(Reuters) -Uber Technologies Inc said on Thursday it was investigating a cybersecurity incident, after a report that its network was breached, forcing the company to shut several internal communications and engineering systems.

A hacker compromised an employee’s account on workplace messaging app Slack and used it to send a message to Uber employees announcing that the company had suffered a data breach, according to a New York Times report https://nyti.ms/3QMveIu on Thursday that cited an Uber spokesperson.

Cybersecurity has been an issue for Uber in the past. It suffered a significant hack in 2016 that exposed the personal information of about 57 million of its customers and drivers.

Shares of the ride-hailing firm were down 5% on Friday, amid broader U.S. market declines. [.N]

It appeared the hacker was able to gain access to other internal systems, posting an explicit photo on an internal information page for employees, the Times report added.

“We are in touch with law enforcement and will post additional updates here as they become available,” Uber said in a tweet https://bit.ly/3qHx2rv, without providing further details.

Uber employees were instructed to not use Slack, which is owned by Salesforce Inc, according to the report. Other internal systems were inaccessible too.

Slack said in a statement to Reuters that the company was investigating the incident and that there was no evidence of a vulnerability inherent to its platform.

“I announce I am a hacker and Uber has suffered a data breach,” the message read, and went on to list several internal databases that were allegedly compromised, the report added.

A person assumed responsibility for the hack and told the paper that he had sent a text message to an Uber employee claiming to be a corporate IT person.

The worker was persuaded to hand over a password that allowed the hacker to gain access to Uber’s systems, the report said.

Uber Chief Executive Officer Dara Khosrowshahi, who took charge a year after the 2016 hack, fired the then chief security officer, who was later charged with trying to cover up the breach.

A U.S. judge last month dismissed the three wire fraud charges against Joseph Sullivan although he still faces two charges of obstructing a U.S. Federal Trade Commission proceeding and failing to report a felony.

Uber’s HackerOne account that the company uses for its bug bounty program, through which the hacker reportedly gained access, appeared to be disabled.

A bounty program is generally seen as a tool to improve security and provide an incentive for hackers to stay within the law.

(Reporting by Shubham Kalia, Maria Ponnezhath and Nivedita Balu in Bengaluru; editing by Uttaresh.V, Rashmi Aich, Saumyadeb Chakrabarty, Kirsten Donovan)