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Using Content Security Policy to Prevent Cross-Site Scripting (XSS)

Note: This post has been crossposted from the SendSafely blog. You can find the original post at  

On SendSafely we make heavy use of many new JavaScript APIs introduced with HTML5. We encrypt files, calculate checksums and upload data using pure JavaScript.  Moving logic like this down to the browser, however, makes the threat of Cross-Site Scripting (XSS) even greater than before. In order to prevent XSS vulnerabilities, our site makes liberal use of pretty aggressive client-side and server-side encoding APIs.  These APIs are based on the OWASP ESAPI library, so we have context-specific encoding methods for pretty much every scenario. Even so, we recognize that it is very difficult to rule out all possible ways to inject code, including human error on our part. For this reason we chose to also implement Content Security Policy (CSP) for SendSafely.

CSP is a new security mechanism supported by modern browsers. It aims to prevent XSS by white-listing URLs the browser can load and execute JavaScript from. The server can, by specifying specific CSP directives, prevent the browser from executing things like in-line JavaScript, eval()setTimeout() or any JavaScript that comes from an untrusted URL. The policy works as a white list, only domains listed are allowed to execute, everything else will be blocked.

The Content Security Policy in SendSafely
In SendSafely, our Javascript files are all loaded from a dedicated host that doesn’t run any dynamic content ( The exceptions to this are for certain third-party JavaScript APIs that we load from an external domain, specifically Google Analytics and reCAPTCHA.  Text-to-JavaScript functions like eval() and setTimeout() are blocked across the board, even if the script is loaded from one of our white-listed hosts, as is any in-line JavaScript

Use of a strict CSP makes it significantly harder to inject executable JavaScript into application pages since the code must come from a trusted server.  The typical XSS attack using un-encoded output on one of our pages won’t work when the CSP is enforced.  In fact, any JavaScript embedded on our content pages (even JavaScript we put there) gets blocked by the policy.  Pretty cool stuff.  

So you may be asking yourself, does this mean XSS is nothing but a memory? Sadly, this is not the case.  For starters, CSP is still fairly new and only supported by recent versions of Firefox, Safari and Chrome. Internet Explorer 10 (IE10) supports a subset of CSP options, but the ability to white list domains is unfortunately not one of them.  Aside from limited browser support, data dynamically loaded into the page from JavaScript is still potentially vulnerable. A strict Content Security Policy should therefore not be considered the end-all solution to XSS . Think of CSP more like a safety belt, which is nice to have when your car crashes.

Dissecting our Policy
Now let’s take a look at the CSP policy we use on and dissect it a bit.  One of the first things to note is that if you are going to implement CSP, you must realize that there are some browser compatibility nuances to deal with.   The main thing to note is that Safari uses ‘X-WebKit-CSP’ as the header name for implementing CSP, while other browsers have standardized on ‘X-Content-Security-Policy’.  Another glitch that affects Safari is that a severe bug in the CSP implementation on Version 5.1 essentially blocks authorized content when a valid CSP is specified.  As a result, you’ll want to specifically detect when Safari is used and send either the ‘X-WebKit-CSP’ header or no header at all (if Version 5.1 is used).

To keep our policy as strict as possible, we use two different policies depending on what the page needs to do.  The stricter policy is used for all pages except the ones that handle encryption and decryption (the reason for this will be discussed in a separate follow up post).  For simplicity, the more strict policy will be explained here.

X-Content-Security-Policy: default-src ‘none’; connect-src ‘self’; script-src; style-src ‘self’ ‘unsafe-inline’ http: https:; img-src ‘self’; report-uri /csp-reports;

The header is divided into different sections that are each separated by a semi-colon. The “default-src” directive defines the security policy for all types of content which are not expressly called out by more specific directives.  We opted to set the default-src value to ‘none’, meaning that by default we allow nothing to load.  If we stopped defining directives here, the site would be completely broken, so now we need to open up the policy and allow specifically what we need to load.  

Now that we’ve explicitly denied everything by default, we need to add back the specific content policy options our site needs.  On SendSafely, we have a hand full of resource categories that we need to add policy settings for.  Each of these are outlined below, along with the CSP directives for each:

  • Ajax Requests - Several pages within our site use the browser’s XMLHttpRequest (XHR) object to make HTTP requests from within our JavaScript code.  In order for us to make these requests we set the “connect-src” attribute to “self”, so scripts on our site can make XHR requests back the server but nowhere else. This attribute is another place where we run into compatibility issues across different browsers.  Specifically, FireFox decided to name this directive “xhr-src” instead of “connect-src”.  To account for this, our CSP code does some basic browser detection and if we detect that FireFox is being used, we change the directive name accordingly.  

  • JavaScript - As mentioned previously, we load all of our internal static JavaScript from a dedicated host ( Additionally, we’ve chosen to load the Google Analytics and ReCaptcha JavaScript files from their origin domains on  Unfortunately the ability to allow just a sub-path of a host (like /scripts/) is not supported. Since ReCaptcha script files get loaded directly off of the main site, our “script-src” directive includes, and

    Having such a large site like in our CSP whitelist is understandably something we are not thrilled about.  The ability to allow sub-paths of a host is slated to be introduced in CSP 1.1, but until then we’ll have to live with it.  The good news is that Google takes security very seriously, and they take great care to avoid script injection bugs on their website.
  • CSS - Our site design makes heavy use of in-line CSS for styling various UI attributes.  As such, the style-src directive includes a value of “self” (that allows us to load CSS files from the same host) and a value of “unsafe-inline”, meaning that we can use in-line CSS from within our HTML pages.  We recognize that by allowing in-line CSS within our pages, there is a minimal increased security risk since someone could potentially be mischievous if they found a way to inject markup into one of our pages.  Given the cost/benefit of refactoring the UI to completely avoid any in-line CSS, however, we decided this is a tolerable risk that we can live with for now.  

  • Images - Our img-src directive specifies both “self” and the two previously mentioned google hosts ( and  as the authorized origin hosts for all image content  For the most part, our site only loads images from the same host.  The exception to this is reCaptcha, however, since reCaptcha loads various images from domain.  

  • The final part of our CSP header is the ‘report-uri’ directive.  This directive tells the browser to send us a report of pages that violate the Content Security Policy. The  violation reports consist of JSON documents sent via an HTTP POST request to the specified URI.  Using this option, we can monitor for events that trigger CSP exceptions and quickly take action if we think there may be a problem with our site.  The reports are also great to use during testing and development in order to debug CSP issues you might encounter.

Final Notes
A few final notes: CSP is a great tool to add an additional layer of protection against Cross-Site Scripting. If you’re building a new application, CSP should be considered as a solid defense in depth security control in the never-ending battle against cross-site scripting. Writing client-side code which is designed to use CSP will save precious developer cycles in the future, if code must be migrated to work with CSP.

Implementing CSP on our site proved to be a very interesting exercise.  We’ll provide more details on some other aspects of our Content Security Policy implementation in a follow up post here on our blog.