Yale New Haven Health has officially confirmed that a March cybersecurity breach impacted over 5.5 million individuals, making it one of the largest healthcare data incidents reported in 2025.
The figure, now listed on the U.S. Department of Health and Human Services’ Office for Civil Rights (HHS OCR) breach portal, significantly expands the scope of the previously disclosed event.
The breach was initially reported on March 11, 2025, following an incident identified on March 8 that disrupted IT systems across Yale New Haven Health System (YNHHS). At the time, officials described the situation as a cybersecurity event that did not compromise electronic medical records or impact clinical operations. The health system swiftly engaged Mandiant, a leading cybersecurity firm, to contain the intrusion and investigate the extent of the breach. Now, more than a month later, the scope of the incident has been clarified through the federal breach notification process.
According to YNHHS’s formal disclosure, the attack involved unauthorized access to network servers and resulted in the exfiltration of sensitive data. While treatment details and financial account information were not involved, the exposed data varied by individual and may have included names, dates of birth, contact details, demographic information such as race or ethnicity, Social Security numbers, patient types, and medical record numbers.
Yale New Haven Health System is Connecticut’s largest healthcare provider, encompassing five hospitals, a broad network of specialists, and Northeast Medical Group. The system is closely affiliated with Yale University and the Yale School of Medicine, positioning it as a major academic healthcare hub in New England. Given its scale, the breach potentially affects a wide demographic across multiple facilities and care networks.
The breach was formally recorded on the HHS OCR portal, listing 5,556,702 individuals as affected by a hacking/IT incident targeting a network server. While the organization had already issued a substitute notice and began notifying patients on April 14, the public acknowledgment of the breach’s full scale comes at a time of heightened scrutiny over data security in healthcare. Just one day earlier, Blue Shield of California revealed that 4.7 million individuals were affected by a separate privacy lapse involving misconfigured analytics tools — highlighting a concerning trend of large-scale healthcare data exposures in 2025.
YNHHS has emphasized that there is no evidence to date of the compromised information being misused. Nonetheless, for patients whose Social Security numbers were included in the breach, the health system is offering complimentary credit monitoring and identity protection services. The organization has also established a dedicated call center to support those impacted and has reiterated its commitment to reinforcing its cybersecurity infrastructure.
To mitigate potential risks, affected individuals are urged to remain vigilant for signs of identity theft, monitor their credit reports, and report any suspicious activity. Reviewing healthcare statements for unauthorized services is also recommended.
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