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Manual Chapter: Setting Up Cross-Domain Request Enforcement
Manual Chapter
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About cross-domain request enforcement

Cross-Origin Resource Sharing (CORS) is an HTML5 feature that enables one website to access the resources of another website using JavaScript within the browser. On occasion, your web application might need to share resources with another external website that is hosted on a different domain. Using Application Security Manager™, you can safely allow CORS by specifying the conditions that state when a foreign web application is allowed to access your web application, after making a cross-domain request. This feature is called cross-domain request enforcement.

You enable cross-domain request enforcement as part of the Allowed HTTP or WebSocket URL properties within a security policy. Then you can specify which domains can access the response generated by requesting this URL (the “resource”). For HTTP URLs, you can also configure how to overwrite CORS response headers that are returned by the web server.

This feature does not affect internal redirection, which is always allowed. For example, Location: /anotherpage/onthisserver/internal_redirect.php would be allowed even if cross-domain request enforcement is enabled on the system.

Setting up cross-domain request enforcement

For this task, the security policy needs to have an allowed HTTP or WebSocket URL.
If you want to allow your application website to access the resources of another website, you can add cross-domain request enforcement to an existing HTTP or WebSocket URL. This procedure shows how to enable Cross-Origin Resource Sharing (CORS) support on your application server for either type of URL.
  1. On the Main tab, click Security > Application Security > URLs .
    The Allowed HTTP URLs screen opens.
  2. In the Current edited security policy list near the top of the screen, verify that the security policy shown is the one you want to work on.
  3. Locate the HTTP or WebSocket URL that needs CORs support:
    1. From the Allowed URLs menu, choose either Allowed HTTP URLs or Allowed WebSocket URLs.
    2. In either the Allowed URLs List or the Allowed WebSocket URLs List, click the URL you want to modify.
    The Allowed HTTP URL Properties screen or WebSocket URL Properties screen for the URL opens.
  4. From either URL Properties list, select Advanced.
  5. Click the HTML5 Cross-Domain Request Enforcement tab.
  6. For Enforcement Mode, specify the option to determine how to handle CORS requests.
    Select this option To do this
    Disabled Do nothing related to cross-domain requests. Pass CORS requests exactly as set by the server.
    Remove all CORS headers Remove all CORS headers from the response. The response is sent to the browser, and the browser does not allow cross-origin requests.
    Replace CORS headers (HTTP URLs only) Replace the CORS header in the response with another header specified on the tab, including allowed origins, allowed methods, allowed headers, and so on. The browser enforces the policy. Then after Replace with specify the protocol, origin, and port for replacing CORS headers.
    Enforce on ASM Allow cross-origin resource sharing as configured in the Allowed Origins setting. CORS requests are allowed from the domains specified as allowed origins. ASM enforces the policy. Specify the protocol, origin, and port of allowed origins
    For maximum security, F5 recommends that you select Enforce on ASM.
    The tab now includes additional settings determined by the option you selected.
  7. For the Allowed Origins setting, add the origins that are allowed to share data returned by this URL.
    1. For Protocol, select the appropriate protocol for the allowed origin.
    2. For Origin Name, type the domain name or IP address with which the URL can share data.
      Wildcards are allowed in the names. For example: *.f5.com will match b.f5.com; however it will not match a.b.f5.com.
    3. For Port, select the port that other web applications can use to request data from your web application, or use the * wildcard for all ports.
    4. If you want to allow sub-domains to receive data, select the Include Sub-Domains check box.
    5. Click Add to add the origins.
      The origins that can share data with the URL are included in the list.
  8. Click Update.
  9. To put the security policy changes into effect immediately, click Apply Policy.
The security policy allows requests for the HTTP or WebSocket URL to access the resources of other websites hosted in a different domain according to the enforcement conditions that you configured.

ASM extracts the Origin (domain) of the request from the Origin header. If the Origin header is missing or has more than one occurrence, ASM issues an Illegal cross-origin request violation if it is set to alarm or block. If the violation is set to block in the URL section of the Learning and Blocking Settings (and the Enforcement Mode of the security policy is set to blocking), the system blocks the request.

If a request comes from a domain that does not belong to the application and is not specified in the list of allowed origins, the system also issues an Illegal cross-origin request violation. If the violation is set to block (and the Enforcement Mode is set to blocking), the request is blocked.

Replacing CORS headers in requests

For this task, the security policy needs to have an allowed HTTP URL. Also, the OPTIONS method must be on the Allowed Methods list.

CORS headers are enforced by all popular browsers. The browser reads the allowed origins from the Access-Control-Allowed-Origin headers in the response. If the subsequent request from that page does not match any of the allowed origins, the browser will not place the request. In many situations, the servers do not populate those headers properly, so you can have ASM™ replace the CORS headers.

