Sanitize all inputs. That includes every bit of data processed by the application, whether loaded from a Facebook user’s profile, loaded from a database, submitted with a form, or received from the query string of an address. Never assume that a given parameter will be clean or of the expected type.
Sanitize all outputs. When displaying a notice or error message, load predetermined strings instead of using dynamic inputs. Never reuse the address of a page without fitering it for injection attempts. Filter any information you output to an application page or via an AJAX interface.
Avoid user-generated HTML. Generally, users should never be allowed to input HTML, FBML, or other rich-text formats. When allowing rich-text data, use pre-built, tested code for processing and displaying it, rathering than trying to create your own filters.
Check every page. Many vulnerabilities appear in secondary pages, such as ad loaders or AJAX interfaces. Verify security precautions in every part of the application. If possible, consider storing secondary files in a folder other than that of the application’s canvas pages.
Verify Facebook sessions. Never rely on a cookie, a query string, or data generated within the application to verify the current user. Facebook provides applications with session information they can always check before making requests or loading information.
Use server whitelisting. If your application does not use AJAX or does not otherwise make requests using the Facebook JavaScript API, take advantage of the server whitelist feature in the application properties and only allow requests from your server.
Understand third-party code. Take the time to examine any code given to you by other developers, such as JavaScript tools or advertising network receiver files, before including them in your application. In particular, third-party code that arnesses a user’s session secret violates rules given by Facebook.
Don’t simply obfuscate. Never rely on JavaScript obfuscation or compression to hide vulnerabilities in application pages. Such techniques may slow down an attacker for a short while, but they can always be worked around or reversed.
Educate your users. Avoid incorporating design patterns that train users to accept bad practices, such as entering third-party passwords. Communicate clearly your policies on privacy, data retention, and information security.
