Content Security Policy Interceptor

• Description
• Parameters
• Report action
• Action aware
• Examples

Description

Interceptor that implements Content Security Policy on incoming requests.

Content Security Policy (CSP) is an added layer of security that helps to detect and mitigate certain types of attacks, including Cross-Site Scripting (XSS) and data injection attacks. These attacks are used for everything from data theft, to site defacement, to malware distribution.

CSP can work in two modes, either enforce or report. In the report mode the Content-Security-Policy-Report-Only header is sent and Content-Security-Policy header is used when using the enforce mode.

CSP is now supported by all major browsers. More information about CSP.

The interceptor adds a nonce value automatically to <s:script> and <s:link> tags. This provides a painless way to implement CSP in a highly secure fashion.

Parameters

• enforcingMode (default false) - When set to “true”, the enforce mode has been enabled, and the provided policy is going to be enforced.
• reportUri - an uri under which the violations will be reported.
• prependServletContext (default true) - a flag to prepend or not the Servlet context to the reportUri (since Struts 6.4.0).
• cspSettingsClassName (default to DefaultCspSettings.class) - a full class name implementing CspSettings interface to allow to define a custom CPS settings. It’s alternative approach of using the CspSettingsAware interface below (since Struts 6.5.0).

Report action

To receive reports about violations against CSP an abstract CspReportAction action has been created, which you can extend to process the reports. When extending the action you must implement processReport(String) to process the report. Read JavaDoc of the action for more details.

Note: the action must always return an HTTP status 204.

Action aware

Since Struts 6.2.0 it is possible to configure the CSP interceptor by providing the an instance of CspSettings interface. Please use CspSettingsAware interface and implement the getCspSettings() method to steer the policy per action.

public class MyAction implements CspSettingsAware {
    
    public String execute() {
        return "success";
    }
    
    public CspSetting getCspSettings() {
      ...
    }
}

Examples

<action  name="someAction" class="com.examples.SomeAction">
    <interceptor-ref name="defaultStack">
        <param name="csp.enforcingMode">true</param>
        <param name="csp.reportUri">/csp-report.action</param>
    </interceptor-ref>
    <result name="success">good_result.ftl</result>
</action>