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Some experience with Content Security Policy

XSSI recently started playing around with Content Security Policy (CSP). CSP is a very neat feature and a good example how to get IT security right.


The main reason CSP exists are cross site scripting vulnerabilities (XSS). Every time a malicious attacker is able to somehow inject JavaScript or other executable code into your webpage this is called an XSS. XSS vulnerabilities are amongst the most common vulnerabilities in web applications.

CSP fixes XSS for good


The approach to fix XSS in the past was to educate web developers that they need to filter or properly escape their input. The problem with this approach is that it doesn't work. Even large websites like Amazon or Ebay don't get this right. The problem, simply stated, is that there are just too many places in a complex web application to create XSS vulnerabilities. Fixing them one at a time doesn't scale.


CSP tries to fix this in a much more generic way: How can we prevent XSS from happening at all? The way to do this is that the web server is sending a header which defines where JavaScript and other content (images, objects etc.) is allowed to come from. If used correctly CSP can prevent XSS completely. The problem with CSP is that it's hard to add to an already existing project, because if you want CSP to be really secure you have to forbid inline JavaScript. That often requires large re-engineering of existing code. Preferrably CSP should be part of the development process right from the beginning. If you start a web project keep that in mind and educate your developers to use restrictive CSP before they write any code. Starting a new web page without CSP these days is irresponsible.


To play around with it I added a CSP header to my personal webpage. This was a simple target, because it's a very simple webpage. I'm essentially sure that my webpage is XSS free because it doesn't use any untrusted input, I mainly wanted to have an easy target to do some testing. I also tried to add CSP to this blog, but this turned out to be much more complicated.


For my personal webpage this is what I did (PHP code):
header("Content-Security-Policy:default-src 'none';img-src 'self';style-src 'self';report-uri /c/");


The default policy is to accept nothing. The only things I use on my webpage are images and stylesheets and they all are located on the same webspace as the webpage itself, so I allow these two things.


This is an extremely simple CSP policy. To give you an idea how a more realistic policy looks like this is the one from Github:
Content-Security-Policy: default-src *; script-src assets-cdn.github.com www.google-analytics... collector-cdn.github.com; object-src assets-cdn.github.com; style-src 'self' 'unsafe-inline' 'unsafe-eval' assets-cdn.github.com; img-src 'self' data: assets-cdn.github.com identicons.github.com www.google-analytics... collector.githubapp.com *.githubusercontent.com *.gravatar.com *.wp.com; media-src 'none'; frame-src 'self' render.githubusercontent.com gist.github.com www.youtube.com player.vimeo.com checkout.paypal.com; font-src assets-cdn.github.com; connect-src 'self' ghconduit.com:25035 live.github.com uploads.github.com s3.amazonaws.com

Reporting feature


You may have noticed in my CSP header line that there's a "report-uri" command at the end. The idea is that whenever a browser blocks something by CSP it is able to report this to the webpage owner. Why should we do this? Because we still want to fix XSS issues (there are browsers with little or no CSP support (I'm looking at you Internet Explorer) and we want to know if our policy breaks anything that is supposed to work. The way this works is that a json file with details is sent via a POST request to the URL given.


While this sounds really neat in theory, in practise I found it to be quite disappointing. As I said above I'm almost certain my webpage has no XSS issues, so I shouldn't get any reports at all. However I get lots of them and they are all false positives. The problem are browser extensions that execute things inside a webpage's context. Sometimes you can spot them (when source-file starts with "chrome-extension" or "safari-extension"), sometimes you can't (source-file will only say "data"). Sometimes this is triggered not by single extensions but by combinations of different ones (I found out that a combination of HTTPS everywhere and Adblock for Chrome triggered a CSP warning). I'm not sure how to handle this and if this is something that should be reported as a bug either to the browser vendors or the extension developers.

Conclusion


If you start a web project use CSP. If you have a web page that needs extra security use CSP (my bank doesn't - does yours?). CSP reporting is neat, but it's usefulness is limited due to too many false positives.


Then there's the bigger picture of IT security in general. Fixing single security bugs doesn't work. Why? XSS is as old as JavaScript (1995) and it's still a huge problem. An example for a simliar technology are prepared statements for SQL. If you use them you won't have SQL injections. SQL injections are the second most prevalent web security problem after XSS. By using CSP and prepared statements you eliminate the two biggest issues in web security. Sounds like a good idea to me.


Buffer overflows where first documented 1972 and they still are the source of many security issues. Fixing them for good is trickier but it is also possible.

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