Microsoft is urging Windows users to immediately install an update after security researchers found a serious vulnerability in the operating system.

The security flaw, known as PrintNightmare, affects the Windows Print Spooler service. Researchers at cybersecurity company Sangfor accidentally published a how-to guide for exploiting it.

The researchers tweeted in late May that they had found vulnerabilities in Print Spooler, which allows multiple users to access a printer.

They published a proof-of-concept online by mistake and subsequently deleted it -- but not before it was published elsewhere online, including developer site GitHub.