AJAX Security Cheat Sheet



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= Introduction =

There is a complete lack of guidelines for AJAX. This document will provide a starting point for AJAX security and will hopefully be updated and expanded reasonably often to provide more detailed information about specific frameworks and technologies.

Use .innerText instead of .innerHtml
The use of .innerText will prevent most XSS problems as it will automatically encode the text.

Don't use eval
Eval is evil, never use it. Needing to use eval usually indicates a problem in your design.

Canonicalize data to consumer (read: encode before use)
When using data to build HTML, script, CSS, XML, JSON, etc. make sure you take into account how that data must be presented in a literal sense to keep it's logical meaning. Data should be properly encoded before used in this manner to prevent injection style issues, and to make sure the logical meaning is preserved.

Check out the OWASP Encoding Project.

Don't rely on client logic for security
Least ye have forgotten the user controls the client side logic. I can use a number of browser plugging to set breakpoints, skip code, change values, etc. Never rely on client logic.

Don't rely on client business logic
Just like the security one, make sure any interesting business rules/logic is duplicated on the server side less a user bypass needed logic and do something silly, or worse, costly.

Avoid writing serialization code
This is hard and even a small mistake can cause large security issues. There are already a lot of frameworks to provide this functionality. Take a look at the JSON page for links.

Avoid building XML dynamically
Just like building HTML or SQL you will cause XML injection bugs, so stay way from this or at least use an encoding library to make attributes and element data safe.

Check out the OWASP Encoding Project.

Never transmit secrets to the client
Anything the client knows the user will also know, so keep all that secret stuff on the server please.

Don't perform encryption in client side code
Use TSL/SSL and encrypt on the server!

Don't perform security impacting logic on client side
This is the overall one that gets me out of trouble in case I missed something :)

Use CSRF protection
Proper CSRF protection ensures that attackers without access to the same origin as the data returned from the server cannot make the server respond with confidential and use JavaScript Hijacking to access it.

Always return JSON with an Object on the outside
Always have the outside primitive be an object for JSON strings:

Exploitable: [{"object": "inside an array"}]

Not exploitable: {"object": "not inside an array"}

Also not exploitable: {"result": [{"object": "inside an array"}]}

Avoid writing serialization code. Remember ref vs. value types!
Look for an existing library that has been reviewed.

Services can be called by users directly
Even though you only expect your AJAX client side code to call those services the users can too. Make sure you validate inputs and treat them like they are under user control (because they are!).

Avoid building XML or JSON by hand, use the framework
Use the framework and be safe, do it by hand and have security issues.

Use JSON And XML Schema for Webservices
You need to use a 3rd party library to validate web services.

Authors and Primary Editors
Til Mas Michael Eddington