- Inline script will be allowed because inline scripting is commonly used (can be disabled if target site does not use this type of scripting),
- eval() function will be allowed in order to not break use of popular JavaScript libraries (ex: JQuery, JQueryUI, Sencha, …) because they use eval() function (it was the case last time I have checked the source code from CDN ;) ),
The support for CSP directives is not the same level in major browsers (Firefox/Chrome/IE). It’s recommanded to check the support provided by target browsers (using site provided in link section of this article) in order to configure CSP policies. The sample below try to provide a set of policies from which your can add policies specific to your application context.
This implementation provide an option to add CSP directives used by Firefox (Mozilla CSP directives).
import java.io.IOException; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException; import java.security.SecureRandom; import java.util.ArrayList; import java.util.List; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.annotation.WebFilter; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.commons.codec.binary.Hex; /** * Sample filter implementation to define a set of Content Security Policies.
* This implementation has a dependency on Commons Codec API.
* This filter set CSP policies using all HTTP headers defined into W3C specification.
*
* This implementation is oriented to be easily understandable and easily adapted.
*/ @WebFilter("/*") public class CSPPoliciesApplier implements Filter /** Configuration member to specify if web app use web fonts */ public static final boolean APP_USE_WEBFONTS = false; /** Configuration member to specify if web app use videos or audios */ public static final boolean APP_USE_AUDIOS_OR_VIDEOS = false; /** * Configuration member to specify if filter must add CSP directive used by Mozilla (Firefox) */ public static final boolean INCLUDE_MOZILLA_CSP_DIRECTIVES = true; /** Filter configuration */ @SuppressWarnings("unused") private FilterConfig filterConfig = null; /** List CSP HTTP Headers */ private ListString> cspHeaders = new ArrayListString>(); /** Collection of CSP polcies that will be applied */ private String policies = null; /** Used for Script Nonce */ private SecureRandom prng = null; /** * Used to prepare (one time for all) set of CSP policies that will be applied on each HTTP * response. * * @see javax.servlet.Filter#init(javax.servlet.FilterConfig) */ @Override public void init(FilterConfig fConfig) throws ServletException // Get filter configuration this.filterConfig = fConfig; // Init secure random try this.prng = SecureRandom.getInstance("SHA1PRNG"); > catch (NoSuchAlgorithmException e) throw new ServletException(e); > // Define list of CSP HTTP Headers this.cspHeaders.add("Content-Security-Policy"); this.cspHeaders.add("X-Content-Security-Policy"); this.cspHeaders.add("X-WebKit-CSP"); // Define CSP policies // Loading policies for Frame and Sandboxing will be dynamically defined : We need to know if context use Frame ListString> cspPolicies = new ArrayListString>(); String originLocationRef = "'self'"; // --Disable default source in order to avoid browser fallback loading using 'default-src' // locations cspPolicies.add("default-src 'none'"); // --Define loading policies for Scripts cspPolicies.add("script-src " + originLocationRef + " 'unsafe-inline' 'unsafe-eval'"); if (INCLUDE_MOZILLA_CSP_DIRECTIVES) cspPolicies.add("options inline-script eval-script"); cspPolicies.add("xhr-src 'self'"); > // --Define loading policies for Plugins cspPolicies.add("object-src " + originLocationRef); // --Define loading policies for Styles (CSS) cspPolicies.add("style-src " + originLocationRef); // --Define loading policies for Images cspPolicies.add("img-src " + originLocationRef); // --Define loading policies for Form cspPolicies.add("form-action " + originLocationRef); // --Define loading policies for Audios/Videos if (APP_USE_AUDIOS_OR_VIDEOS) cspPolicies.add("media-src " + originLocationRef); > // --Define loading policies for Fonts if (APP_USE_WEBFONTS) cspPolicies.add("font-src " + originLocationRef); > // --Define loading policies for Connection cspPolicies.add("connect-src " + originLocationRef); // --Define loading policies for Plugins Types cspPolicies.add("plugin-types application/pdf application/x-shockwave-flash"); // --Define browser XSS filtering feature running mode cspPolicies.add("reflected-xss block"); // Target formating this.policies = cspPolicies.toString().replaceAll("(\\[|\\])", "").replaceAll(",", ";") .trim(); > /** * Add CSP policies on each HTTP response. * * @see javax.servlet.Filter#doFilter(javax.servlet.ServletRequest, * javax.servlet.ServletResponse, javax.servlet.FilterChain) */ @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain fchain) throws IOException, ServletException HttpServletRequest httpRequest = ((HttpServletRequest) request); HttpServletResponse httpResponse = ((HttpServletResponse) response); /* Step 1 : Detect if target resource is a Frame */ // Customize here according to your context. boolean isFrame = true; /* Step 2 : Add CSP policies to HTTP response */ StringBuilder policiesBuffer = new StringBuilder(this.policies); // If resource is a frame add Frame/Sandbox CSP policy if (isFrame) // Frame + Sandbox : Here sandbox allow nothing, customize sandbox options depending on // your app. policiesBuffer.append(";").append("frame-src 'self';sandbox"); if (INCLUDE_MOZILLA_CSP_DIRECTIVES) policiesBuffer.append(";").append("frame-ancestors 'self'"); > > // Add Script Nonce CSP Policy // --Generate a random number String randomNum = new Integer(this.prng.nextInt()).toString(); // --Get its digest MessageDigest sha; try sha = MessageDigest.getInstance("SHA-1"); > catch (NoSuchAlgorithmException e) throw new ServletException(e); > byte[] digest = sha.digest(randomNum.getBytes()); // --Encode it into HEXA String scriptNonce = Hex.encodeHexString(digest); policiesBuffer.append(";").append("script-nonce ").append(scriptNonce); // --Made available script nonce in view app layer httpRequest.setAttribute("CSP_SCRIPT_NONCE", scriptNonce); // Add policies to all HTTP headers for (String header : this.cspHeaders) httpResponse.setHeader(header, policiesBuffer.toString()); > /* Step 3 : Let request continue chain filter */ fchain.doFilter(request, response); > /** * * * @see javax.servlet.Filter#destroy() */ @Override public void destroy() // Not used > > Tools
There’s a number of free tools that can assist with the generating, evaluation and monitoring of content security policy.
It’s very useful to include these types of tools into a web application development process in order to perform a regular automatic first level check (do not replace an manual audit and manual audit must be also conducted regularly).
- CSP Tester (browser extension) to build and test the policy for your web application.
- CSP Generator for automatically generating policies (chrome/firefox extension).
- CSP Evaluator for evaluating existing content security policies for security misconfigurations.
- Csper report collector for monitoring a content security policy using report-uri.
Information Links
- W3C Specifications: CSP 1.0, CSP 1.1
- Introduction to CSP
- CSP browser support
- CSP readiness browser testing
- CSP Quick Reference
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