JSONP is said to "by-pass" the same origin policy. I take this to mean that using it allows the script to load from a remote server on a different domain and run locally on a website.
I'm not clear however on what exactly can be done by the script while it is running.
Is it just as privileged at JavaScript loaded from the same domain? Are there any additional restrictions imposed on JSONP that one can count on?
In a browser, <script> tags may reference scripts located in any domain. Script resources are not subject to the same-origin restrictions that an Ajax request is.
As such, you can dynamically insert a script tag that will refer to a script on any domain and it will load successfully and not be blocked by same origin restrictions.
This is how JSONP works - by making a cross origin request by requesting a remote script to load and run. It is not a blanket bypass of the same origin restrictions because you cannot just directly make a remote API call using JSONP as the server you are contacting must explicitly support JSONP because it's a completely different type of response. So, in essence, the server you are contacting must explicitly support and allow this cross origin request via JSONP.
I'm not clear however on what exactly can be done by the script while
it is running.
Once you request a JSONP script, that script is just a piece of Javascript running in your page. It can literally do anything that any script running in your page can do.
Is it just as privileged at JavaScript loaded from the same domain?
Are there any additional restrictions imposed on JSONP that one can
count on?
It's just a script running in your page. It can do anything that any script running in your page can do. By convention, it is supposed to call a callback function that was specified in the URL and pass it some data (the result of the request), but it could do any other thing that Javascript in your page can do.
There are no additional restrictions on JSONP scripts. The browser doesn't know if a script is JSONP script or not. It's just a script that can do exactly the same things as any other script.
JSONP is basically a hack that solved a problem before browsers supported CORS. I can't think of any reason these days to design a solution based on JSONP any more if you can use CORS. CORS is more secure as it doesn't let some other site run random Javascript in the context of your page.
JSONP works by employing ordinary <script> tags and content fetched as JavaScript via an HTTP GET. The browser basically thinks it's just fetching another script for use by the page, so the code with which the third-party site responds can do anything it wants to.
"Well-behaved" JSONP sources send back, by convention, a call to the function whose name you give as the "callback" parameter in the URL. That is, the server responds with a JavaScript statement:
yourCallback({ name: value, ... });
There's really no way for your code on the page to tell whether that's really all that happened, however.
Related
We can simply call GET request for any page on the web using html tags from another origin:
<script src="http://example.com/user/post?txt=sample"></script>
XHR other origin is blocked because of security reason, as an instance, attacker can post behalf of a user using GET request(Consider the fact that it is not possible because of lack of cookies). However, the above script tag will do the same(Same, cookies are not available). So why XHR GET request is not allowed?
GET requests are not supposed to change anything on the server. From RFC 2616 section 9.1.1:
In particular, the convention has been established that the GET and HEAD methods SHOULD NOT have the significance of taking an action other than retrieval. These methods ought to be considered "safe".
In your example of posting to a site using a GET request, the site shouldn't even allow that, and the same-origin policy isn't really meant to prevent it.
The reason XHR is treated differently is that XHR returns the HTTP response to the JavaScript code that made the request, so it has the potential to leak information. For example, if cross-domain XHR GET requests were allowed, a malicious script could query your bank's website to find out how much money is in your account.
Other methods of performing GET requests don't leak information. In particular:
You can add a <script> tag to the document, but the browser will try to run the response as a script. Unless the response is a valid script that's specifically designed to provide data using the JSONP convention, your code can't "see" anything that was in the response.
You can add an <img> tag to the document and maybe load some of the user's personal photos from another site, but the image will only appear on the screen; you can't access the pixel data from JavaScript.
I don't see how the callback function in JSONP is any different from the success callback function in AJAX.
Given #1, I don't see how it is fundamentally more secure.
So is the only difference an artificial same-domain constraint with AJAX?
Why can't AJAX just allow cross-domain requests; if this can cause a security hole, wouldn't the attack just XSS a JSONP request?
Confused,
Max
An ajax call is an actual HTTP request from your client directly to a server. Ajax calls can be synchronous (blocking until they complete) or asynchronous. Because of same-origin security protections, ajax calls can only be made to the same server that the web page came from unless the target server explicitly allows a cross origin request using CORS.
