Working with cross-site scripting match conditions

Note

This is AWS WAF Classic documentation. You should only use this version if you created AWS WAF resources, like rules and web ACLs, in AWS WAF prior to November 2019, and you have not migrated them over to the latest version yet. To migrate your resources, see Migrating your AWS WAF Classic resources to AWS WAF.

For the latest version of AWS WAF, see AWS WAF.

Attackers sometimes insert scripts into web requests in an effort to exploit vulnerabilities in web applications. You can create one or more cross-site scripting match conditions to identify the parts of web requests, such as the URI or the query string, that you want AWS WAF Classic to inspect for possible malicious scripts. Later in the process, when you create a web ACL, you specify whether to allow or block requests that appear to contain malicious scripts.

Creating cross-site scripting match conditions

When you create cross-site scripting match conditions, you specify filters. The filters indicate the part of web requests that you want AWS WAF Classic to inspect for malicious scripts, such as the URI or the query string. You can add more than one filter to a cross-site scripting match condition, or you can create a separate condition for each filter. Here's how each configuration affects AWS WAF Classic behavior:

• More than one filter per cross-site scripting match condition (recommended) – When you add a cross-site scripting match condition that contains multiple filters to a rule and add the rule to a web ACL, a web request must match only one of the filters in the cross-site scripting match condition for AWS WAF Classic to allow or block the request based on that condition.

  For example, suppose you create one cross-site scripting match condition, and the condition contains two filters. One filter instructs AWS WAF Classic to inspect the URI for malicious scripts, and the other instructs AWS WAF Classic to inspect the query string. AWS WAF Classic allows or blocks requests if they appear to contain malicious scripts either in the URI or in the query string.

• One filter per cross-site scripting match condition – When you add the separate cross-site scripting match conditions to a rule and add the rule to a web ACL, web requests must match all the conditions for AWS WAF Classic to allow or block requests based on the conditions.

  Suppose you create two conditions, and each condition contains one of the two filters in the preceding example. When you add both conditions to the same rule and add the rule to a web ACL, AWS WAF Classic allows or blocks requests only when both the URI and the query string appear to contain malicious scripts.

Note

When you add a cross-site scripting match condition to a rule, you also can configure AWS WAF Classic to allow or block web requests that do not appear to contain malicious scripts.

To create a cross-site scripting match condition

1. Sign in to the AWS Management Console and open the AWS WAF console at https://console.aws.amazon.com/wafv2/.

   If you see Switch to AWS WAF Classic in the navigation pane, select it.

2. In the navigation pane, choose Cross-site scripting.

3. Choose Create condition.

4. Specify the applicable filter settings. For more information, see Values that you specify when you create or edit cross-site scripting match conditions.

5. Choose Add another filter.

6. If you want to add another filter, repeat steps 4 and 5.

7. When you're done adding filters, choose Create.

Values that you specify when you create or edit cross-site scripting match conditions

When you create or update a cross-site scripting match condition, you specify the following values:

Name

The name of the cross-site scripting match condition.

The name can contain only the characters A-Z, a-z, 0-9, and the special characters: _-!"#`+*},./ . You can't change the name of a condition after you create it.

Part of the request to filter on

Choose the part of each web request that you want AWS WAF Classic to inspect for malicious scripts:

Header

A specified request header, for example, the User-Agent or Referer header. If you choose Header, specify the name of the header in the Header field.

HTTP method

The HTTP method, which indicates the type of operation that the request is asking the origin to perform. CloudFront supports the following methods: DELETE, GET, HEAD, OPTIONS, PATCH, POST, and PUT.

Query string

The part of a URL that appears after a ? character, if any.

Note

For cross-site scripting match conditions, we recommend that you choose All query parameters (values only) instead of Query string for Part of the request to filter on.

URI

The URI path of the request, which identifies the resource, for example, /images/daily-ad.jpg. This doesn't include the query string or fragment components of the URI. For information, see Uniform Resource Identifier (URI): Generic Syntax.

Unless a Transformation is specified, a URI is not normalized and is inspected just as AWS receives it from the client as part of the request. A Transformation will reformat the URI as specified.

Body

The part of a request that contains any additional data that you want to send to your web server as the HTTP request body, such as data from a form.

Note

If you choose Body for the value of Part of the request to filter on, AWS WAF Classic inspects only the first 8192 bytes (8 KB). To allow or block requests for which the body is longer than 8192 bytes, you can create a size constraint condition. (AWS WAF Classic gets the length of the body from the request headers.) For more information, see Working with size constraint conditions.

Single query parameter (value only)

Any parameter that you have defined as part of the query string. For example, if the URL is "www.xyz.com?UserName=abc&SalesRegion=seattle" you can add a filter to either the UserName or SalesRegion parameter.

