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Configure Request Filtering in IIS

Published: February 29, 2012

Updated: February 29, 2012

Applies To: Windows Server 2012, Windows Server 2012 R2



This document shows you how to use common request-filter settings to improve the security of your IIS 8 web server.

Request filters restrict the types of HTTP requests that IIS 8 processes. By blocking specific HTTP requests, request filters help prevent potentially harmful requests from reaching the server. The request filter module scans incoming requests and rejects requests that are unwanted based upon the rules that you set up.

By default, IIS rejects requests to browse critical code segments. It also rejects requests for some file name extensions.

You can configure a request filter at the server-wide level and then override the configuration at a website level.

This guide was written for, and tested on, the following operating systems:

  • Windows Server® 2012

  • Windows® 8

The general settings include such settings as the following:

  1. Whether to allow access to a file with an extension that is not listed for request filter.

  2. Whether to allow requests that use HTTP verbs that are not listed.

  3. Whether to allow requests that contain high-bit characters (non-ASCII).

  4. Whether to allow requests that are double encoded.

  5. Maximum length of the content requested.

  6. Maximum length of the URL

  7. Maximum size of a query string

  1. Open IIS Manager and select the level for which you want to configure request filter.

  2. In Features View, double-click Request Filtering.

  3. In the Actions pane, click Edit Feature Settings.

  4. In the Edit Request Filtering Settings dialog, edit the settings as desired, and then click OK.

  • To configure high-bit characters, use the following syntax:

    appcmd set config /section:requestfiltering /allowhighbitcharacters:true | false

    For example, to allow high-bit characters, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /allowhighbitcharacters:true

  • To configure double escaping, use the following syntax:

    appcmd set config /section:requestfiltering /allowdoubleescaping:true | false

    For example, to enable double escaping, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /allowdoubleescaping:true

  • To configure a value for the maximum allowed length of content, use the following syntax:

    appcmd set config /section:requestfiltering /requestlimits.maxallowedcontentlength: unit

    The variable requestlimits.maxallowedcontentlength unit specifies the maximum length of content.

    For example, to specify 30000000 as the maximum length of content, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /requestlimits.maxallowedcontentlength:30000000

  • To configure a value for the maximum allowed length of an incoming URL, use the following syntax:

    appcmd set config /section:requestfiltering /requestlimits.maxurl: unit

    The variable requestlimits.maxurl unit specifies the maximum length of an incoming URL.

    For example, to specify 4096 as the maximum incoming URL length, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /requestlimits.maxurl:4096

  • To configure a value for the maximum allowed length of an incoming query string, use the following syntax:

    appcmd set config /section:requestfiltering /requestlimits.maxquerystring: unit

    The variable requestlimits.maxquertystring unit specifies the maximum length of an incoming query string.

    For example, to specify 2048 as the maximum incoming query string, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /requestlimits.maxquerystring:2048

  • To configure a size limit for a specific HTTP header, use the following syntax:

    appcmd set config /section:requestfiltering /+requestlimits.headerLimits.[header=' string ',sizelimit=' unit ']

    The variable header string specifies the header this restriction applies to. The variable sizelimit unit specifies the maximum size of this header.

    For example, to specify a maximum size of 2048 for headers that include a value of contoso.com, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /+requestlimits.headerLimits.[header='contoso.com',sizelimit='2048']

For each file name extension you add, you can indicate whether to allow or reject requests for that type of file.

  1. Open IIS Manager and select the level for which you want to configure request filter.

  2. In Features View, double-click Request Filtering.

  3. Select the File Name Extensions tab.

  4. In the Actions pane, click either Allow File Name Extension or Deny File Name Extension.

  5. Type the file name extension in the box, and then click OK.

  • To configure how IIS deals with unlisted file name extensions, use the following syntax:

    appcmd set config /section:requestfiltering /fileExtensions.allowunlisted:true | false

    For example, to deny unlisted file name extensions, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /fileExtensions.allowunlisted:false

  • To configure whether file name extensions apply to WebDAV requests, use the following syntax:

    appcmd set config /section:requestfiltering /fileExtensions.applyToWebDAV:true | false

    For example, to configure IIS so that file name extensions do not apply to WebDAV requests, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /fileExtensions.applyToWebDAV:false

  • To add a file name extension, use the following syntax:

    appcmd set config /section:requestfiltering /+fileExtensions.[fileextension=' .string ',allowed='true | false']

    The variable fileextension string is the file name extension you want to allow or deny.

