TL;DR Combining response-type switching, invalid state and redirect-uri quirks using OAuth, with third-party javascript-inclusions has multiple vulnerable scenarios where authorization codes or tokens could leak to an attacker. This could be used in attacks for single-click account takeovers. Frans Rosén, Security Advisor at Detectify goes through three different scenarios found in the wild below.
TLDR; I was able to create a malicious page that would reconnect your Slack WebSocket to my own WebSocket to steal your private Slack token. Slack fixed the bug in 5 hours (on a Friday) and paid me $3,000 for it.
AddThis is a share button used by over a million sites. They were all vulnerable to XSS earlier this year. In my previous post I described the pitfalls of the postMessage API. This post will describe how I identified and exploited them on the AddThis widget.
The postMessage API is an alternative to JSONP, XHR with CORS headers and other methods enabling sending data between origins. It was introduced with HTML5 and like many other cross-document features it can be a source of client-side vulnerabilities.