Year Of Security for Java – Week 7 – Content Security Policy

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What is it and why should I care?
Content Security Policy (CSP) is a new(ish) technology put together by Mozilla that web apps can use as an additional layer of protection against Cross Site Scripting (XSS), which is the primary goal of the technology. A secondary goal is to protect against clickjacking.

XSS is quite a complex issue, as is evidenced by the recommendations in the OWASP prevention cheatsheets for XSS in general and DOM based XSS. CSP does several things to help app developers deal with XSS.

Whitelist Content Locations
One reason XSS is quite harmful is that the browsers implicitly trust the content received from the server, even if that content has been manipulated and is loading from an unintended location. This is where CSP comes in: CSP allows app developers to declare a whitelist of trusted locations for content. Browsers that understand CSP will then respect that list and only load content from there, and will ignore content that references locations outside the list.

No Inline Scripts
Adding scripts inline to content is the way XSS is done, made possible because browsers have no way of telling whether the site actually sent that content, or if the attacker added the script to the site content. CSP prevents this entirely by forcing the separation of content and code (Great design!). This means that you have to move all your scripts to external files, which will require work for most apps, though it can be done (Twitter is one example of a high-profile site using CSP). The upside is in order for an attack to be successful with CSP, an attacker has to be able to:
Step 1) Inject a script tag at the head of your page.
Step 2) Make that script tag load from a trusted site within your whitelist.
Step 3) Control the referenced script at that trusted site.

This makes an XSS attack significantly more difficult.

Note: one common question here is what about event handling. CSP does still allow event handling through the onXXX handlers or the addEventListener mechanism.

No Code From Strings (eval dies)
Another welcome addition is the blacklisting of functions that create code from strings. This means that usage of the evil eval is out (along with a few others). Creating code from strings is a popular attack technique and is rather difficult to trace, so the removal of all these functions is actually quite helpful.

Another common question stemming from this is how to deal with JSON parsing. The right way to do this from a security perspective has always been to actually parse the JSON instead of eval it anyhow, and this functionality is still available, so nothing changes there.

Policy Violation Reporting
A rather cool feature of CSP is that you can configure your site to have a violation reporting handler, and then have that data available whether you run in report-only or in enforcing mode. In report only mode, you can get reports of locations in your site that will be prevented from execution when you enable CSP (a nice way to test). In enforcing mode, you can still get this data, but in production, you can also use this as a simple XSS detection mechanism (bad guy tried XSS and it didn’t run).

What should I do about it?

Well, you should use it! Actually, CSP doesn’t really seem to have a downside. It’s there to make your site safer, and even if a client’s browser doesn’t support it, it’s entirely backwards compatible, so your site won’t break for the client.

In general, I think the basic approach should be:
Step 1) Solve XSS through your standard security development practices (you should already be doing this)
Step 2) Learn about CSP – read the specs thoroughly
Step 3) Make the changes to your site and test and re-test (normal dev process)
Step 4) Run in report-only mode and monitor any violations to find areas you still need to fix. (could be avoided if you have a full functional test suite you can execute against your app in testing – good for you if you do!)
Step 5) After you’re confident your site is working properly, turn it on in enforcing mode.

As for how you actually implement it, you have 2 basic options: an HTTP header and a META tag. The header option is preferred, and an example is listed below.

Content-Security-Policy: default-src 'self'; 
    img-src *;
    object-src *;

The example above says the following:
Line 1: By default, only allow content from ‘self’ or the site represented by the current url.
Line 2: Allow images from anywhere.
Line 3: Allow objects from only the listed urls.
Line 4: Allow scripts from only the listed url.
Line 5: Use the listed url to report any violations of the specified policy.

CSP is a very interesting and useful new technology to help battle XSS. It’s definitely a useful tool to add to your arsenal.

Update (2/15/2012) Here’s an interesting post on using ModSecurity to add CSP to your site – a nice look for the administrators. (h/t @ryancbarnett)


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