Microsoft is warning users about a bug impacting all supported versions of Windows.
Microsoft
issued
an advisory on a Windows security vulnerability today after exploit code
for the bug went public.
The bug, which lies in the MHTML (MIME Encapsulation of Aggregate HTML)
protocol handler, can be exploited to cause data leakage. Though
proof-of-concept code for the vulnerability has already gone public, the
company said it is unaware of any attempts to exploit the bug.
"The impact of an attack on the vulnerability would be similar to that of
server-side cross-site-scripting (XSS) vulnerabilities," blogged Angela
Gunn, security response communications manager for Microsoft Trustworthy
Computing. "For instance, an attacker could construct an HTML link
designed to trigger a malicious script and somehow convince the targeted user
to click it."
If a user clicked that link, the malicious script would run on the
user's computer for the rest of the current Internet Explorer session, and
could be used to collect user information, spoof content displayed in the
browser or for other acts, Gunn added.
According to the advisory, the vulnerability affects all supported editions
of Microsoft Windows. The
vulnerability
exists due to the way MHTML interprets MIME-formatted requests for content
blocks within a document. Even though proof-of-concept code is available, the
bug is difficult for attackers to exploit in a successful and useful way,
opined Andrew Storms, director of security operations for nCircle.
"To create a successful attack with this bug the attacker needs to entice a
user to click on a specific URL and he also needs control over the user's Web
server," he said. "These factors, together with the fact that the
mitigation is easy to install, make successful exploits less likely.
"The risk with this bug is not huge, especially since Microsoft has already
provided easy and fairly safe mitigation techniques," he added.
Microsoft offered a few workarounds to protect users. For example,
users can follow instructions in the advisory to lock down the MHTML
protocol and set Internet and local intranet security zone settings to high to
block ActiveX controls and active scripting.
"This bug looks like a strong candidate for the regular patch process,"
he added. "So far, there are no active exploitations. You also have to consider
that the bug is in the Windows OS and it affects all versions of Windows,
so testing is going to take some time. Microsoft is not going to rush out a
patch just yet."
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