Victims of a massive global hack of Microsoft email server software — estimated in the tens of thousands by cybersecurity responders — hustled Monday to shore up infected systems and try to diminish chances that intruders might steal data or hobble their networks.

The White House has called the hack an “active threat” and said senior national security officials were addressing it.

The breach was discovered in early January and attributed to Chinese cyber spies targeting U.S. policy think tanks. Then in late February, five days before Microsoft issued a patch on March 2, there was an explosion of infiltrations by other intruders, piggybacking on the initial breach. Victims run the spectrum of organizations that run email servers, from mom-and-pop retailers to law firms, municipal governments, health care providers and manufacturers.

While the hack doesn’t pose the kind of national security threat as the more sophisticated SolarWinds campaign, which the Biden administration blames on Russian intelligence officers, it can be an existential threat for victims who didn’t install the patch in time and now have hackers lingering in their systems. The hack poses a new challenge for the White House, which even as it prepares to respond to the SolarWinds breach, must now grapple with a formidable and very different threat from China.

“I would say it’s a serious economic security threat because so many small companies out there can literally have their business destroyed through a targeted ransomware attack,” said Dmitri Alperovitch, former chief technical officer of the cybersecurity firm CrowdStrike.

He blames China for the global wave of infections that began February 26, though other researchers say it’s too early to confidently attribute them. It’s a mystery how those hackers got wind of the initial breach because no one knew about this except a few researchers, Alperovitch said.

After the patch was released, a third wave of infections began, a piling on that typically occurs in such cases because Microsoft dominates the software market and offers a single point of attack.

Cybersecurity analysts trying to pull together a complete picture of the hack said their…