JSONP calls are an interesting hack with the <script> tag that allows cross-origin communication. In a JSONP call, the client creates a script tag and puts a URL on it with an callback=xxxx query parameter on it. That script request (via the script tag insertion) is sent by the browser to the foreign server. The browser just thinks it's requesting some javascript code. The server then creates some special javascript for the purposes of this call and in that javascript that will get executed by the browser when it's returned, the server puts a function call to the function named in the callback=xxxx query parameter. By either defining variables of by passing data to that function, the server can communicate data back to the client. For JSONP, both client and server must cooperate on how the JSONP call works and how the data is defined. A client cannot make a JSONP call to a server that doesn't explicitly support JSONP because the exact right type of JSONP response has to be built by the server or it won't work.
So, the two communication methods work completely differently. Only ajax calls can be synchronous. By the nature of the <script> tag insertion, JSONP calls are always asynchronous.
In an Ajax call, the response comes back in a ajax event handler.
In a JSONP call, the response comes when the returned Javascript calls a function of yours.
In some ways, JSONP is a security hole that bypasses the cross-origin security mechanism. But, you can only call servers that explicitly choose to support a JSONP-like mechanism so if a server doesn't want you to be able to call it cross-origin, it can prevent it by not supporting JSONP. You can't make regular ajax calls to these other servers.
The browser makers can't really close this loophole because if they did zillions of web pages would break that either already use JSONP or load scripts from other domains. For example, every page on the web that uses jQuery off the Google or Microsoft CDNs would break because the browser wouldn't be allowed to download javascript from cross-origin domains.
JSONP was largely invented as a work-around to be able to make cross-origin requests. But, since JSONP requires explicit server support in order to work, it wasn't really a security problem because a JSONP call can only be made to a server that explicitly decided to allow that type of cross origin call. JSONP is used much less now than it used to be because CORS was invented as a more elegant way to control/allow this. CORS stands for Cross Origin Resource Sharing and it provides a means for a target server to tell a web browser exactly what type of cross origin requests are allowed and even to tell it which web page domains are allowed to make such requests. It is has much finer control available than JSONP and all modern browsers now support CORS.
Here's an example of how a cross-origin call causes problems. If you could load any arbitrary web page from any other web page or make any arbitrary ajax call, then imagine you were already logged into your webmail interface on Yahoo in so some other browser window. This means that your cookies are set to allow requests from your browser to fetch data from Yahoo. If the javascript in some other web page was allowed to make a webmail request to Yahoo (that would automatically have your cookies attached), it could then fetch all your webmail data and send it back to it's own site. One web site could rip off all the logged-in data from any other web site. All web security would be broken.
But, the way we have it today, as long as Yahoo doesn't support a JSONP interface that uses those same web cookies, it is safe from unauthorized JSONP requests.
Here are some other good writeups on the dangers of cross-origin ajax and why it has to be prevented:
Why the cross-domain Ajax is a security concern?
Why Cross-Domain AJAX call is not allowed?
Why are cross-domain AJAX requests labelled as a "security risk"?
JSONP's callback is not an actual callback. Rather, JSONP works by script injection. E.g., if you want to make a JSONP call, you insert this script element into the DOM:
<script src="http://example.com/ajaxendpoint?jsonp=parseResponse"></script>
The server's response will be something like this:
parseResponse({"json":"value"});
It will be evaluated in the window's global scope. So essentially JSONP is like a remote exec(), where the server is advised what string to create to execute.
This is very different from Ajax: with JSONP, the response is evaluated in the script's global scope; with XMLHttpRequest, the response is received as a string and not evaluated.
(Also, JSONP can only be used with GET, whereas AJAX allows any http method.)
Thus to your second issue, "I don't see how it is fundamentally more secure." Well, you are right, JSONP is actually extremely insecure. The server can return any script it wants, and do anything it wants to your browser!
Cross-domain requests are insecure because they can be used to reveal information about the current page to a page on another domain.
And you are right that any XSS attack can just use JSONP. The purpose of CORS is not to prevent XSS (if you have untrusted scripts running on your page you are hosed anyway).
