# HTTP Header Validation

## Contents
Overview Configuring HTTP Header Regular Expressions Configuring Threatening Content Regular Expressions

## Overview
The Enterprise Gateway can check HTTP header values for threatening content. This ensures that only properly configured name-value pairs appear in the HTTP request headers. Regular expressions are used to test HTTP header values. This enables you to make decisions on what to do with the message (for example, if the HTTP header value is `X`, route to service `X`). You can configure the following sections on the Validate HTTP Headers screen: Enter Regular Expression: HTTP header values can be checked using regular expressions. You can select regular expressions from the global White list or enter them manually. For example, if you know that an HTTP header must have a value of `ABCD`, a regular expression of `^ABCD$` is an exact match test. Enter Threatening Content Regular Expression: You can select threatening content regular expressions from the global Black list to run against all HTTP headers in the message. These regular expressions identify common attack signatures (for example, SQL injection attacks). You can configure the global White list and Black list libraries of regular expressions on the Policies tab in the Policy Studio.
## Configuring HTTP Header Regular Expressions
The Enter Regular Expression table displays the list of configured HTTP header names together with the White list of regular expressions that restrict their values. For this filter to run successfully, all required headers must be present in the request, and all must have values matching the configured regular expressions. The Name column shows the name of the HTTP header. The Regular Expression column shows the name of the regular expression that the Enterprise Gateway uses to restrict the value of the named HTTP header. A number of common regular expressions are available from the global White list library. Configuring a Regular Expression You can configure regular expressions by selecting the Add, Edit, and Delete buttons. The Configure Regular Expression dialog enables you to add or edit regular expressions to restrict the values of HTTP headers. To configure a regular expression, perform the following steps: Enter the name of the HTTP header in the Name field. Select whether this header is Optional or Required using the appropriate radio button. If it is Required, the header must be present in the request. If the header is not present, the filter fails. If it is Optional, the header does not need to be present for the filter to pass. You can enter the regular expression to restrict the value of the HTTP header manually or select it from the global White list library of regular expressions in the Expression Name drop-down list. A number of common regular expressions are provided (for example, alphanumeric values, dates, and email addresses). You can use properties representing the values of message attributes to compare the value of an HTTP header with the value contained in a message attribute. Enter the `$` character in the Regular Expression field to view a list of available attributes. At runtime, the property is expanded to the corresponding attribute value, and compared to the HTTP header value that you want to check. You can add a regular expression to the library by selecting the Add/Edit button. Enter a Name for the expression followed by the Regular Expression. Advanced Settings The Advanced section enables you to extract a portion of the header value which is run against the regular expression. The extracted substring can be Base64 decoded if necessary. This section is specifically aimed towards HTTP Basic authentication headers, which consist of the `Basic` prefix (with a trailing space), followed by the Base64-encoded username and password. The following is an example of the HTTP Basic authentication header:
    Authorization: Basic dXNlcjp1c2Vy
The Base64-encoded portion of the header value is what you are interested in running the regular expression against. You can extract this by specifying the string that occurs directly before the substring you want to extract, together with the string that occurs directly after the substring. To extract the Base64-encoded section of the `Authorization` header above, enter `Basic` (with a trailing space) in the Start substring field, and leave the End substring field blank to extract the entire remainder of the header value. Important Note: You must select the start and end substrings to ensure that the exact substring is extracted. For example, in the HTTP Basic example above, you should enter `Basic` (with a trailing space) in the Start substring field, and not `Basic` (with no trailing space). By specifying the correct substrings, you are left with the Base64-encoded header value (`dXNlcjp1c2Vy`). However, you still need to Base64 decode it before you can run a regular expression on it. Make sure to select the Base64 decode checkbox. The Base64-decoded header value is `user:user`, which conforms to the standard format of the `Authorization` HTTP header. This is the value that you need to run the regular expression against. The following example shows an example of an HTTP Digest authentication header:
    Authorization: Digest username="user", realm="oracle.com", qop="auth",
    algorithm="MD5", uri="/editor", nonce="Id-00000109924ff10b-0000000000000091",
    nc="1", cnonce="ae122a8b549af2f0915de868abff55bacd7757ca",
    response="29224d8f870a62ce4acc48033c9f6863"
You can extract single values from the header value. For example, to extract the `realm` field, enter `realm="` (including the `"` character), in the Start substring field and `"` in the End substring field. This leaves you with `oracle.com` to run the regular expression against. In this case, there is no need to Base64 decode the extracted substring. Note: If both Start substring and End substring fields are blank, the regular expression is run against the entire header value. Furthermore, if both fields are blank and the Base64 decode checkbox is selected, the entire header value is Base64 encoded before the regular expression is run against it. While the above examples deal specifically with the HTTP authentication headers, the interface is generic enough to enable you to extract a substring from other header values.
## Configuring Threatening Content Regular Expressions
The regular expressions entered in this section guard against the possibility of an HTTP header containing malicious content. The Enter Threatening Content Regular Expression table lists the Black list of regular expressions to run to ensure that the header values do not contain threatening content. For example, to guard against an SQL `DELETE` attack, you can write a regular expression to identify SQL syntax and add it to this list. The Threatening Content Regular Expressions are listed in a table. All of these expressions are run against all HTTP header values in an incoming request. If the expression matches any of the values, the filter fails. Important Note: If any regular expressions are configured in the Configuring HTTP Header Regular Expressions section, these expressions are run before Threatening Content Regular Expressions (TCRE) are run. For example, if you already configured a regular expression to extract the Base64-decoded value of the `Authentication` header value in the example above, the TCRE is run against this value instead of the attribute value that appears in the HTTP header. You can add threatening content regular expressions using the Add button. You can edit or remove existing expressions by selecting them in the drop-down list, and clicking the Edit or Delete button. You can enter the regular expressions manually or select them from the global Black list library of threatening content regular expressions. This library is pre-populated with a number of regular expressions that scan for common attack signatures. These include expressions to guard against common SQL injection-style attacks (for example, SQL `INSERT`, SQL `DELETE`, and so on), buffer overflow attacks (content longer than 1024 characters), and the presence of control characters in attribute values (ASCII control characters). Enter or select an appropriate regular expression to restrict the value of the specified HTTP header. You can add a regular expression to the library by selecting the Add/Edit button. Enter a Name for the expression followed by the Regular Expression.