A newly discovered vulnerability in a widely used software library is causing mayhem on the Internet, forcing cyber defenders to scramble as hackers rush to exploit the weakness.

The vulnerability, known as Log4j, comes from a popular open source product that helps software developers track changes in applications that they build. It is so popular and embedded across so many companies’ programs that security executives expect widespread abuse.

“The Apache Log4j Remote Code Execution Vulnerability is the single biggest, most critical vulnerability of the last decade,” said Amit Yoran, chief executive of Tenable, a network security firm, and the founding director of the U.S. Computer Emergency Readiness Team.

The U.S. government on Friday sent a warning to the private sector about the Log4j vulnerability and the looming risk it poses.

“The Apache Log4j Remote Code Execution Vulnerability is the single biggest, most critical vulnerability of the last decade”—Tenable CEO Amit Yoran

Much of the software affected by Log4j, which bears names like Hadoop or Solr, may be unfamiliar to the public at large. But as with the SolarWinds program at the center of a massive Russian espionage operation last year, the ubiquity of these workhorse programs makes them ideal jumping-off points for digital intruders.

Juan Andres Guerrero-Saade, principal threat researcher with cybersecurity firm SentinelOne, called it “one of those nightmare vulnerabilities that there’s pretty much no way to prepare for.”

While a partial fix for the vulnerability was released on Friday by Apache, the maker of Log4j, affected companies and cyber defenders will need time to locate the vulnerable software and properly implement patches.

In practice, this flaw allows an outsider to enter active code into the record-keeping process. That code then tells the server hosting the software to execute a command giving the hacker control.

The issue was first publicly disclosed by a security researcher working for Chinese technology company Alibaba Group Holding Ltd, Apache noted in its security advisory.

So far, no major disruptive cyber incidents have been publicly documented as a result of the vulnerability, but researchers are seeing an alarming uptick in hacking groups trying to take advantage of the bug for espionage.

What many experts now fear is that the bug could be used to deploy malware that either destroys data or encrypts it, like what was used against U.S. pipeline operator Colonial Pipeline Co in May, which led to shortages of gasoline in some parts of the United States.

Guerrero-Saade said his firm already had seen Chinese hacking groups moving to take advantage of the vulnerability.

U.S. cybersecurity firms Mandiant and Crowdstrike also said they found sophisticated hacking groups leveraging the bug to breach targets. Mandiant described those hackers as “Chinese government actors” in an email to Reuters.

(Reporting by Christopher Bing and Raphael Satter in Washington and Joseph Menn in San Francisco Editing by Matthew Lewis)