If you want ASM to replace CORS headers when enforcing HTML5 cross-domain requests, you can update an existing HTTP URL. This task does not apply to WebSocket URLs, only HTTP URLs.
  1. On the Main tab, click Security > Application Security > URLs .
    The Allowed HTTP URLs screen opens.
  2. In the Current edited security policy list near the top of the screen, verify that the security policy shown is the one you want to work on.
  3. From the Allowed HTTP URLs List, click the name of the URL you want to modify.
    The Allowed HTTP URL Properties screen opens.
  4. From the Allowed URL Properties list, select Advanced.
  5. On the HTML5 Cross-Domain Request Enforcement tab, for Enforcement Mode, select Replace CORS headers.
    The tab now includes additional settings where you define how to overwrite CORS response headers returned by the web server.
  6. In the Allowed Origins setting, add the origins that are allowed to share data returned by this URL.
    Select Replace with, then specify the origin names:
    1. For Protocol, select the appropriate protocol for the allowed origin.
    2. For Origin Name, type the domain name or IP address that you want to allow to share your data with.
      Wildcards are allowed in the names. For example: *.f5.com will match b.f5.com, but it will not match a.b.f5.com.
    3. For Port, select the port that other web applications can use to request data from your web application, or use the * wildcard for all ports.
    4. If you want to allow sub-domains to receive data, select the Include Sub-Domains check box.
    5. Click Add to add the origins.
      The origins that can share data with the URL are included in the list.
  7. Optionally, for Allowed Methods, specify which methods other applications may use when requesting this URL from another domain. Select Replace with, then move the methods to allow from the Available Methods to the Allowed Methods list.
    Important: Any method you allow here must also be in the Allowed Methods list in the security policy ( Security > Application Security > Headers > Methods ).
  8. Optionally, for Allowed Headers, select Replace with, then type the headers that other applications can use when requesting this URL from another domain.
    Allowed headers are request headers sent by clients. For example, to allow clients to send Ajax requests, type X-Requested-With, and to allow XML requests, type Content-Type.
  9. Optionally, for Exposed Headers, select Replace with, then specify the headers that JavaScript can expose and share with other applications when requesting this URL from another domain.
    Exposed headers are the headers the server returns in the response. For example, to discover server side web application technology, type X-Powered-By.
  10. Optionally, for Allow Credentials, select Replace with, then specify whether requests from applications in another domain can include user credentials.
  11. Optionally, for Maximum Age, select Replace with, then specify the number of seconds that the results of a preflight request can be cached or use the default.
  12. Click Update.
  13. To add methods, such as OPTIONS, required to replace headers:
    1. Click Security > Application Security > Headers > Methods .
    2. Click Create.
    3. In the Method setting, select OPTIONS.
    4. Click Create.
  14. To put the security policy changes into effect immediately, click Apply Policy.
The security policy passes the CORS request to the application server. ASM replaces the header of the response with the header you specified, and returns the response.

If this request is authorized by the web server, the browser allows the foreign domain to send its original request. If the request from that page does not match any of the allowed origins, the browser declines the request.

How cross-domain request enforcement works

If you enable cross-domain request enforcement, the system must authorize requests (typically AJAX requests) made from one domain to another. When a client makes a request to another origin, the browser sends a preflight request to determine whether JavaScript from another domain may access your resource.

When processing a modification request, the browser sends a preflight request if it has no previously cached allowed origins (that is, this is the first time the browser goes to the foreign domain for such requests). The preflight request uses an OPTIONS HTTP method and CORS-related headers to check whether the server authorizes that origin.

The CORS-related headers that are included in a preflight request are:

Header Description
Origin Determines requesting origin.
Access-Control-Request-Method Indicates which methods are used in the actual request (other than simple methods).
Access-Control-Request-Headers Indicates which headers are used in the actual request (other than simple headers).

In response to the preflight request, the system uses these CORS response headers:

Header Description
Access-Control-Allow-Origin List of origins the resource may be shared among (support wildcard).
Access-Control-Allow-Credentials Indicates whether actual request may include user credentials (true/false).
Access-Control-Allow-Methods Indicates which methods can be used during the actual request.
Access-Control-Allow-Headers Indicates which request headers can be used during the actual request.
Access-Control-Max-Age Indicates how long (in seconds) to cache the results of a preflight request in the browser.
Access-Control-Expose-Headers Indicates which response headers are safe to expose to JavaScript.

The browser uses the response to determine whether to allow the JavaScript to make the actual request. If the cross-domain request is authorized, the server processes the actual requests by rechecking the origin and including another response header:

Header Description
Access-Control-Expose-Headers Indicates which response headers are safe to expose to JavaScript.

The browser then allows the foreign domain to send its original requests.

If you do not enable cross-domain request enforcement, the system removes all cross-origin request headers and CORS is not allowed for the URL.

If you select Enforce on ASM as the CORS Enforcement Mode, ASM™ permits access according to the allowed origins. So, when using this option, there is no need for a preflight request because ASM itself checks the origin. Unlike using the Replace CORS headers setting, ASM, not the browser, does the enforcement.

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