Windows flaw could lead to forged digital signatures

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 PLC but was sold earlier this year to SecureNet Ltd., 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 Inc. 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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John Fontana

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