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POIf I store a static session token in JS for use with AJAX, will it be secure against CSRF?
 

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  1. POIf I store a static session token in JS for use with AJAX, will it be secure against CSRF?
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    <p>I've been reading about CSRF and XSS vulnerabilities for a few days now, and trying to come up with a solution that's 1) easy to implement and use, 2) uses Javascript to do a lot of heavy lifting, and 3) makes it virtually impossible to perform a CSRF attack against.</p> <p>I haven't seen a solution like the one I'm about to describe. I'm hoping this doesn't mean that it's leaky.</p> <p>I've come up with the following solution, based on my knowledge of AJAX and JS. This code assumes the user has passed through a login screen, and a session variable has been set on the server <strong>and</strong> in a cookie, with the same values. </p> <p>It's easier to paste the code in and document it, rather than explain what it's doing. This code would be run in the page the user sees immediately after logging in:</p> <pre><code>&lt;script&gt; // this is the constructor: function Controller(){ //the following 2 variables are private, and inaccessible via JS calls var secretToken; //this holds the session token, but cannot be read by the browser //returns the session token from the server var x = new ajaxObject('AJAX/retrieve_session_cookie.lasso'); x.callback = function(responseText, responseStatus){ secretToken = responseText; } //this is a private function, again inaccessible via JS calls function getCookie(){ x.update(); } //the following 2 functions are publicly accessible //just a test function to ensure that secretToken is invisible this.tell = function(){ alert(secretToken); } //privileged function that calls a private function, to load the token into a private variable this.initialize = function(){ getCookie(); } } E = new Controller(); E.initialize(); &lt;/script&gt; </code></pre> <p>The variable <code>secretToken</code> can't be read by the user, as it's a private member variable of the controller object. </p> <p>In <code>retrieve_session_cookie.lasso</code>, I'm checking for a valid session, and matching the session variable with the browser's cookie. If both these conditions are met, the session variable is returned in plain text, where it's set as <code>secretToken</code> in the object <code>E</code>. By double-checking to see if the cookie matches the session token, I would hope that it would be impossible obtain the session token via CSRF, as it can't forge a cookie. Typing in 'AJAX/retrieve_session_cookie.lasso' would return nothing, unless it was typed in by the user while in a valid session, and from the user's computer only.</p> <p>Also, now that my controller has local access to the session token, I could 'burn in' the session token with every AJAX request, so I don't even have to think about it passing a token anymore, each time an AJAX file is requested. All the AJAX objects and requests would be initialized as private members in the controller object's constructor, so nobody could access / modify the callback functions to disclose the session token. </p> <p>Passing the session token with every AJAX call would protect every other AJAX file, as they would all perform the same check for the cookie matching the session token before returning any data. And, in my programming, there would be one less variable to worry about.</p> <p>If I were to move ahead with a controller implemented this way, would there be ANY way for the token to be accessed / exploited, either by the user or a malicious coder via CSRF?</p>
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    1. USStickByAtlas
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    1. COAre you using ASP.NET server side? Is the issue you're trying to solve the fact that the WebService you're calling isn't session enabled? If so, you could just decorate your web service method with [WebMethod(EnableSession = true)]. Then you have access to the logged in user session.
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    2. COYou just said a user with a valid session can go to `AJAX/retrieve_session_cookie.lasso` and view the token. Other than that, it seems rather sound, but I'm sure someone with a reason to break your security, could still manage...
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    3. CO@Berhard : I'm actually programming in Lasso (it's an obscure server-side language). It does support sessions, so I thought it would be a good idea to set a session variable for an AJAX token, that was identical to a token value in the cookie.
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