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CERT Advisory CA-2000-08 Inconsistent Warning Messages in Netscape

   Original release date: May 26, 2000
   Last Revised: --
   Source: CERT/CC
   A complete revision history is at the end of this file.
Systems Affected

     * Systems running Netscape Navigator, up to and including Navigator
       4.73, without the Personal Security Manager installed

   A flaw exists in Netscape Navigator that could allow an attacker to
   masquerade as a legitimate web site if the attacker can compromise the
   validity of certain DNS information. This is different from the
   problem reported in CERT Advisory CA-2000-05, but it has a similar
   impact. This vulnerability was recently discovered by Kevin Fu of of
   the Massachusetts Institute of Technology and, independently, by Jon
   If a user visits a web site in which the certificate name does not
   match the site name and proceeds with the connection despite the
   warning produced by Netscape, then subsequent connections to any sites
   that have the same certificate will not result in a warning message.
   It should be noted that neither this vulnerability, nor the one
   described in CERT Advisory CA-2000-05 represent a weakness or
   vulnerability in SSL. Rather, these problems are a result of the
   fundamentally insecure nature of the DNS system, combined with an
   over-reliance on web browsers to do "sanity checking." In both cases,
   it is (and has been) within the power of the user to validate
   connections by examining certificates and verifying the certificates
   against their expectations.
   Netscape and other browsers take steps to warn users when the DNS
   information appears to be suspicious; the browser may not be able to
   do all the checks necessary to ensure that the user is connecting to
   the correct location. Therefore, as a general practice, the CERT/CC
   recommends validating certificates before any sensitive transactions.
I. Description

   Digital certificates are small documents used to authenticate and
   encrypt information transmitted over the Internet. One very common use
   of digital certificates is to secure electronic commerce transactions
   through SSL. The kind of certificates used in e-commerce transactions
   are called X.509 certificates. The X.509 certificates help a web
   browser and the user ensure that any sensitive information transmitted
   over the Internet is readable only by the intended recipient. This
   requires verifying the recipient's identity and encrypting data so
   that only the recipient can decrypt it.
   The "padlock" icon used by Netscape, Internet Explorer, and other
   browsers is an indication that an SSL-secured transaction has been
   established to someone. It does not necessarily indicate to whom the
   connection has been established. Netscape and other browsers take
   steps to warn users when DNS-based information conflicts with the
   strongly authenticated information contained in the X.509 certificates
   used in SSL transactions. These warnings are supplemental information
   to help users decide if they're connecting to whom they think they are
   connecting. These steps and warnings are designed to protect against
   attacks on the DNS information.
   If you rely solely on the warning dialogs provided by web browsers to
   determine if the connection is with whom you think it is or if you do
   not fully understand the implications of the dialogs, then you may be
   subject to the attacks described in this document and CA-2000-05.
   The essence of the problem is this: Within one Netscape session, if a
   user clicks on "continue" in response to a "hostname does not match
   name in certificate" error, then that certificate is incorrectly
   validated for future use in the Netscape session, regardless of the
   hostname or IP address of other servers that use the certificate.
   For example, suppose that an attacker constructs a web site named, authenticated by a certificate that does not match, and convinces a victim to navigate there. Netscape will
   present a warning dialog indicating that the site to which the user
   thinks she's navigating ( does not match the
   information presented in the certificate. If the user does not intend
   to provide any sensitive information to, she may
   choose to continue with the connection (i.e., she may choose to click
   "OK" in response to the warning dialog), possibly attributing the
   warning dialog to a benevolent misconfiguration on the part of or failing to understand the implications of the warning
   Then, within the same session, no warning dialogs will be presented
   under the following circumstances:
     * the attacker co-opts the DNS system in some fashion to cause the
       DNS name of a legitimate site to resolve to the IP address of a
       system under the control of the attacker
     * the system under the control of the attacker is authenticated
       using the same certificate as, which the user
       previously accepted in the warning dialog mentioned above
     * the victim attempts to connect to the legitimate site (but instead
       gets directed to the site under the control of the attacker by
       virtue of the attack on DNS)
   This allows the attacker to bypass the ordinary "sanity checking" done
   by Netscape, and the result is that the user may provide sensitive
   information to the attacker.
II. Impact

