Computer systems around the world are at risk of being hacked after Microsoft acknowledged a flaw in its operating system. The company released a security update to patch the bug, but experts say many systems may still be vulnerable.

The flaw, which Microsoft has been aware of for months, affects all versions of Windows operating system released in the past 17 years. It allows hackers to remotely take control of a computer, giving them access to its files, passwords and other sensitive information.

Microsoft says it has not seen any evidence that the flaw has been exploited, but security experts …