The bug is found in the function jQuery.ajaxSetup, jquery-1.6.2.js - lines 6901 - 6919

Before v 1.5 function jQuery.ajaxSetup used to merge custom settings for each call with the standard jQuery.ajaxSettings using a shallow copy, for example:

jQuery.extend( jQuery.ajaxSettings, settings );

Beginning with v 1.5 function jQuery.ajaxSetup started using deep copy (recursive) flag in it, for example:

jQuery.extend( true, target, jQuery.ajaxSettings, settings );

There are 2 problems with this approach.

First - when context is supplied within the settings, then if for example it is a "body" node of the document, then the deep copy would spend time making copy of an entire document into the target. And it is unnecessary, because one of the next steps in the original code goes and replaces the context and url with the references to the original objects. But until the copied data is not picked up by the garbage collector, it consumes memory, not even speaking of some extra resources spent for each ajax request.

Second - it is possible that as a context, an object containing circular references could be passed. That will lead to out of stack space errors.

I've created a simple jsfiddle case to demo the error.

http://jsfiddle.net/MBwUx/3/

It produces errors on at least IE8, Firefox 5, Chrome12. I have not tested on other browsers but in my opinion the problem will 100% reproduce in any browser because it is not a browser-specific issue.

In my jQuery copy I rewrote the function in the following way. Sorry I did not use GitHub:

/*  Creates a full fledged settings object into target
    with both ajaxSettings and settings fields.
    If target is omitted, writes into ajaxSettings.
*/
ajaxSetup: function ( target, settings ) {
    /*  Create an empty object to hold an independent copy of 
        jQuery.ajaxSettings if target is provided
    */
    
    var _ajaxSettings = {};

    /* no target provided --> assign settings and make jQuery.ajaxSettings a target */
    
    if (!settings) {
        settings = target;
        target = jQuery.ajaxSettings;
    }
    /*  else create a copy of jQuery.ajaxSettings so context and url 
        could be deleted from it without modifying jQuery.ajaxSettings itself
    */

    else {
        jQuery.extend(_ajaxSettings, jQuery.ajaxSettings);
    }
    /* Flatten fields we don't want deep extended */

    for (var field in { context: 1, url: 1 }) {
        if (field in settings) {
            target[field] = settings[field];
        } else if (field in _ajaxSettings) {
            target[field] = _ajaxSettings[field];
        }
        /* drop properties so they don't get involved in the jQuery.extend step */

        delete settings[field];
        delete _ajaxSettings[field];
    }

    jQuery.extend(true, target, _ajaxSettings, settings);

    return target;
},

Thank you.

Roman Tumaykin