The hackers behind the powerful set of digital intrusion tools exposed this week have racked up a worrying number of victims, the White House said Friday, the latest indication that the cyber espionage campaign targeting Microsoft Corp’s Exchange email software poses a serious threat.

“This is a significant vulnerability that could have far reaching impacts,” White House press secretary Jen Psaki told reporters. “We’re concerned that there’re a large number of victims.”

Wielding tools that exploited four previously unknown vulnerabilities, the allegedly Chinese group that Microsoft dubs “Hafnium” has been breaking into email servers since January, remotely and silently siphoning information from their inboxes without having to send a single malicious email or rogue attachment.

Few victims of the hackers have been made public so far. Microsoft said this week that targets included infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks, and non-governmental groups.

On Tuesday researchers at Dell Technologies’ Secureworks said the pace of break-ins began spiking overnight last Sunday, something others have read as an indication that the hackers ramped up their activity because they knew they were about to be exposed.

Much of the activity was concentrated in the United States, but victims have popped up around the world.

Reuters