Security experts around the world raced Friday to patch one of the worst computer vulnerabilities discovered in years, a critical flaw in open-source code widely used across industry and government in cloud services and enterprise software.

“I’d be hard-pressed to think of a company that’s not at risk,” said Joe Sullivan, chief security officer for Cloudflare, whose online infrastructure protects websites from malicious actors. Untold millions of servers have it installed, and experts said the fallout would not be known for several days.

New Zealand’s computer emergency response team was among the first to report that the flaw in a Java-language utility for Apache servers used to log user activity was being “actively exploited in the wild” just hours after it was publicly reported Thursday and a patch released.