Prevent XSS “on” Attribute Attacks in CKEditor 3.x

By  on  

CKEditor 3.x had issues with XSS /security issues with on attributes.  For example, you could trigger malicious code via an onerror attribute -- ouch!  Of course the problem has been fixed in CKEditor 4 but upgrading can be an issue if you have custom plugins.  Here's how the issue can be solved!

The JavaScript

We'll use prototype monkey-patching to accomplish this security fix:

// Prevent bad on* attributes (https://github.com/ckeditor/ckeditor-dev/commit/1b9a322)
var oldHtmlDataProcessorProto = CKEDITOR.htmlDataProcessor.prototype.toHtml;
CKEDITOR.htmlDataProcessor.prototype.toHtml = function(data, fixForBody) {
    function protectInsecureAttributes(html) {
        return html.replace( /([^a-z0-9<\-])(on\w{3,})(?!>)/gi, '$1data-cke-' + CKEDITOR.rnd + '-$2' );
    }
    
    data = protectInsecureAttributes(data);
    data = oldHtmlDataProcessorProto.apply(this, arguments);
    data = data.replace( new RegExp( 'data-cke-' + CKEDITOR.rnd + '-', 'ig' ), '' );

    return data;
};

The toHtml method of CKEDITOR.htmlDataProcessor is modified to remove the troublesome on attributes during HTML render within the editor, but the attributes are indeed kept within the editor contents value and will display when you switch CKEditor to source mode.  Problem solved!

Recent Features

  • By
    An Interview with Eric Meyer

    Your early CSS books were instrumental in pushing my love for front end technologies. What was it about CSS that you fell in love with and drove you to write about it? At first blush, it was the simplicity of it as compared to the table-and-spacer...

  • By
    5 More HTML5 APIs You Didn&#8217;t Know Existed

    The HTML5 revolution has provided us some awesome JavaScript and HTML APIs.  Some are APIs we knew we've needed for years, others are cutting edge mobile and desktop helpers.  Regardless of API strength or purpose, anything to help us better do our job is a...

Incredible Demos

  • By
    Event Delegation with MooTools

    Events play a huge role in JavaScript. I can't name one website I've created in the past two years that hasn't used JavaScript event handling on some level. Ask yourself: how often do I inject elements into the DOM and not add an...

  • By
    Chris Coyier&#8217;s Favorite CodePen Demos

    David asked me if I'd be up for a guest post picking out some of my favorite Pens from CodePen. A daunting task! There are so many! I managed to pick a few though that have blown me away over the past few months. If you...

Discussion

  1. I also noticed that, so what next ????

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!