https://www.theregister.co.uk/2019/12/16/slack_filesharing_vulnerability_post_sharing/

Security researchers have uncovered a flaw in messaging app Slack that allows a file shared in a private channel to be viewed by anyone in that workspace – even guests.

Folk from Israeli cloud security outfit Polyrize uncovered the vuln, that they say exposes files shared through the IRC-for-millennials application, which boasts millions of users.