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A security hole deemed "critical" by Microsoft could allow an attacker to run code of their choice on a vulnerable PC by exploiting a problem in Microsoft's MSN Chat, MSN Messenger, and Exchange Instant Messenger products, Microsoft says in a security alert Wednesday. The company has patched the vulnerability.

The vulnerability comes in an ActiveX control that allows users of MSN Chat, Messenger, and Exchange Instant Messenger to group together in a single "space" to chat in real time, Microsoft says.

A buffer overflow in the component could allow an attacker to assault the flaw and run any code against vulnerable machines, Microsoft says. An attacker could also exploit the vulnerability using a malicious Web site or through e-mail formatted in HTML, the Redmond, Washington, company says.

The ActiveX component that allows the chat feature is included with MSN Messenger 4.5 and higher and Exchange Instant Messenger, Microsoft says. MSN Chat users and Windows XP users, however, would only be vulnerable to the bug if they had downloaded and installed the chat component separately, Microsoft says.

E-Mail Protection

E-mail users are protected if they have downloaded the Outlook E-Mail Security Update or are running Outlook 2002 or Outlook Express, the company says.

More information about the vulnerability, and a patch to repair it, are available on Microsoft's Web site.

Microsoft's chat problem comes three days after the announcement of a security hole in rival America Online's Instant Messenger application that could have allowed attackers to run programs of their choice. That flaw has since been patched by AOL.