If you are using Firefox 4, you now have an extra layer of security when accessing mobile.twitter.com.
Over the past few weeks we’ve been testing a new security feature for our mobile site. It is called a Content Security Policy, or CSP. This policy is a standard developed by Mozilla that aims to thwart cross site scripting (XSS) attacks at their point of execution, the browser. The upcoming release of Firefox 4 implements CSP, and while the mobile site may not get a high volume of desktop browser traffic (the desktop users hitting that site typically have low bandwidth connections), it has given us an opportunity to test out a potentially powerful anti-XSS tool in a controlled setting.
The policy also contains a ‘reporting URI’ to which the browser sends JSON reports of any violations. This feature not only assists debugging of the CSP rules, it also has the potential to alert a site’s owner to emerging threats.
We began our explorations by restricting the changes to browsers that support CSP (currently only Firefox 4) in order to lessen the impact on users. Next, we identified all the possible locations of our assets and built a rule set to encompass those; for example, things such as user profile images and stylesheets from our content delivery network.
Today CSP is one hundred percent live on mobile.twitter.com and we are logging and evaluating incoming violation reports.
Over the next couple of months we plan to implement a Content Security Policy across more of Twitter, and we encourage you to request support for this standard in your preferred browser.
The following people at Twitter contributed to the CSP effort: John Adams, Jacob Hoffman-Andrews, Kevin Lingerfelt, Bob Lord, Mark Percival, and Marcus Philips
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