At least 10 different hacking groups are using a recently discovered flaw in Microsoft Corp’s mail server software to break in to targets around the world, cybersecurity company ESET said in a blog post on Wednesday. The breadth of the exploitation adds to the urgency of the warnings being issued by authorities in the United States and Europe about the weaknesses found in Microsoft’s Exchange software. The security holes in the widely used mail and calendaring solution leave the door open to industrial-scale cyber espionage, allowing malicious actors to steal emails virtually at will from vulnerable servers. Tens of thousands of organizations have already been compromised, Reuters reported last week, and new victims are being made public daily. Earlier on Wednesday, for example, Norway’s parliament announced data had been “extracted” in a breach linked to the Microsoft flaws. Germany‘s cybersecurity watchdog agency also said on Wednesday two federal authorities had been affected by the hack, although it declined to identify them.