In a recent announcement, Slack, the online services company, made it known to everyone that it was going to reset all passwords from accounts that were not secured after the data breach the company suffered in 2015.

Apparently, this announcement was followed by various reactions and questions from users, and in turn, Slack informed that this reset will affect approximately 1% of accounts. In addition, a company stated that this process only affects users who created accounts before March 2015, who have not changed their password since then, and whose account does not require logging in through a single-sign-on (SSO) provider.
For the record, in March 2015, the company detected unauthorized access to a database containing information about user accounts, i.e. names, emails, passwords, personal phone numbers, etc. Naturally, this access was carried out by hackers, who also provided the company's system with code. Fortunately, they were unable to steal financial information.

Slack then restored the passwords of some users, but recommended that the rest immediately change their passwords and enable 2FA. Recently, the company realized that other users' data may have been compromised. Usually this is achieved either by malware or by reusing the same passwords across other services. The latter seems to be the more likely scenario, according to the company.
Slack immediately verified the accounts of such users, reset their passwords , and notified each user who was affected in any way with a corresponding announcement.
For now, Slack is still investigating the incident with the support of the appropriate team of people and will provide further updates and information when the process is complete.