If you choose Single query parameter (value only), you will also specify a Query parameter name. This is the parameter in the query string that you will inspect, such as UserName or SalesRegion. The maximum length for Query parameter name is 30 characters. Query parameter name is not case sensitive. For example, it you specify UserName as the Query parameter name, this will match all variations of UserName, such as username and UsERName.

All query parameters (values only)

Similar to Single query parameter (value only), but rather than inspecting the values of a single parameter, AWS WAF Classic inspects all parameter values within the query string for possible malicious scripts. For example, if the URL is "www.xyz.com?UserName=abc&SalesRegion=seattle," and you choose All query parameters (values only), AWS WAF Classic will trigger a match if either the value of UserName or SalesRegion contain possible malicious scripts.

Header

If you chose Header for Part of the request to filter on, choose a header from the list of common headers, or enter the name of a header that you want AWS WAF Classic to inspect for malicious scripts.

Transformation

A transformation reformats a web request before AWS WAF Classic inspects the request. This eliminates some of the unusual formatting that attackers use in web requests in an effort to bypass AWS WAF Classic.

You can only specify a single type of text transformation.

Transformations can perform the following operations:

None

AWS WAF Classic doesn't perform any text transformations on the web request before inspecting it for the string in Value to match.

Convert to lowercase

AWS WAF Classic converts uppercase letters (A-Z) to lowercase (a-z).

HTML decode

AWS WAF Classic replaces HTML-encoded characters with unencoded characters:

• Replaces " with &

• Replaces   with a non-breaking space

• Replaces < with <

• Replaces > with >

• Replaces characters that are represented in hexadecimal format, &#xhhhh;, with the corresponding characters

• Replaces characters that are represented in decimal format, &#nnnn;, with the corresponding characters

Normalize white space

AWS WAF Classic replaces the following characters with a space character (decimal 32):

• \f, formfeed, decimal 12

• \t, tab, decimal 9

• \n, newline, decimal 10

• \r, carriage return, decimal 13

• \v, vertical tab, decimal 11

• non-breaking space, decimal 160

In addition, this option replaces multiple spaces with one space.

Simplify command line

For requests that contain operating system command line commands, use this option to perform the following transformations:

• Delete the following characters: \ " ' ^

• Delete spaces before the following characters: / (

• Replace the following characters with a space: , ;

• Replace multiple spaces with one space

• Convert uppercase letters (A-Z) to lowercase (a-z)

URL decode

Decode a URL-encoded request.

Adding and deleting filters in a cross-site scripting match condition

You can add or delete filters in a cross-site scripting match condition. To change a filter, add a new one and delete the old one.

To add or delete filters in a cross-site scripting match condition

1. Sign in to the AWS Management Console and open the AWS WAF console at https://console.aws.amazon.com/wafv2/.

   If you see Switch to AWS WAF Classic in the navigation pane, select it.

2. In the navigation pane, choose Cross-site scripting.

3. Choose the condition that you want to add or delete filters in.

4. To add filters, perform the following steps:

   1. Choose Add filter.

   2. Specify the applicable filter settings. For more information, see Values that you specify when you create or edit cross-site scripting match conditions.

   3. Choose Add.

5. To delete filters, perform the following steps:

   1. Select the filter that you want to delete.

   2. Choose Delete filter.

Deleting cross-site scripting match conditions

If you want to delete a cross-site scripting match condition, you must first delete all filters in the condition and remove the condition from all the rules that are using it, as described in the following procedure.

To delete a cross-site scripting match condition

1. Sign in to the AWS Management Console and open the AWS WAF console at https://console.aws.amazon.com/wafv2/.

   If you see Switch to AWS WAF Classic in the navigation pane, select it.

2. In the navigation pane, choose Cross-site scripting.

3. In the Cross-site scripting match conditions pane, choose the cross-site scripting match condition that you want to delete.

4. In the right pane, choose the Associated rules tab.

   If the list of rules using this cross-site scripting match condition is empty, go to step 6. If the list contains any rules, make note of the rules, and continue with step 5.

5. To remove the cross-site scripting match condition from the rules that are using it, perform the following steps:

   1. In the navigation pane, choose Rules.

   2. Choose the name of a rule that is using the cross-site scripting match condition that you want to delete.

   3. In the right pane, select the cross-site scripting match condition that you want to remove from the rule, and choose Remove selected condition.

   4. Repeat steps b and c for all the remaining rules that are using the cross-site scripting match condition that you want to delete.

   5. In the navigation pane, choose Cross-site scripting.

   6. In the Cross-site scripting match conditions pane, choose the cross-site scripting match condition that you want to delete.

6. Choose Delete to delete the selected condition.