From bleepingcomputer.com
Slack has started sending out emails informing users that their account passwords have been reset due to being compromised during the Slack 2015 security incident. Slack has stated that this is only affecting 1% of their users.
In 2015, Slack was hacked and an unauthorized user was able to gain access to their infrastructure, including a database containing hashed passwords. The attackers also injected a script that would capture passwords in plain text as they were entered by users logging into their workspaces.