Security Flaw with Digital signatures in Microsoft Outlook

From: Roberto Franceschetti (roberto_remove_n.o.s.p.a.m_tag_at_logsat.com)
Date: 02/17/05

  • Next message: T. Sean Weintz: "Re: Stopping Spam" 
    Date: Thu, 17 Feb 2005 21:09:56 GMT

    This report is also available graphically at
    http://www.logsat.com/Signatures

    On 10/21/2004 the following vulnerability was reported to Microsoft:

    Security Flaw with Digital signatures in Microsoft Outlook -
    Emails in Microsoft Outlook digitally signed with S/MIME using either a
    commercial personal certificate like Verisign or using a certificate issued
    by MS Certificate Server can be altered. Outlook will not show any warnings
    about the email being changed, the digital signature will still be
    reported valid even though the message content has been modified and
    parties involved in the signatures changed.
    This is an extremely serious flaw as I can change any digitally signed
    emails I want without Outlook ever noticing.
    After several emails with Microsoft and CERT during the months that
    followed, no fixes have been issued to correct this security flaw. It is
    only now that I am making this information public after all my attempts to
    have Microsoft resolve the problem have failed.

    The following are 3 digitally signed messages. The 1st one is a valid,
    unmodified email from Roberto Franceschetti (roberto at logsat.com) to
    support at logsat.com: (follow the hyperlinks for the email's source and
    screenshots)

    Screenshot at http://www.logsat.com/Signatures/Valid.gif
    Email's source at http://www.logsat.com/Signatures/Valid.msg

    The following one has been "hacked" so that the sender now appears to be
    "Hackers Franceschetti" (hackers@logsat.com). Note that Outlook states that
    the email is absolutely valid, and that the certificate is Valid and
    Trusted. This is most definitely not the case, as I've altered the original
    message to make it appear as a different person actually sent it. Imagine
    the scenario where a digital signature is supposed to unequivocally identify
    a sender, but now this email that appears to be sent by "hackers" appears
    legitimate, and a poor victim will trust it and send the hacker any
    confidential information he is asked for... (follow the hyperlinks for the
    email's source):

    Screenshot at http://www.logsat.com/Signatures/Hacked1.gif
    Email's source at http://www.logsat.com/Signatures/Hacked1.msg

    This 3rd email is yet another variation showing how a digitally signed email
    can further be forget without Outlook ever raising warning flags (follow the
    hyperlinks for the email's source):

    Screenshot at http://www.logsat.com/Signatures/Hacked2.gif
    Email's source at http://www.logsat.com/Signatures/Hacked2.msg

    The full emails with the conversations between myself, Microsoft and CERT
    can be found here (http://www.logsat.com/Signatures/emails.asp). I hope that
    by making this information public all the users who rely on digital
    signatures will be aware of this severe security flaw in Microsoft Outlook,
    and will take other precautions to ensure the identity of users in digitally
    signed emails they receive.
    Roberto Franceschetti
    LogSat Software
    roberto at sign logsat.com

  • Next message: T. Sean Weintz: "Re: Stopping Spam"

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