How it was found

Description

Vulnerable parameter: user Vulnerable script: https://wakatime.com/static/html/github-btn.html Vulnerable code:

  var params = function () {
    var vars = [], hash;
    var hashes = window.location.href.slice(window.location.href.indexOf("?") + 1).split("&");
    for(var i = 0; i < hashes.length; i++) {
      hash = hashes[i].split('=');
      vars.push(hash[0]);
      vars[hash[0]] = hash[1];
    }
    return vars;
  }()
  var user = params.user,
      repo = params.repo,
      type = params.type,
 ...
  } else if (type == "follow") {
    mainButton.className += " github-me";
    text.innerHTML = "Follow @" + user;

PoCs

PoC #1 HTML Injection for Chrome, Internet Explorer

https://wakatime.com/static/html/github-btn.html#&user=<h1><marquee>HTML<br>HTML<br>HTML<br>HTML<br>HTML<br>HTML<br>&type=follow

PoC #2 Using this script it is also possible to use DOM Based XSS in Internet Explorer, as I did in these reports #200753 #200826 But since the X-Frame-Options: SAMEORIGIN header is used for all HTTP responses, this could not be done in this case.

Another vulns

Also, this script has potential vulnerabilities using user controlled parameters in the path to the JS script. But for the exploitation it requires additional vulnerability on api.github.com (for example, Open Redirect or user controlled output).

  function jsonp(path) {
    var el = document.createElement('script');
    el.src = path + '?callback=callback';
    head.insertBefore(el, head.firstChild);
  }
...
  if (type == 'follow') {
    jsonp('https://api.github.com/users/' + user);
  } else {
    jsonp('https://api.github.com/repos/' + user + '/' + repo);
  }

PoC

https://wakatime.com/static/html/github-btn.html?#&user=../../another/endpoint&repo=../../another/endpoint&type=fork

Result

https://api.github.com/another/endpoint?callback=callback