   Attackers can trick users into disclosing information (such as credit
   card numbers, personal data, or other sensitive information) intended
   for a legitimate web site - if the user has previously accepted a
   certificate in which the name recorded in the certificate does not
   match the DNS name of the web site to which the user is connecting.
III. Solution

Check Certificates

   The CERT/CC recommends that prior to providing any sensitive
   information over SSL, you check the name recorded in the certificate
   to be sure that it matches the name of the site to which you think you
   are connecting. For example, in Netscape, click on the "padlock" icon
   to engage the "Security Info" dialog box. Then click on the "View
   Certificate" button. A dialog box will appear, listing the certificate
   authority that signed the certificate and the server for which it was
   issued. If you do not trust the certificate authority or if the name
   of the server does not match the site to which you think you're
   connecting, be suspicious.
Validate Certificates Independently

   Web browsers come configured to trust a variety of certificate
   authorities. If you delete the certificates of all the certificate
   authorities in your browser, then whenever you encounter a new SSL
   certificate, you will be prompted to validate the certificate
   yourself. You can do this by validating the fingerprint on the
   certificate through an alternate means, such as the telephone. That
   is, the same dialog box mentioned above also lists a fingerprint for
   the certificate. If you wish to validate the certificate yourself,
   call the organization for which the certificate was issued and ask
   them to confirm the fingerprint on the certificate.
   Deleting the certificates of the certificate authorities in your
   browser will cause the browser to prompt you for validation whenever
   you encounter a new site certificate. This may be inconvenient and
   cumbersome, but it provides you with greater control over which
   certificates you accept.
   It is also important to note that this sort of verification is only
   effective if you have an independent means through which to validate
   the certificate. This sort of validation is called out-of-band
   validation. For example, calling a phone number provided on the same
   web page as the certificate does not provide any additional security.
   The CERT/CC encourages all organizations engaging in electronic
   commerce to train help desk or customer support personnel to answer
   questions about certificate fingerprints.
Reject certificates that don't match the host name

   As a specific defense against this vulnerability, we recommend not
   accepting certificates that don't match the host name. The most likely
   cause of a non-matching certificate is a configuration error on the
   part of the web server administrator. However, a user is unable to
   distinguish between a benign misconfiguration and a malicious attack.
   Even if the user does not intend to provide any sensitive information
   to a site with a non-matching certificate, answering "OK" to this
   dialog may permit an attacker to successfully carry out the exploit.
Stay up-to-date with patches, workarounds, and certificate management

   Apply a patch from your vendor. Appendix A contains vendor
Appendix A Vendor Information


   [...] the potential exploit in question can be completely prevented if
   the user does not click "continue" as stated above. Because of this
   safety measure, we do not feel an emergency release is necessary.
   However, we are planning on fixing this in a future release of
   Communicator, scheduled for release later this year.
   Additionally, this flaw was fixed in PSM approximately 6 months before
   [the initial report of the vulnerability].
   The CERT Coordination Center thanks Kevin Fu of MIT and Jon Guyer for
   initially discovering and reporting this vulnerability, and their help
   in constructing this advisory.
   Shawn Hernan was the primary author of this document.
   This document is available from:
CERT/CC Contact Information

   Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
          Phone: +1 412-268-7090 (24-hour hotline)
          Fax: +1 412-268-6989
          Postal address:
          CERT Coordination Center
          Software Engineering Institute
          Carnegie Mellon University
          Pittsburgh PA 15213-3890
   CERT personnel answer the hotline 08:00-20:00 EST(GMT-5) / EDT(GMT-4)
   Monday through Friday; they are on call for emergencies during other
   hours, on U.S. holidays, and on weekends.
Using encryption

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   If you prefer to use DES, please call the CERT hotline for more
Getting security information

   CERT publications and other security information are available from
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   * "CERT" and "CERT Coordination Center" are registered in the U.S.
   Patent and Trademark Office.
   Any material furnished by Carnegie Mellon University and the Software
   Engineering Institute is furnished on an "as is" basis. Carnegie
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   Conditions for use, disclaimers, and sponsorship information
   Copyright 2000 Carnegie Mellon University.
   Revision History
May 26, 2000: initial release

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