Marriott revealed in November 2018 that an unknown third-party had gained access to its Starwood guest reservation system by exploiting an unpatched vulnerability from 2014.
Marriott noted that compromised information of its customers may include their contact details, loyalty account information, additional personal details such as gender, birthday day and month, and room as well as language preferences among others.
"At the end of February 2020, we identified that an unexpected amount of guest information may have been accessed using the login credentials of two employees at a franchise property", Marriott said in a statement.
The company first realized the breach in February, it said, and the hackers' actions are believed to have started in the middle of January.
When the firm learned of this activity, the login credentials were disabled, and the company began an investigation, before raising the level of monitoring and arranging resources to inform and assist guests.
In addition, Marriott Bonvoy members who had their information potentially exposed in the incident had their passwords disabled and will be requested to change their password on the next login, as well as prompted to enable multi-factor authentication. Phone numbers, email addresses and physical addresses were accessed, but details such as credit card information, account passwords or PIN numbers, passport information, ID numbers or driver's license numbers weren't accessed.
Marriott International notified affected guests on Tuesday via email, as well as set up call centers and a web portal for people to find out if they were impacted.
It is unclear whether the impact of the latest breach is limited to certain markets or customers across the globe.