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.
This is a walkthrough of a hard-to-reproduce bug I found in Slack a few months back. Even though the payload was only working because of a legacy migration, by utilizing Python’s AppKit to insert data into Chrome’s rich text format clipboard, I was able to add and modify the XSS payload already inside Slack.
Developers are leaking access tokens for Slack widely on GitHub, in public repositories, support tickets and public gists. They are extremely easy to find due to their structure. It is clear that the knowledge about what these tokens can be used for with malicious intent is not on top of people’s minds…yet. The Detectify team shows the impact, with examples, and explains how this could be prevented.