Last week Microsoft issued an unscheduled security update that fixes a serious security vulnerability in Internet Explorer 9, 10, and 11.

According to Microsoft, this vulnerability is currently being exploited on the web, which means that malicious activity that takes advantage of the security hole has been observed.

Details of the vulnerability can be found in Microsoft’s Security Update Guide.

Anyone who still uses Internet Explorer for web browsing should install this update by running Windows Update in the Windows Control Panel or system settings.