Manual Chapter :
Setting Up Cross-Domain Request Enforcement
Applies To:
Show VersionsBIG-IP ASM
- 17.1.2, 17.1.1, 17.1.0, 17.0.0
Setting Up Cross-Domain Request Enforcement
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.
- On the Main tab, click.The Allowed HTTP URLs screen opens.
- In theCurrent edited security policylist near the top of the screen, verify that the security policy shown is the one you want to work on.
- Locate the HTTP or WebSocket URL that needs CORs support:
- From the Allowed URLs menu, choose either Allowed HTTP URLs or Allowed WebSocket URLs.
- 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. - From eitherURL Propertieslist, selectAdvanced.
- Click the HTML5 Cross-Domain Request Enforcement tab.
- ForEnforcement Mode, specify the option to determine how to handle CORS requests.Select this optionTo do thisDisabledDo nothing related to cross-domain requests. Pass CORS requests exactly as set by the server.Remove all CORS headersRemove 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 afterReplace withspecify the protocol, origin, and port for replacing CORS headers.Enforce on ASMAllow cross-origin resource sharing as configured in theAllowed Originssetting. CORS requests are allowed from the domains specified as allowed origins. ASM enforces the policy. Specify the protocol, origin, and port of allowed originsFor maximum security, F5 recommends that you selectEnforce on ASM.The tab now includes additional settings determined by the option you selected.
- For theAllowed Originssetting, add the origins that are allowed to share data returned by this URL.
- ForProtocol, select the appropriate protocol for the allowed origin.
- ForOrigin Name, type the domain name or IP address with which the URL can share data.Wildcards are allowed in the names. For example:*.f5.comwill matchb.f5.com; however it will not matcha.b.f5.com.
- ForPort, select the port that other web applications can use to request data from your web application, or use the * wildcard for all ports.
- If you want to allow sub-domains to receive data, select theInclude Sub-Domainscheck box.
- ClickAddto add the origins.The origins that can share data with the URL are included in the list.
- ClickUpdate.
- To put the security policy changes into effect immediately, clickApply 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.
- On the Main tab, click.The Allowed HTTP URLs screen opens.
- In theCurrent edited security policylist near the top of the screen, verify that the security policy shown is the one you want to work on.
- From the Allowed HTTP URLs List, click the name of the URL you want to modify.The Allowed HTTP URL Properties screen opens.
- From theAllowed URL Propertieslist, selectAdvanced.
- On the HTML5 Cross-Domain Request Enforcement tab, forEnforcement Mode, selectReplace CORS headers.The tab now includes additional settings where you define how to overwrite CORS response headers returned by the web server.
- In theAllowed Originssetting, add the origins that are allowed to share data returned by this URL.SelectReplace with, then specify the origin names:
- ForProtocol, select the appropriate protocol for the allowed origin.
- ForOrigin 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.comwill matchb.f5.com, but it will not matcha.b.f5.com.
- ForPort, select the port that other web applications can use to request data from your web application, or use the * wildcard for all ports.
- If you want to allow sub-domains to receive data, select theInclude Sub-Domainscheck box.
- ClickAddto add the origins.The origins that can share data with the URL are included in the list.
- Optionally, forAllowed Methods, specify which methods other applications may use when requesting this URL from another domain. SelectReplace with, then move the methods to allow from theAvailable Methodsto theAllowed Methodslist.Any method you allow here must also be in the Allowed Methods list in the security policy ().
- Optionally, forAllowed Headers, selectReplace with, then type the headers that other applications can use when requesting this URL from another domain.Allowed headersare request headers sent by clients. For example, to allow clients to send Ajax requests, typeX-Requested-With, and to allow XML requests, typeContent-Type.
- Optionally, forExposed Headers, selectReplace with, then specify the headers that JavaScript can expose and share with other applications when requesting this URL from another domain.Exposed headersare the headers the server returns in the response. For example, to discover server side web application technology, typeX-Powered-By.
- Optionally, forAllow Credentials, selectReplace with, then specify whether requests from applications in another domain can include user credentials.
- Optionally, forMaximum Age, selectReplace with, then specify the number of seconds that the results of a preflight request can be cached or use the default.
- ClickUpdate.
- To add methods, such as OPTIONS, required to replace headers:
- Click.
- ClickCreate.
- In theMethodsetting, selectOPTIONS.
- ClickCreate.
- To put the security policy changes into effect immediately, clickApply 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 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.