[NT] Flaw in Outlook 2002's Way of Handling V1 Exchange Server Security Certificates Leads To Information Disclosure

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Date: 01/23/03

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      Flaw in Outlook 2002's Way of Handling V1 Exchange Server Security
    Certificates Leads To Information Disclosure
    ------------------------------------------------------------------------

    SUMMARY

    Microsoft Outlook 2002 provides the facility to encrypt e-mails sent
    between e-mail recipients. Encryption is used to prevent parties other
    than the intended recipients from reading the contents of an e-mail.
    Outlook uses public key certificates to facilitate the exchange of the
    cryptographic keys that are used in the encryption process, and Outlook
    offers a number of different options as to what type of certificates can
    be used. S/MIME certificates are the most commonly used (and are not
    affected by the vulnerability that is the subject of this bulletin), but
    there are other certificate options including V1 Exchange Server Security
    certificates.

    A vulnerability exists because there is a flaw in the way Outlook 2002
    handles a V1 Exchange Server Security certificate when using it to encrypt
    e-mail. As a result of this flaw, Outlook fails to encrypt the mail
    correctly and the message will be sent in plain text. This could cause the
    information in the e-mail to be exposed when the user believed it to be
    protected through encryption.

    DETAILS

    Vulnerable systems:
     * Microsoft Outlook 2002

    Mitigating factors:
     * This vulnerability only affects encryption when a V1 Exchange Server
    Security certificate is used. S/MIME encryption, which is the most widely
    used form of e-mail encryption used by Outlook, is not affected.
     * This vulnerability only affects Outlook 2002 and only when sending HTML
    e-mail.

    Patch availability:
    Download locations for this patch
     * Microsoft Outlook 2002:
        
    <http://microsoft.com/downloads/details.aspx?FamilyId=F20A2E4B-E458-48F0-B0CB-7E73C0BB4884&displaylang=entp://microsoft.com/downloads/details.aspx?FamilyId=F20A2E4B-E458-48F0-B0CB-7E73C0BB4884&displaylang=en
        <http://www.microsoft.com/office/ork/xp/journ/olk1006a.htm>
    http://www.microsoft.com/office/ork/xp/journ/olk1006a.htm (administrative
    update only)

    Note: This and other Office updates can be obtained at
    <http://office.microsoft.com/productupdates>
    http://office.microsoft.com/productupdates.

    What's the scope of the vulnerability?
    This vulnerability could result in a user, who had Outlook 2002 configured
    to use a V1 Exchange Server Security certificate for e-mail encryption,
    sending e-mail that the user believed to be encrypted when it in fact was
    not.

    If an attacker were able to intercept the e-mail, the e-mail could be read
    as plain text, instead of being protected through encryption.

    What is a digital certificate?
    Digital certificates are a familiar fixture within public-key
    cryptography. In public-key cryptography, there are two keys: the private
    key, which must be kept secret, and the public key, which is intended to
    be shared with the world. In order for the public key to be shared
    effectively, there needs to be a way to learn whose it is, how it can be
    used, and to verify that the information is bona fide. Digital
    certificates provide a way to do this.

    A digital certificate combines a public key with information about it -
    who owns it, what purposes it can be used for, when it expires, and so
    forth. When a user needs a digital certificate, he or she gets it from an
    organization known as a Certificate Authority (CA). The CA not only
    creates the certificate, it also digitally signs it, thereby vouching for
    the information in it and preventing it from being modified without
    detection.

    What is a V1 Exchange Server Security Certificate?
    A V1 Exchange Server Security certificate is one of the encryption
    certificate options that is available when Outlook 2002 is used in
    conjunction with a Microsoft Exchange mail server. In this configuration,
    the Exchange mail server can act as a CA and issue certificates to the
    Outlook clients. One type of certificate it can issue is the V1 Exchange
    Server Security certificate - however the default certificate type in this
    configuration is an S/MIME certificate.

    Is there a flaw in V1 Exchange Server Security certificates?
    No - it is important to recognize that the flaw is in the way Outlook
    handles this type of certificate, and not in the certificates themselves.

    You've mentioned S/MIME - what is it?
    S/MIME stands for Secure/Multipurpose Internet Mail Extensions. S/MIME
    provides a consistent way to send and receive MIME encoded data securely.
    Based on the popular Internet MIME standard, S/MIME provides the following
    cryptographic security services for electronic messaging applications:
    authentication, message integrity and non-repudiation of origin (using
    digital signatures) and privacy and data security (using encryption).

    For a description of S/MIME and information about digital certificates,
    see Microsoft Knowledge Base article Q195724, RFC 2633 section 1, and the
    article Working with the Outlook 2000 Security Model.

    Is S/MIME affected by this vulnerability?
    No - this vulnerability is restricted to situations where V1 Exchange
    Server Security certificates are being used for encryption.

    How do I tell what type of certificate Outlook is using to send and
    receive encrypted e-mail?
    You can tell what type of certificate Outlook 2002 is using for encryption
    by checking Tools|Options|Security|Security Settings. If the security
    settings show that the "secure message format" is "Exchange Server
    Security", then Outlook 2002 is using a V1 Exchange Server Security
    certificate.

    You've talked about encryption, but certificates can be used for digitally
    signing e-mail as well. Is this application of certificates affected?
    No, digital signing is unaffected - this vulnerability only affects
    encryption using Exchange Server Security certificates.

    What causes the vulnerability?
    The vulnerability results because Outlook 2002 does not correctly use V1
    Exchange Server Security certificates when such certificates are selected
    for encryption. The result of the error is that the e-mail message is not
    encrypted

    What's wrong with encryption in Outlook 2002 when the V1 Exchange Server
    Security Certificate is used?
    There is a flaw in the way Outlook processes a request to use the V1
    Exchange Server Security certificate to encrypt an e-mail message. As a
    result of this flaw, the certificate is not used properly and the message
    is sent in plain text.

    What could this vulnerability enable an attacker to do?
    It would allow an attacker who had the ability to intercept e-mail
    messages between two parties to read those e-mails even if the parties
    believed them to be encrypted.

    How could an attacker exploit this vulnerability?
    To exploit this vulnerability, an attacker would need to have the ability
    to intercept e-mail between parties who were using V1 Exchange Server
    Security certificates for encryption, or have access to the infrastructure
    where those e-mails were being stored, such as access to mail servers.
    Given this access, an attacker could read e-mails that users believed to
    be protected by encryption.

    What does the patch do?
    The patch eliminates the vulnerability by ensuring that Outlook 2002 uses
    the V1 Exchange Server Security certificate correctly when it is selected
    as the certificate to be used for encryption operations.

    ADDITIONAL INFORMATION

    The information has been provided by
    <mailto:0_43316_E51E4D7D-DECD-43AE-9A29-36080E8D4C3C_US@Newsletters.Microsoft.com> Microsoft Product Security.

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