Microsoft has released a security alert highlighting 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.
In its 58th security alert of the year, Microsoft rated the flaw as
critical for end users, but low for both Internet and intranet
servers.
Outlook Express is the simplified version of Microsoft's Outlook
e-mail application that is distributed with many versions of
Microsoft Windows.
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.
The flawed code 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 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.
Outlook Express version 5.5 Service Pack 2 (SP2) and 6.0 SP1, which
is included in Windows XP SP1, are not affected by the
vulnerability, nor is Microsoft Outlook, according to the alert and
a statement from Aviram Jenik, a researcher at BeyondSecurity.com,
which discovered the vulnerability and reported it to Microsoft.