Patients receiving care at several tribal healthcare clinics in California have been warned that a cyber incident led to the exposure of both personal identification details and private medical information. The clinics are operated by a regional health organization that runs multiple facilities across the Sierra Foothills and primarily serves American Indian communities in that area.
A ransomware group known as Rhysida has publicly claimed responsibility for a cyberattack that took place in November 2025 and affected the MACT Health Board. The organization manages several clinics in the Sierra Foothills region of California that provide healthcare services to Indigenous populations living in nearby communities.
In January, the MACT Health Board informed an unspecified number of patients that their information had been involved in a data breach. The organization stated that the compromised data included several categories of sensitive personal information. This exposed data may include patients’ full names and government-issued Social Security numbers. In addition to identity information, highly confidential medical details were affected. These medical records can include information about treating doctors, medical diagnoses, insurance coverage details, prescribed medications, laboratory and diagnostic test results, stored medical images, and documentation related to ongoing care and treatment.
The cyber incident caused operational disruptions across MACT clinic systems starting on November 20, 2025. During this period, essential digital services became unavailable, including phone communication systems, platforms used to process prescription requests, and scheduling tools used to manage patient appointments. Telephone services were brought back online by December 1. However, as of January 22, some specialized imaging-related services were still not functioning normally, indicating that certain technical systems had not yet fully recovered.
Rhysida later added the MACT Health Board to its online data leak platform and demanded payment in cryptocurrency. The amount requested was eight units of digital currency, which was valued at approximately six hundred sixty-two thousand dollars at the time the demand was reported. To support its claim of responsibility, the group released sample files online, stating that the materials were taken from MACT’s systems. The files shared publicly reportedly included scans of passports and other internal documents.
The MACT Health Board has not confirmed that Rhysida’s claims are accurate. There is also no independent verification that the files published by the group genuinely originated from MACT’s internal systems. At this time, it remains unclear how many individuals received breach notifications, what method was used by the attackers to access MACT’s network, or whether any ransom payment was made. The organization declined to provide further information when questioned.
In its written notification to affected individuals, MACT stated that it experienced an incident that disrupted its information technology operations. The organization reported that an internal investigation found that unauthorized access occurred to certain files stored on its systems during a defined time window between November 12 and November 20, 2025.
The health organization is offering eligible individuals complimentary identity monitoring services. These services are intended to help patients detect possible misuse of personal or financial information following the exposure of sensitive records.
Rhysida is a cybercriminal group that first became active in public reporting in May 2023. The group deploys ransomware designed to both extract sensitive data from victim organizations and prevent access to internal systems by encrypting files. After carrying out an attack, the group demands payment in exchange for deleting stolen data and providing decryption tools that allow victims to regain access to locked systems. Rhysida operates under a ransomware-as-a-service model, in which external partners pay to use its malware and technical infrastructure to carry out attacks and collect ransom payments.
The group has claimed responsibility for more than one hundred confirmed ransomware incidents, along with additional claims that have not been publicly acknowledged by affected organizations. On average, the group’s ransom demands amount to several hundred thousand dollars per incident.
A significant portion of Rhysida’s confirmed attacks have targeted hospitals, clinics, and other healthcare providers. These healthcare-related incidents have resulted in the exposure of millions of sensitive records. Past cases linked to the group include attacks on healthcare organizations in multiple U.S. states, with ransom demands ranging from over one million dollars to several million dollars. In at least one case, the group claimed to have sold stolen data after a breach.
Researchers tracking cybersecurity incidents have recorded more than one hundred confirmed ransomware attacks on hospitals, clinics, and other healthcare providers across the United States in 2025 alone. These attacks collectively led to the exposure of nearly nine million patient records. In a separate incident reported during the same week, another healthcare organization confirmed a 2025 breach that was claimed by a different ransomware group, which demanded a six-figure ransom payment.
Ransomware attacks against healthcare organizations often involve both data theft and system disruption. Such incidents can disable critical medical systems, interfere with patient care, and create risks to patient safety and privacy. When hospitals and clinics lose access to digital systems, staff may be forced to rely on manual processes, delay or cancel appointments, and redirect patients to other facilities until systems are restored. These disruptions can increase operational strain and place patients and healthcare workers at heightened risk.
The MACT Health Board is named after the five California counties it serves: Mariposa, Amador, Alpine, Calaveras, and Tuolumne. The organization operates approximately a dozen healthcare facilities that primarily serve American Indian communities in the region. These clinics provide a range of services, including general medical care, dental treatment, behavioral health support, vision and eye care, and chiropractic services.
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