According to the report, the company identified a security issue and following an investigation linked the credentials to a hack it suffered in 2015 when hackers were able to insert keylogging code into its software that was capable of reading user's passwords as they entered them.
Back in 2015, hackers gained access to a Slack database that stored user profile information, including usernames and encrypted passwords, according to the company.
No financial data was exposed and Slack was quick to patch up the situation and enabled a two factor authentication as well as a password kill switch for the teams - this particular security measure allowed team admins to kick everyone out of a Slack room and force them to reset their passwords, whether they like it or not.
The organization reset the passwords after it discovered that a collection of user email addresses and password combinations had been undermined.
The Discord-for-Suits developer said on Thursday that it was resetting the passwords for roughly 1 per cent of its 10 million or so accounts after an investigation revealed that stolen credentials were being sold online.
Slack made a decision to perform today's password reset after receiving a tip about a trove of compromised user credentials.
Arriving as a tip through Slack's bug bounty program, the stolen account credentials were originally thought to be the result of isolated malware infections or phishing operations.
Slack, the collaboration app popular with developers and anyone exasperated by Skype, has reset thousands of user passwords after belatedly admitting that a March 2015 security breach also compromised credentials.
The 100,000-plus accounts that Slack reset were all created before March 2015. Users who have changed their password since March 2015 were also unaffected.
If you were impacted by Slack's 2015 data breach and received an email from the provider, make sure you check your access logs within the application to see what IPs were used to access your account.
"We have no reason to believe that any of these accounts were compromised, but we believe that this precaution is worth any inconvenience the reset may cause".