The personal details of more than 10.6 million users who stayed at MGM Resorts hotels have been published on a hacking forum this week.

Besides details for regular tourists and travelers, included in the leaked files are also personal and contact details for celebrities, tech CEOs, reporters, government officials, and employees at some of the world's largest tech companies.

ZDNet verified the authenticity of the data today, together with a security researcher from Under the Breach, a soon-to-be-launched data breach monitoring service.

A spokesperson for MGM Resorts confirmed the incident via email.

WHAT WAS EXPOSED

According to our analysis, the MGM data dump that was shared today contains personal details for 10,683,188 former hotel guests.

Included in the leaked files are personal details such as full names, home addresses, phone numbers, emails, and dates of birth.

ZDNet reached out to past guests and confirmed they stayed at the hotel, along with their timeline, and the accuracy of the data included in the leaked files.

We got confirmation from international business travelers, reporters attending tech conferences, CEOs attending business meetings, and government officials traveling to Las Vegas branches.

MGM RESORTS SAYS THEY NOTIFIED CUSTOMERS LAST YEAR

Once we verified the data, ZDNet also reached out to MGM Resorts.

Within an hour after we reached out to the company, we were in a conference call with the hotel chain's security team. Within hours, the MGM Resorts team was able to verify the data and track it to a past security incident.

An MGM spokesperson told ZDNet the data that was shared online this week stems from a security incident that took place last year.

"Last summer, we discovered unauthorized access to a cloud server that contained a limited amount of information for certain previous guests of MGM Resorts," MGM told ZDNet.

"We are confident that no financial, payment card or password data was involved in this matter."

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The hotel chain said it promptly notified all impacted hotel guests in accordance with applicable state laws.

While we were not able to track down one of these notifications personally, some users appear to have posted online about receiving one in August last year.

Also, MGM Resorts told us it retained two cybersecurity forensics firms to conduct an internal investigation into last year's server exposure.

"At MGM Resorts, we take our responsibility to protect guest data very seriously, and we have strengthened and enhanced the security of our network to prevent this from happening again," the company said.

According to Irina Nesterovsky, Head of Research at threat intel firm KELA, the data of MGM Resorts hotel guests had been shared in some closed-circle hacking forums since at least July, last year. The hacker who released this information is believed to have an association, or be a member of GnosticPlayers, a hacking group that has dumped more than one billion user records throughout 2019.

A POTENTIAL DANGER OF SIM SWAPPING AND SPEAR-PHISHING

However, while MGM's security incident went under the radar last year, the publication of this data dump on a very popular and openly accessibly hacking forum this week has brought it to many other hackers' attention.

Under the Breach, the company that spotted this leak and notified this reporter was the one who highlighted the highly sensitive nature of the breach.

The leaked data is a treasure trove for contact details for many high-profile users, working for big tech firms and governments all over the world. These users now face a higher risk of receiving spear-phishing emails, and being SIM swapped, Under the Breach told ZDNet.

Twitter CEO Jack Dorsey, pop star Justin Bieber, and DHS and TSA officials are some of the big names Under the Breach spotted in the leaked files.

MGM Resorts told ZDNet that the data was old. We can confirm this statement as from all the hotel guests we called today, none stayed at the hotel past 2017. Some of the phone numbers we called were disconnected, but many were also valid, and the right person answered the phone.

The size and the severity of this MGM Resorts security incident pale in comparison to the massive data breach that impacted Marriott hotels in 2017 when the details of hundreds of millions of users were stolen by Chinese state-sponsored hackers.