Slack has started sending out emails informing users that their account passwords have been reset. Slack has stated that this is only affecting 1% of their users. 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 Slack’s workspaces. Slack also recommends that users enable 2-factor authentication on their accounts to make them more secure.
Source: https://www.bleepingcomputer.com/news/security/slack-resets-account-passwords-compromised-during-2015-hack/