Yahoo Inc. has issued a "highly critical" update to its popular Messenger online chat software after finding a flaw that could let an attacker hijack a computer running the program.

An alert issued Friday by Danish security company Secunia stated that the problem exists in all versions of the tool for Windows PCs that were downloaded before Nov. 2.

Users are being advised to update to the latest version of the software, Yahoo Messenger 8.1, available from Yahoo's website.

The problem, discovered by Yahoo, could let an attacker remotely run software on a machine over the internet, crash programs such as Internet Explorer, or force a person to be logged out of chat or the Messenger tool itself, according to the internet company's own alert, issued Dec. 8.

"These impacts could only be possible if an attacker is successful in prompting someone to view malicious HTML code, most likely executed by getting a person to visit their web page," Yahoo said in its security alert. "To our knowledge, there have been no known executable code exploits related to this issue."

The flaw lies in Messenger's ActiveX control, which can be forced to experience a so-called buffer overflow in which too much data is stored in a temporary area, causing the computer to crash or expose the system to attackers. ActiveX is a set of Microsoft technologies used to share information among different programs.

Secunia called the flaw 'highly critical" — its second-most severe rating on a five-level scale.