read, Write" />
    </authorization>
    <requestFiltering>
      <fileExtensions allowUnlisted="true">
        <add fileExtension=".exe" allowed="false" />
        <add fileExtension=".bat" allowed="false" />
        <add fileExtension=".cmd" allowed="false" />
      </fileExtensions> <requestLimits maxAllowedContentLength="1000000" maxUrl="1024" /> <hiddenSegments> <add segment="_vti_bin" /> </hiddenSegments>
    </requestFiltering>
<ipSecurity enableReverseDns="false" allowUnlisted="true">
<add ipAddress="127.0.0.1" allowed="true" />
<add ipAddress="169.254.0.0" subnetMask="255.255.0.0" allowed="false" />
</ipSecurity>
  </security>
  </system.ftpServer>
</location>

Sample Code

The following examples specify FTP request filtering options that deny *.exe, *.com, *.bat, and *.cmd files.

AppCmd.exe

appcmd.exe set config "Default Web Site" -section:system.ftpServer/security/requestFiltering /+"fileExtensions.[fileExtension='.exe',allowed='False']" /commit:apphost

appcmd.exe set config "Default Web Site" -section:system.ftpServer/security/requestFiltering /+"fileExtensions.[fileExtension='.com',allowed='False']" /commit:apphost

appcmd.exe set config "Default Web Site" -section:system.ftpServer/security/requestFiltering /+"fileExtensions.[fileExtension='.cmd',allowed='False']" /commit:apphost

appcmd.exe set config "Default Web Site" -section:system.ftpServer/security/requestFiltering /+"fileExtensions.[fileExtension='.bat',allowed='False']" /commit:apphost

Note: You must be sure to set the commit parameter to apphost when you use AppCmd.exe to configure these settings. This commits the configuration settings to the appropriate location section in the ApplicationHost.config file.

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.GetApplicationHostConfiguration(); ConfigurationSection requestFilteringSection = config.GetSection("system.ftpServer/security/requestFiltering", "Default Web Site"); ConfigurationElement fileExtensionsElement = requestFilteringSection.GetChildElement("fileExtensions"); ConfigurationElementCollection fileExtensionsCollection = fileExtensionsElement.GetCollection(); ConfigurationElement addElement = fileExtensionsCollection.CreateElement("add"); addElement["fileExtension"] = @".exe"; addElement["allowed"] = false; fileExtensionsCollection.Add(addElement); ConfigurationElement addElement1 = fileExtensionsCollection.CreateElement("add"); addElement1["fileExtension"] = @".com"; addElement1["allowed"] = false; fileExtensionsCollection.Add(addElement1); ConfigurationElement addElement2 = fileExtensionsCollection.CreateElement("add"); addElement2["fileExtension"] = @".cmd"; addElement2["allowed"] = false; fileExtensionsCollection.Add(addElement2); ConfigurationElement addElement3 = fileExtensionsCollection.CreateElement("add"); addElement3["fileExtension"] = @".bat"; addElement3["allowed"] = false; fileExtensionsCollection.Add(addElement3); 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.GetApplicationHostConfiguration Dim requestFilteringSection As ConfigurationSection = config.GetSection("system.ftpServer/security/requestFiltering", "Default Web Site") Dim fileExtensionsElement As ConfigurationElement = requestFilteringSection.GetChildElement("fileExtensions") Dim fileExtensionsCollection As ConfigurationElementCollection = fileExtensionsElement.GetCollection Dim addElement As ConfigurationElement = fileExtensionsCollection.CreateElement("add") addElement("fileExtension") = ".exe" addElement("allowed") = False fileExtensionsCollection.Add(addElement) Dim addElement1 As ConfigurationElement = fileExtensionsCollection.CreateElement("add") addElement1("fileExtension") = ".com" addElement1("allowed") = False fileExtensionsCollection.Add(addElement1) Dim addElement2 As ConfigurationElement = fileExtensionsCollection.CreateElement("add") addElement2("fileExtension") = ".cmd" addElement2("allowed") = False fileExtensionsCollection.Add(addElement2) Dim addElement3 As ConfigurationElement = fileExtensionsCollection.CreateElement("add") addElement3("fileExtension") = ".bat" addElement3("allowed") = False fileExtensionsCollection.Add(addElement3) serverManager.CommitChanges() End Sub End Module

