BOSTON (AP) — Federal agencies warned that cybercriminals could unleash a wave of data-scrambling extortion attempts against the U.S. health care system, an effort that, if successful, could paralyze hospital information systems just as nationwide cases of COVID-19 are spiking.

In a joint alert Wednesday, the FBI and two federal agencies said they had credible information of “an increased and imminent cybercrime threat” to U.S. hospitals and health care providers. The alert said malicious groups are targeting the sector with attacks aiming for “data theft and disruption of healthcare services.”

The impact of the expected attack wave is difficult to assess.

It involves a particular strain of ransomware, which scrambles a target’s data into gibberish until they pay up. Previous such attacks on health care facilities have impeded care and, in one case in Germany, led to the death of a patient, but such consequences are still rare.

The federal warning itself could help stave off the worst consequences, either by leading hospitals to take additional precautions or by expanding efforts to knock down the systems cybercriminals use to launch such attacks.

The offensive coincides with the U.S. presidential election, although there is no immediate indication the cybercriminals involved are motivated by anything but profit. The federal alert was co-authored by the Department of Homeland Security and the Department of Health and Human Services.

Independent security experts say the ransomware, called Ryuk, has already impacted at least five U.S. hospitals this week and could potentially affect hundreds more. Four health care institutions have been reported hit by ransomware so far this week, three belonging to the St. Lawrence Health System in upstate New York and the Sky Lakes Medical Center in Klamath Falls, Oregon.

Sky Lakes acknowledged the ransomware attack in an online statement, saying it had no evidence that patient information was compromised. It said emergency and urgent care “remain available.”

The St. Lawrence system also acknowledged a Tuesday ransomware attack, noting in a statement released Thursday that no patient or employee data appeared to have been accessed or compromised. Matthew Denner, the emergency services director for St. Lawrence County, told the Adirondack Daily Enterprise that the hospital owner instructed the county to divert ambulances from two of the affected hospitals for a few hours Tuesday. The company did not return requests for comment on that report.

Alex Holden, CEO of Hold Security, which has been closely tracking Ryuk for more than a year, said the attack could be unprecedented in magnitude for the U.S. In a statement, Charles Carmakal, chief technical officer of the security firm Mandiant, said the cyberthreat could be the “most significant” the country has ever seen.

The U.S. has seen a plague of ransomware over the past 18 months or so, with major cities from Baltimore to Atlanta hit and local governments and schools walloped especially hard.

In September, a ransomware attack hobbled all 250 U.S. facilities of the hospital chain Universal Health Services, forcing doctors and nurses to rely on paper and pencil for record-keeping and slowing lab work. Employees described chaotic conditions impeding patient care, including mounting emergency room waits and the failure of wireless vital-signs monitoring equipment.

Also in September, the first known fatality related to ransomware occurred in Duesseldorf, Germany, when an IT system failure forced a critically ill patient to be routed to a hospital in another city.

Holden said the Russian-speaking group behind recent attacks was demanding ransoms well above $10 million per target and that criminals involved on the dark web were discussing plans to try to infect more than 400 hospitals, clinics and other medical facilities.

Carmakal said the criminal gang “is deliberately targeting and disrupting U.S. hospitals, forcing them to divert patients to other healthcare providers” and producing prolonged delays in critical care.

While no one has proven suspected ties between the Russian government and gangs that use the Trickbot platform that distributes Ryuk and other malware, Holden said he has “no doubt that the Russian government is aware of this operation.” Microsoft has been engaged since early October in trying to knock Trickbot offline.

Dmitri Alperovitch, co-founder and former chief technical officer of the cybersecurity firm CrowdStrike, said there are “certainly lot of connections between Russian cyber criminals and the state,” with Kremlin-employed hackers sometimes moonlighting as cyber criminals.

Increasingly, ransomware criminals are stealing data from their targets before encrypting networks, using it for extortion. They often sow the malware weeks before activating it, waiting for moments when they believe they can extract the highest payments, said Brett Callow, an analyst at the cybersecurity firm Emsisoft.

A total of 59 U.S. health care providers or systems have been impacted by ransomware in 2020, disrupting patient care at up to 510 facilities, Callow said.

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