Are there any security issues that should be considered when using JSONP?
Answer
Update: JSONP is a common hack to do cross-domain requests. Modern browsers now have Cross Origin Resource Sharing, and IE8+ have XDomainRequest which is similar. See http://enable-cors.org/ for more info.
JSONP is just a script include that allows you to use a callback. You should however be aware of Cross-site request forgery (CSRF).
As long as you control the script and the server, JSONP isn't anymore insecure than a script include. Unless you have a JSONP-service that returns sensitive data to logged in users. A malicious site can send a request to the service (hoping that the user is logged in on your site), and retreive the data. The service can check the referrer of the request, but it is possible to spoof the referrer using flash (thanks Chris Moschini).
Imagine this senario: - A user logs into his internet banking account. Storing a session cookie in the users browser. This site has a jsonp service with sensitive info about the user and his accounts. - Other sites won't know that the user is logged in, but they could do a wild guess and try to access the jsonp service. Since the user has a session cookie, the browser will get a response, and there's nothing stopping the site from doing an ajax post to save the sensitive data on their server.
Update June 28th 2012: If you want to protect against CSRF attacks you should read this in depth blog post by a security expert: http://erlend.oftedal.no/blog/?blogid=130