Practical JSONP Injection

JSONP injection is a lesser known but quite widespread and dangerous vulnerability and it surfaced in the last years due to the high rate of adoption of JSON, web APIs and the urging need for cross-domain communications.

What is JSONP?

Assuming everybody knows what JSON is, let’s talk a little about JSONP. JSONP comes from JSON with Padding and it was created in order to bypass common restrictions such as Same-origin Policy which is enforced for XMLHttpRequest (AJAX requests).

Let’s take an example. Our online banking application, http://verysecurebank.ro, has implemented an API call that returns the current user’s transactions.

An HTTP request to the http://verysecurebank.ro/getAccountTransactions endpoint presents us with the transactions, JSON formatted:

json-transactions

If our reports application, accessible at http://reports.verysecurebank.ro wants to get the transaction details, an AJAX call to the page won’t be possible, due to Same-origin Policy being in effect (different host).

sop-json

To get around this problem, JSONP came into play. Since Cross-domain script inclusion (mostly used to externally load JavaScript libraries such as jQuery, AngularJS etc.) is allowed, but not recommended, a smart trick apparently solved the entire equation: prepending the response with a callback.

Note: even if it might be obvious, it’s worth mentioning that when including a script cross-domain, it will run in the context of the including application, not in the source’s context.

Adding a callback to the API response, wrapped around the JSON formatted data, allows us to load the API response between script tags and get its content by defining our own callback function to handle it.

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