The fundamental difference is that, for some reason, it's perfectly fine to load javascript files located on other domains (via script tag), but it is not OK by default to load other cross domain resources.
I'm with you, in that the delineation seems rather arbitrary. In jQuery, when you do a JSONP call, effectively you are creating a script tag, loading the resource, and then the jQuery library executes your script by calling the function defined in that JSONP result.
In my eyes, I cannot think of an additional vector of attack introduced by allowing cross domain AJAX which is not already gaping wide by allowing cross domain script loading, which is a common practice used everywhere (jQuery by googleCDN, advertising scripts, google analytics, and countless others).
From wikipedia
In addition, many legacy cross-domain operations predating JavaScript are not subjected to same-origin checks; one such example is the ability to include scripts across domains, or submit POST forms.
Is it possible to do AJAX calls from inside an iframe that has a different domain source?
I've tried script injection but it doesn't work because the iframe's source is secure.
I made a simple fiddle with California DMV website here.
I'm getting DOM exception 8 error. Is it a security issue?
It is not possible to modify or make JS calls in an iframe with a different domain source. This is restricted in all browsers for security reasons.
See the "Same Origin Policy" for a description of how inter frame security works. In a nutshell, there is very little communication allowed between frames on a different domain for security reasons. You cannot make any direct Javascript calls between frames on different domains.
There is a way to make cross domain ajax calls and it involves using JSONP. Basically, you inject a script tag into your own frame and that script tag points to server endpoint anywhere on the web. Since the src value of a script tag is not restricted by the same origin policy, you can reach that server. But, now you need to have a way to get that result back. That is done using JSONP where you specify in your server request a javascript function that you want the returned javascript to call. That returned javascript can have javascript data in it that is then passed to the desired function. JSONP requires cooperation between both client code and the server code since a normal ajax call might not support the extra part of JSONP. But, with this cooperation of both sides, you can get around the same origin policy for server endpoints that support JSONP.
HTML5 has a new messaging system that can safely communicate data (not direct JS calls) between cooperating frames in different domains. See here and here for a description of how the HTML5 messaging works.
Yes it's a security issue because of the Same Origin Policy enforced by most browsers: http://en.wikipedia.org/wiki/Same_origin_policy .
You can look into JSONP http://niryariv.wordpress.com/2009/05/05/jsonp-quickly/ which is specifically designed to get around this.
Is it possible through jQuery (or plain javascript) to test if a webpage on another domain is available?
I tried getting the response headers with an ajax-call but I get an error no matter what site outside my own domain I test.
So do I really need a proxy script on my server or would I be able to skip that request?
Is it possible through jQuery (or plain javascript) to test if a webpage on another domain is available?
Due to same origin policy restriction you need a proxy/bridge on your server unless the remote server implements JSONP which obviously we cannot assume for the general case.
You can create an <img> tag that points to an existing image on the external domain.
If the onerror event fires, image, and perhaps the entire site, is down.
If it fires after 5 seconds or so, it probably timed out, so the entire site is likely to be down.
Yes, you need to use a proxy script on your server. JavaScript cannot be used in a browser to request resources across domains, as per the same-origin policy.
I need to find a way to notify a 3rd party website about an user action on my website. A server sided connection is not desired. Hashing with private keys is used to sign the request so users can't abuse it.
My question is how I can send this request safely.
tracking image: XSA possible
iframe: XSA, frame breaker
script include: evilness at its best
JSONP (with jQuery): ??
others?
Does someone know if it's possible to inject Javascript in JSONP answers? I mean to bypass browser boundaries JSONP is Javascript that calls a function with the JSON as parameter but it could also contain other javascript calls. Does jQuery somehow check if there is malicious content in jsonp callbacks?
If you only need to target modern browsers, and you control all the domains, you can create an HTTP access control policy to allow them to communicate with each other. However, since that doesn't appear to be the case, you're going to be stuck with JSONP.
It's funny that you mention "script include" as "evilness at its best", because that's exactly what JSONP is. Since, until recently, browsers were incapable of cross-domain requests, the only way to get anything from a 3rd-party client-side was to include a script from that 3rd party. JSONP simply takes advantage of this workaround returning the JSON inside of a function definition, which your script can then call to get the included data.