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There has been a good deal of discussion and speculation recently
about a reported security vulnerability involving how Internet Explorer
identifies secure web sites. The Microsoft Security Response Center has
investigated the report and we"d like to provide information about the
issue and our plans for addressing it. The report discusses a
problem in the way Internet Explorer establishes secure web sessions
via the Secure Socket Layer (SSL) protocol. SSL provides a number of
security features, but of particular interest in this case is its
ability to verify that a web site is indeed the site it purports to be.
A flaw in the SSL implementation could enable an attacker to create a
web site that bypasses this protection, and masquerades as a different
web site – one that the user might trust and provide with personal
information such as credit card numbers. The flaw could enable an
attacker who has been issued a valid SSL digital certificate to create
a seemingly valid additional certificate that purports to belong to a
different web site. When a user visited the site, the attacker could
present the second certificate in an attempt to convince the user that
he or she was actually at the site it claimed. While Microsoft
has confirmed that the flaw does exist, it"s important to note that
actually exploiting it would be difficult, for several reasons: | • | The attack scenario is narrow.
If a user arrived at the attacker"s web site in the belief that it was
actually a different, legitimate site, the flaw could allow the
attacker to bolster this belief. But it provides no way to make the
user actually arrive at the attacker"s site, let alone in the belief
that it is a different site. Doing this would likely require that the
attacker be able to modify the Internet infrastructure that the user
transited, via a technique such as DNS cache poisoning. However, such
techniques are difficult, temporary, and generally require favorable
network topology. | | • | The identity of the attacker could easily be determined.
To exploit the vulnerability, the attacker would require a valid SSL
digital certificate, issued by a trusted Certificate Authority.
However, most commercial Certificate Authorities require substantial
proof of identity before issuing such a certificate, thereby making it
possible for law enforcement authorities to determine who the attacker
was. (Information on verifying certificates can be found here). | | • | The user would always have the ability to determine the truth.
Anytime an SSL session has been established, an icon shaped like a lock
is present in the lower right corner of the screen. By double-clicking
on the icon, the user can see information about the site"s digital
certificate, including the identity of the issuer. This would clearly
show that, in contrast to the norm, this one hadn"t been directly
issued by a commercial Certificate Authority. |
Despite
the many challenges associated with exploiting the flaw, there is
indeed a flaw here and Microsoft has developed a patch that will
eliminate it. For further information and to download the patch, please
see Microsoft Security Bulletin MS02-050: Certificate Validation Flaw Could Enable Identity Spoofing (329115). We
regret any anxiety that customers may have experienced regarding this
issue. Clearly, it would have been best if a balanced assessment of the
issue and its risk had been available from the start. However, the
report, which neglected to discuss any of the challenges associated
with actually exploiting the vulnerability, was made public without any
advance warning to Microsoft. Responsible security researchers have the
safety of users in mind and work with vendors to ensure that the
information published about potential vulnerabilities is balanced and,
above all, correct. Had this been done in this case, all users"
interests would have been better served.
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