    For example, to add an allow rule for the file name extension .xxx, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /+fileExtensions.[fileextension=' .xxx ',allowed='true']

  • To remove a rule for the file name extension .xxx, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /-fileExtensions.[fileextension=' .xxx ']

IIS 8 permits you to define custom filter rules that apply to incoming requests. Using this feature, you can define filters that can do the following:

  1. Scan the request URL.

  2. Scan for query strings contained in the URL.

  3. Scan for specific header fields.

  4. Define what file name extensions the filter applies to.

  5. Define strings you want to deny.

  1. Open IIS Manager and select the level for which you want to configure request filter.

  2. In Features View, double-click Request Filtering.

  3. Select the Rules tab.

  4. In the Actions pane, double-click Add Filtering Rule.

  5. In the Name box, type a name for the filtering rule.

  6. If you want the URL scanned, select the Scan Url check box.

  7. If you want the query string scanned, select the Scan query string check box.

  8. Under Scan Headers, type one or more headers to scan for.

  9. Under Applies To, type one or more file name extensions that the rule applies to.

  10. Under the Deny Strings, type one or more strings you want to deny.

  11. Click OK.

This feature allows you to reject requests that contain a URL segment (for example, a folder name).

  1. Open IIS Manager and select the level for which you want to configure request filter.

  2. In Features View, double-click Request Filtering.

  3. Select the Hidden Segments tab.

  4. In the Actions pane, click Add Hidden Segment.

  5. Type the URL segment in the box, and then click OK.

  • To configure whether hidden segments apply to WebDAV requests, use the following syntax:

    appcmd set config /section:requestfiltering /hiddensegments.applyToWebDAV:true | false

    For example, to configure IIS so that hidden segments do not apply to WebDAV requests, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /hiddensegments.applyToWebDAV:false

  • To configure a hidden segment, use the following syntax:

    appcmd set config /section:requestfiltering /+hiddensegments.[segment=' string ']

    The variable segment string specifies a URL segment that is hidden.

    For example, to specify that /bin is a hidden segment, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /+hiddensegments.[segment='/bin']

You can configure IIS to accept a specified URL. In addition, you can configure it to deny a specified URL sequence.

  1. Open IIS Manager and select the level for which you want to configure request filter.

  2. In Features View, double-click Request Filtering.

  3. Select the URL tab.

  4. In the Actions pane, select either Allow URL or Deny Sequence.

  5. Type the URL or the URL sequence in the box, and click OK.

  • To deny a URL sequence, use the following syntax:

    appcmd set config /section:requestfiltering /+denyurlsequences.[sequence=' string ']

    The variable sequence string specifies a sequence of characters in a URL that IIS is never allowed to parse.

    For example, to specify that IIS never parse URLs that contain two periods, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /+denyurlsequences.[sequence='..']

You can define a list of verbs that IIS 8 accepts as part of a request. When IIS rejects a request based on this feature, the error code logged is 404.6.

  1. Open IIS Manager and select the level for which you want to configure request filter.

  2. In Features View, double-click Request Filtering.

  3. Select the HTTP Verbs tab.

  4. In the Actions pane, click either Allow Verb or Deny Verb.

  5. Enter the verb in the box, and then click OK.

  • To configure how IIS deals with unlisted verbs, use the following syntax:

    appcmd set config /section:requestfiltering /verbs.allowunlisted:true | false

    For example, to deny unlisted verbs, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /verbs.allowunlisted:false

  • To configure whether verb filtering applies to WebDAV requests, use the following syntax:

    appcmd set config /section:requestfiltering /verbs.applyToWebDAV:true | false

    For example, to configure IIS so that verb filtering does not apply to WebDAV requests, type the following at the command prompt and then press ENTER:

    appcmd set config /section:requestfiltering /verbs.applyToWebDAV:false

  • To configure a verb to filter, use the following syntax:

    appcmd set config /section:requestfiltering /+verbs.[verb=' string ',allowed='true | false']

    The variable verb string specifies the verb this restriction applies to.

    For example, to specify GET is allowed, type the following at the command prompt, and then press ENTER:

    appcmd set config /section:requestfiltering /+verbs.[verb='GET',allowed='true']

You can limit the size of HTTP request headers to improve performance and security. Headers are name/value pairs that define the operating parameters of an HTTP transaction.

  1. Open IIS Manager and select the level for which you want to configure request filter.

  2. In Features View, double-click Request Filtering.

  3. Select the Headers tab, and click Add Header.

  4. In the Header box, type the header field name.

  5. In the Size Limit box, type a positive integer that represents the header size limit in bytes.

  6. Click OK.

You can configure IIS 8 to allow or deny specific query strings contained in the requested URL. For example, if a denied query string is found in a request URL, the request is denied.

  1. Open IIS Manager and select the level you want to configure request filter for.

  2. In Features View, double-click Request Filtering.

  3. Select the Query Strings tab, and click either Allow Query String or Deny Query String.

  4. In the Query string box, type the query string.

  5. Click OK.

You can use IIS logging to evaluate and optimize your request filter configuration.

The following table shows the request filter error codes that you see in the log:

 

Error Description Status Code

Request Filtering: URL Sequence denied

404.5

Request Filtering: Verb denied

404.6

Request Filtering: File name extension denied

404.7

Request Filtering: Denied by hidden segment

404.8

Request Filtering: Denied because request header is too long.

404.10

Request Filtering: Denied because URL doubled escaping

404.11

Request Filtering: Denied because of high bit characters

404.12

Request Filtering: Denied because content length too large

404.13

Request Filtering: Denied because URL too long

404.14

Request Filtering: Denied because query string too long

404.15

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