Microsoft says a security gap in Internet Explorer could allow an attacker to take complete control of a computer if the user clicks on a link to a malicious website.
The vulnerability affects versions 6 through 11 of the Web browser.
Microsoft Corp. said Saturday that it was aware of "limited, targeted attacks" that tried to exploit the security gap. The company is working on a safety fix which it will provide in an upcoming software update.
In the meantime, Microsoft encourages customers to enable a firewall, apply all software updates and install anti-malware software.
A division of the Homeland Security Department says it is aware of the exploitation. It recommends users and administrators download software called the Enhanced Mitigation Experience Toolkit (EMET) from Microsoft, or use another browser until an update becomes available.