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Microsoft has released a security alert acknowledging a serious security hole in its Outlook Express e-mail client. The vulnerability, found in Outlook Express versions 5.5 and 6.0, could allow a remote attacker to take control of PCs running Outlook Express using malicious code embedded in an e-mail message.

Microsoft rated the severity of the flaw as critical for end users, but low for both Internet and intranet servers. The company also issued a patch for the vulnerability.

Outlook Express is a simplified version of Microsoft's Outlook e-mail application that is distributed with many versions of Microsoft Windows. Microsoft's announcement of the most recent Outlook Express vulnerability is the 58th security alert the company has posted since the beginning of the year.

Buffer Overflow Again

According to a security alert posted on Microsoft's Web site, the vulnerability was discovered in Outlook Express code that is used to support Secure/Multipurpose Internet Mail Extensions (S/MIME), an e-mail security standard that allows Internet users to send and receive encrypted e-mail messages.

Ironically, the security hole was found in code that is used by Outlook Express to generate a message warning users that problems occurred when trying to verify the authenticity of an incoming e-mail. According to the Microsoft alert, the code to generate the error message can be used to create a buffer overflow condition on the machine running Outlook Express.

Buffer overflow attacks allow attackers to circumvent program code designed to prevent an application from executing extraneous or "arbitrary" code. In this case, the buffer overflow would allow an attacker to place and execute code on the machine running Outlook Express, causing the e-mail client to fail or letting the attacker take advantage of the current user's permissions to execute commands.

And, because Outlook Express contains a "preview pane" feature that displays the contents of an e-mail message without requiring the user to open it, users could unwittingly launch an attack just by selecting the e-mail message in their inbox, displaying the e-mail's contents in the preview pane and running the malicious code.

New Versions Unaffected

Outlook Express version 5.5 Service Pack 2 (SP2) and 6.0 SP1 (the latter included in Windows XP SP1) are not affected by the vulnerability, nor is Microsoft Outlook, the e-mail client that is sold as part of Microsoft's popular Office suite of products, according to the Microsoft security alert and a statement from Aviram Jenik, a researcher at BeyondSecurity.com, which first discovered the vulnerability and reported it to Microsoft.

Microsoft has come under increased scrutiny for the security vulnerabilities of its products. In January, Microsoft Chair and Chief Software Architect Bill Gates announced a "trustworthy computing initiative" designed to produce software that is "as available, reliable, and secure as electricity, water services, and telephony," according to Gates.

Since that time, however, Microsoft has struggled to respond to a number of viruses and worms--including the Klez and more recent BugBear worms--that target its products and operating systems. Often these viruses have exploited two or more known security vulnerabilities in sophisticated attacks.

The company has also been called upon to patch a number of serious security holes in its Windows operating systems, Internet Explorer Web browser, SQL Server relational database product, and other applications.

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