Microsoft warns of 'critical' flaw in Outlook Express


By Paul Roberts, IDG News Service
OCTOBER 11, 2002
 

Microsoft Corp. released a security alert yesterday acknowledging a serious security hole in its Outlook Express e-mail client. The vulnerability, which was found in Outlook Express versions 5.5 and 6.0, could allow a remote attacker to take control of machines 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.

Outlook Express is a simplified version of Microsoft's Outlook e-mail application that is distributed with many versions of Microsoft Windows.

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, 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 used to generate the error message can be exploited and 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 taking advantage of the 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 in-box, displaying the e-mail's contents in the preview pane and running the malicious code.

Outlook Express Version 5.5 Service Pack 2 and 6.0 Service Pack 1, which is included in Windows XP Service Pack 1, aren't 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 Inc., which first discovered the vulnerability and reported it to Microsoft.

In addition to the security alert, Microsoft issued a patch for the vulnerability on its Web site. However, the company today acknowledged that the patch itself contains an error that could confuse Outlook Express 6.0 users who have installed its service packs for Windows XP or Internet Explorer 6.

The fix for the Outlook Express flaw is included in those service packs, freeing users that are running them from the need to install the new patch, Microsoft said. But according to an update that was added to the "Caveats" section of yesterday's advisory, the patch displays an incorrect warning message when users with the service packs try to install it.

The warning message should notify those users that the patch isn't needed. Instead, Microsoft said, it reads: "This update requires Internet Explorer 6.0 to be installed."