"GEO implemented several containment and remediation measures to address the incident, restore its systems and reinforce the security of its networks and information technology systems," the company said.
GEO said it recovered its data but did not say if this meant restoring from backups or paying the ransomware gang to decrypt its files.
Exposed personal details could include name, address, date of birth, Social Security number, employee ID number, driver's license number, medical treatment information, and other health-related information.
The incident impacted only a small portion of the GEO Group's network, which includes 123 private prisons, processing centers, and community reentry centers in the United States, Australia, South Africa, and the United Kingdom.
US government contracts amounted for more than half of the GEO Group's 2019 revenue, according to the company's yearly 10-K form filed with the SEC.
The company's stock price fell 14% from $9.76 at the end of trading on Tuesday to $8.38 the next day, after GEO disclosed the incident.