Windows flaw could be used to forge digital signatures in Outlook

[InfoSec News <isn () c4i org>]

http://www.nwfusion.com/news/2002/0903winflaw.html

By John Fontana
Network World Fusion 
09/03/02 

An independent researcher who last month documented a flaw in
Microsoft's cryptography software now says the same vulnerability
could be used to forge digital signatures on e-mail sent to users of
the Outlook email program.

The researcher says attackers could create what looks like a signed
and secure message and trick recipients using Outlook into believing
that they are having a secure conversation with another party.

"There is no difference between signed and unsigned e-mail in
Outlook," says Mike Benham, an independent security researcher who
originally reported to Microsoft what is called the certificate chain
spoofing attack. "For five or so years people in corporate
environments think they have been exchanging secure mail using Outlook
but that is not the case."

A Microsoft spokesman said the company is aware of the new flaw and is
investigating it.

Outlook's Secure/Multipurpose Internet Mail Extensions (S/MIME), a
standard for secure mail created by the Internet Engineering Task
Force, is susceptible to the flaw, according to Benham.

A source confirmed that a product called MailSecure, which has a
plug-in for Outlook, was tested and found vulnerable to the problem.  
The product was originally marketed by Baltimore Technologies but was
sold earlier this year to SecureNet Limited, an Australian security
software vendor.

The attack lets hackers create a phony security certificate that can
be used to digitally sign e-mail. When a user of Outlook opens the
mail, the software does not check the validity of the certificate and
presents the e-mail as a digitally signed communication.

For example, an attacker, especially from inside a corporation, could
spoof the e-mail address of the CEO and use a bogus certificate to
digitally sign a message sent to an employee telling him or her she is
fired.

A month ago when Benham found the original flaw, which Microsoft has
yet to patch, the company verified its existence but said it only
affected Internet Explorer. Microsoft is working on patches for
Windows 98, ME, NT4, 2000 and XP.

But Benham's further testing found the flaw affects Outlook Express 5
on Windows 2000 with Service Pack 3. He is uncertain if all versions
of Outlook are vulnerable, but said in all likelihood that they are.

"This S/MIME attack is the same attack. It is applied a little
differently but it can be used to forge digital signatures," says
Benham. "And there is a chance that it might affect other applications
running on the OS."

The flaw, which resides in the cryptography mechanism of the OS, does
not validate SSL certificate chains, the hierarchy of trust that
cascades from certificate authorities such as VeriSign. That means the
software does not check the validity of the certificates it uses for
security purposes.

The IE attack lets a hacker create a bogus digital certificate from a
valid certificate and use the bogus certificate to place himself in
the middle of an encrypted Secure Socket Layer (SSL) session and
intercept and read data. SSL is a standard for securing online
transactions and electronic commerce.

In order to avoid the digital signature attack, users need to manually
inspect the chain of validation for digital certificates.

Last month, Microsoft and VeriSign said that exploiting the
vulnerability would be difficult. Attackers must have a valid
certificate from which to create the bogus certificate and VeriSign,
which is the certificate authority for 400,000 certificates in
circulation today, claims that makes attackers easy to track.

Critics contend the exploit is not that difficult from both a
programming perspective and from the perspective of tricking users
onto rogue Web sites or fooling them with spoofed e-mail addresses.




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