JavaScript

var adminManager = new ActiveXObject('Microsoft.ApplicationHost.WritableAdminManager');
adminManager.CommitPath = "MACHINE/WEBROOT/APPHOST";
var requestFilteringSection = adminManager.GetAdminSection("system.ftpServer/security/requestFiltering", "MACHINE/WEBROOT/APPHOST/Default Web Site");
var fileExtensionsElement = requestFilteringSection.ChildElements.Item("fileExtensions");
var fileExtensionsCollection = fileExtensionsElement.Collection;

var addElement = fileExtensionsCollection.CreateNewElement("add");
addElement.Properties.Item("fileExtension").Value = ".exe";
addElement.Properties.Item("allowed").Value = false;
fileExtensionsCollection.AddElement(addElement);

var addElement1 = fileExtensionsCollection.CreateNewElement("add");
addElement1.Properties.Item("fileExtension").Value = ".com";
addElement1.Properties.Item("allowed").Value = false;
fileExtensionsCollection.AddElement(addElement1);

var addElement2 = fileExtensionsCollection.CreateNewElement("add");
addElement2.Properties.Item("fileExtension").Value = ".cmd";
addElement2.Properties.Item("allowed").Value = false;
fileExtensionsCollection.AddElement(addElement2);

var addElement3 = fileExtensionsCollection.CreateNewElement("add");
addElement3.Properties.Item("fileExtension").Value = ".bat";
addElement3.Properties.Item("allowed").Value = false;
fileExtensionsCollection.AddElement(addElement3);

adminManager.CommitChanges();

VBScript

Set adminManager = createObject("Microsoft.ApplicationHost.WritableAdminManager")
adminManager.CommitPath = "MACHINE/WEBROOT/APPHOST"
Set requestFilteringSection = adminManager.GetAdminSection("system.ftpServer/security/requestFiltering", "MACHINE/WEBROOT/APPHOST/Default Web Site")
Set fileExtensionsElement = requestFilteringSection.ChildElements.Item("fileExtensions")
Set fileExtensionsCollection = fileExtensionsElement.Collection

Set addElement = fileExtensionsCollection.CreateNewElement("add")
addElement.Properties.Item("fileExtension").Value = ".exe"
addElement.Properties.Item("allowed").Value = False
fileExtensionsCollection.AddElement(addElement)

Set addElement1 = fileExtensionsCollection.CreateNewElement("add")
addElement1.Properties.Item("fileExtension").Value = ".com"
addElement1.Properties.Item("allowed").Value = False
fileExtensionsCollection.AddElement(addElement1)

Set addElement2 = fileExtensionsCollection.CreateNewElement("add")
addElement2.Properties.Item("fileExtension").Value = ".cmd"
addElement2.Properties.Item("allowed").Value = False
fileExtensionsCollection.AddElement(addElement2)

Set addElement3 = fileExtensionsCollection.CreateNewElement("add")
addElement3.Properties.Item("fileExtension").Value = ".bat"
addElement3.Properties.Item("allowed").Value = False
fileExtensionsCollection.AddElement(addElement3)

adminManager.CommitChanges()

The following examples specify FTP request limits for a maximum content length of 1000000 bytes and maximum URL length of 1024 bytes.

AppCmd.exe

appcmd.exe set config "Default Web Site" -section:system.ftpServer/security/requestFiltering /requestLimits.maxAllowedContentLength:"1000000" /requestLimits.maxUrl:"1024" /commit:apphost

Note: You must be sure to set the commit parameter to apphost when you use AppCmd.exe to configure these settings. This commits the configuration settings to the appropriate location section in the ApplicationHost.config file.

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.GetApplicationHostConfiguration(); ConfigurationSection requestFilteringSection = config.GetSection("system.ftpServer/security/requestFiltering", "Default Web Site"); ConfigurationElement requestLimitsElement = requestFilteringSection.GetChildElement("requestLimits"); requestLimitsElement["maxAllowedContentLength"] = 1000000; requestLimitsElement["maxUrl"] = 1024; 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.GetApplicationHostConfiguration Dim requestFilteringSection As ConfigurationSection = config.GetSection("system.ftpServer/security/requestFiltering", "Default Web Site") Dim requestLimitsElement As ConfigurationElement = requestFilteringSection.GetChildElement("requestLimits") requestLimitsElement("maxAllowedContentLength") = 1000000 requestLimitsElement("maxUrl") = 1024 serverManager.CommitChanges() End Sub End Module

JavaScript

var adminManager = new ActiveXObject('Microsoft.ApplicationHost.WritableAdminManager');
adminManager.CommitPath = "MACHINE/WEBROOT/APPHOST";

var requestFilteringSection = adminManager.GetAdminSection("system.ftpServer/security/requestFiltering", "MACHINE/WEBROOT/APPHOST/Default Web Site");
var requestLimitsElement = requestFilteringSection.ChildElements.Item("requestLimits");
requestLimitsElement.Properties.Item("maxAllowedContentLength").Value = 1000000;
requestLimitsElement.Properties.Item("maxUrl").Value = 1024;

adminManager.CommitChanges();

VBScript

Set adminManager = createObject("Microsoft.ApplicationHost.WritableAdminManager")
adminManager.CommitPath = "MACHINE/WEBROOT/APPHOST"

Set requestFilteringSection = adminManager.GetAdminSection("system.ftpServer/security/requestFiltering", "MACHINE/WEBROOT/APPHOST/Default Web Site")
Set requestLimitsElement = requestFilteringSection.ChildElements.Item("requestLimits")
requestLimitsElement.Properties.Item("maxAllowedContentLength").Value = 1000000
requestLimitsElement.Properties.Item("maxUrl").Value = 1024

adminManager.CommitChanges()
Deprecated Elements