-----BEGIN PGP SIGNED MESSAGE-----
AUSCERT External Security Bulletin Redistribution
ESB-2000.108 -- CERT Advisory CA-2000-08
Inconsistent Warning Messages in Netscape Navigator
29 May 2000
AusCERT Security Bulletin Summary
Product: Netscape Navigator
Operating System: N/A
Impact: Access Confidential Data
Access Required: Remote
- --------------------------BEGIN INCLUDED TEXT--------------------
- -----BEGIN PGP SIGNED MESSAGE-----
CERT Advisory CA-2000-08 Inconsistent Warning Messages in Netscape
Original release date: May 26, 2000
Last Revised: --
A complete revision history is at the end of this file.
* 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.
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
example.com, authenticated by a certificate that does not match
example.com, 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 (www.example.com) does not match the
information presented in the certificate. If the user does not intend
to provide any sensitive information to www.example.com, 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
example.com 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 www.example.com, 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.
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.
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
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
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.
We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from
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
our web site
To be added to our mailing list for advisories and bulletins, send
email to firstname.lastname@example.org and include SUBSCRIBE
your-email-address in the subject of your message.
* "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
Mellon University makes no warranties of any kind, either expressed or
implied as to any matter including, but not limited to, warranty of
fitness for a particular purpose or merchantability, exclusivity or
results obtained from use of the material. Carnegie Mellon University
does not make any warranty of any kind with respect to freedom from
patent, trademark, or copyright infringement.
Conditions for use, disclaimers, and sponsorship information
Copyright 2000 Carnegie Mellon University.
May 26, 2000: initial release
- -----BEGIN PGP SIGNATURE-----
Version: PGP for Personal Privacy 5.0
- -----END PGP SIGNATURE-----
- --------------------------END INCLUDED TEXT--------------------
This security bulletin is provided as a service to AusCERT's members. As
AusCERT did not write the document quoted above, AusCERT has had no control
over its content. The decision to use any or all of this information is
the responsibility of each user or organisation, and should be done so in
accordance with site policies and procedures.
NOTE: This is only the original release of the security bulletin. It may
not be updated when updates to the original are made. If downloading at
a later date, it is recommended that the bulletin is retrieved directly
from the original authors to ensure that the information is still current.
Contact information for the authors of the original document is included
in the Security Bulletin above. If you have any questions or need further
information, please contact them directly.
Previous advisories and external security bulletins can be retrieved from:
If you believe that your system has been compromised, contact AusCERT or
your representative in FIRST (Forum of Incident Response and Security
Internet Email: email@example.com
Facsimile: (07) 3365 7031
Telephone: (07) 3365 4417 (International: +61 7 3365 4417)
AusCERT personnel answer during Queensland business hours
which are GMT+10:00 (AEST).
On call after hours for emergencies.
-----BEGIN PGP SIGNATURE-----
-----END PGP SIGNATURE-----