U.S. social media aggregation network Reddit Wednesday said its system has been hacked by a hacker who illegally accessed a variety of data including user emails, source code and internal files.

Reddit, the fifth most trafficked website in the U.S., said the hackers broke into its systems In June and stole all Reddit data from 2007 and before.

"On June 19, we learned that between June 14 and June 18, an attacker compromised a few of our employees' accounts with our cloud and source code hosting providers," Reddit Chief Technology Officer Christopher Slowe wrote in an official blog post.

A complete copy of an old database backup containing very early Reddit user data -- from the site' s launch in 2005 through May 2007 was illegally accessed, and most of the information breached in account credentials are user names, email passwords, and pubic or some private messages, he said.

He noted the attack was conducted by intercepting text messages that were supposed to reach Reddit's employees containing single-use login codes.

"We learned that SMS-based authentication is not nearly as secure as we would hope," Slowe wrote, urging every Reddit employees to move to token-based two-factor authentication.

While acknowledging the attack as a serious one, Slowe said the hacker did not get gain write access to Reddit systems and only accessed in a read-only manner some systems contained backup data, source code and other logs.

Reddit says that it will notify users whose current account password relates to credentials compromised in the breach, and will prompt those affected individuals to change their passwords.

The social media network said it has reported the issue to law enforcement and is cooperating with their investigation.

As of February 2018, Reddit had 542 million monthly visitors, ranking as the third most visited website in U.S. and the sixth in the world, according to Alexa Internet. 57.4 percent of its user base comes from the United States, followed by Britain at 7.5 percent and Canada at 6.3 percent.