Slack, the popular work communications app, reported on Friday that it had been hacked.
According to an internal investigation, the security breach occurred in February and lasted up to four days, affecting Slack's central user database containing usernames, email addresses, encrypted passwords and other sensitive information.
Although Slack said users' passwords were unreadable to the hackers, the startup admitted it found “suspicious activity” from a small, unspecified group of Slack user accounts, suggesting at least some data had been compromised.
"Since the compromised system was first discovered, we have been working 24 hours a day to methodically examine, rebuild and test each component of our system to ensure it is safe," the company said in an email to Slack users on Friday.
"We are very aware that our service is essential to many teams," added Slack's VP of policy and compliance strategy, Anne Toth, in a company blog post.
"We deeply regret this incident and apologize to you and to everyone who relies on Slack for the inconvenience."
As a result of the breach, Slack implemented two-factor authentication, a security protocol also offered by tech companies like Google for logging into Gmail. After Slack users enter their typical login information, they will be prompted to enter a one-time passcode sent to their phones.
While security breaches have become commonplace, the news may be particularly damaging for Slack, which counts many businesses and organizations among its 500,000 or so daily active users, including Mashable.
The six-year-old startup, which was cofounded run by Stewart Butterfield, recently signed a deal to raise an additional $160 million in funding from new investors including DST Global, Index Ventures, Venture Partners and Horizons Ventures.
The deal is expected to close in the next few weeks, according to a The Wall Street Journal report, valuing Slack at $2.76 billion -- more than twice the valuation it earned after raising $120 million five months earlier. It also propels Slack into the so-called billion dollar startup club, edging out companies such as Lyft, Instacart and DocuSign, and making Slack one of the highest valued business startups in the U.S.