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  • CERT Advisory CA-2000-05 Netscape Navigator Improperly Validates SSL Sessions

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Original release date: May 12, 2000

A complete revision history is at the end of this file.

Systems Affected

  • Systems running Netscape Navigator 4.72, 4.61, and 4.07. Other versions less than 4.72 are likely to be affected as well.


The ACROS Security Team of Slovenia has discovered a flaw in the way Netscape Navigator validates SSL sessions.

I. Description

The text of the advisory from ACROS is included below. It includes information CERT/CC would not ordinarily publish, including specific site names and exploit information. However, because it is already public, we are including it here as part of the complete text provided by ACROS.


   ACROS Security Problem Report #2000-04-06-1-PUB
   Bypassing Warnings For Invalid SSL Certificates In Netscape Navigator
   FULL REPORT                                                        PUBLIC

   Affected System(s): Netscape Navigator & Communicator
              Problem: Bypassing Warnings For Invalid SSL Certificates
             Severity: High
             Solution: Installing the Personal Security Manager or
                       Installing the newest Netscape Communicator (v4.73)
           Discovered: April 3, 2000
      Vendor notified: April 4, 2000
          Last update: May 10, 2000
            Published: May 10, 2000


Our team has discovered a flaw in Netscape Navigator that allows bypassing
of warning about an invalid SSL certificate. SSL protection is used in most
major Internet-based financial services (e-banking, e-commerce). The flaw
we have found effectively disables one of the two basic SSL functionalities:
to assure users that they are really communicating with the intended web
server - and not with a fake one.
Using this flaw, the attacker can make users send secret information (like
credit card data and passwords) to his web server rather than the real one -

INTRODUCTION (skip this section if you already understand how SSL works)

When a web browser tries to connect to a SSL-protected server, a so-called
SSL session is  established. At the beginning of this session the server
presents his SSL certificate containing his public key. At this point,
browser checks the certificate for the following conditions (*):

1) Certificate must be issued by a certificate authority trusted by browser
   (some are default: Verisign, Thawte etc.)
2) Certificate must not be expired (its expiry date:time must be later than
   the current system date:time on the computer browser is running on)
3) Certificate must be for the server that browser is connecting to (if
   browser is connecting to, the certificate must be for

All three conditions must be met for browser to accept the certificate. For
every condition not met, browser should display a warning to the user and
then user can decide whether connection should be established or not.
These three conditions combined provide user with assurance that his browser
is really connecting to the correct server and not to some fake server
placed on the Internet by malicious individual(s) trying to trick users to
give them credit card information, passwords and other secret information.

For example, let's take a look at a sample web e-banking system that doesn't
use SSL certificates and requires one-time password tokens for user
authentication. User connects to Browser asks DNS
server for IP address of and gets Browser
then connects to and user is presented with login form
asking for his username and one-time password. He enters this data and
starts using e-banking services.
A simple attack (called web-spoofing) on this system is to attack the DNS
server and "poison" its entry for with attacker's IP address Attacker sets up a web server at that web-wise
looks exactly like the original server. User trying to
connect to will now instead connect to the attacker's server
and provide it with his one-time password. Attacker's server will use this
password to connect to the real server at and transfer all
of the user's money to his secret Swiss bank account ;-).

This attack is successfully disabled by using SSL protocol. In that
case, when browser falsely connects to at rather
than to, attacker's server must provide a valid certificate
for, which it can't unless the attacker has stolen the secret
key and the certificate from the real server. Let's look at three

1) Attacker could issue a certificate for himself (on his own
   CA). That wouldn't work since his CA is not trusted by user's browser.
2) Attacker could use a stolen expired key and certificate (those are often
   not protected as strongly as valid ones since one could think they can't
   be used any more). That wouldn't work since browser will notice that
   certificate is expired.
3) Attacker could use a valid key and certificate for some other site (e.g. That wouldn't work since browser will accept only
   valid certificates for

It would seem that this problem of web-spoofing is successfully solved with
SSL certificates.


There is a flaw in implementation of SSL certificate checks in Netscape

The Flaw

Netscape Navigator correctly checks the certificate conditions (*) at the
beginning of a SSL session it establishes with a certain web server.
The flaw is, while this SSL session is still alive, all HTTPS
connections to *THAT SERVER'S IP ADDRESS* are assumed to be a part of this
session (and therefore certificate conditions are not checked again).
Instead of comparing hostnames to those of currently open sessions, Navigator
compares IP addresses. Since more than one hostname can have the same IP
address, there is a great potential for security breach.
This behavior is not in compliance with SSL specification.


The following will try to demonstrate the flaw. It is assumed that for
redirecting user's web traffic, the attacker will generally use "DNS
poisoning" or reconfiguring routers, while in our demonstration we will
use the HOSTS file on client computer to get the same effect and make it
easier to reproduce the flaw.

In this demonstration, we will make Navigator open Thawte's homepage over
secure (HTTPS) connection while requesting Verisign's home address at
Thawte's and Verisign's homepages are used as examples - this would work
just the same on any other secured web sites.

1) First, add the following line to the local HOSTS file on the computer
running the Navigator and save it:

This will make the computer (and, consequently, the browser) think that IP
address of (which is actually is in fact (which is actually IP address of

At this point it is important to note that SSL, if correctly implemented,
provides protection against such "domain name spoofing", because while the
browser will connect to the wrong server, that server will not be able to
provide a valid SSL certificate and the SSL session will not be
established (not without user being warned about the certificate).

2) Close all instances of Navigator to clean any cached IP addresses.

3) Open Navigator and go to It works as it should -
Thawte's server provides a valid SSL certificate for its hostname
( and so the SSL session is established.

4) With the same instance of Navigator, go to Now
watch the Thawte's homepage appear again WITHOUT ANY WARNINGS!

What happened here? In step 3), Navigator looked up the IP address for (from the DNS server) and found It tried to
establish a SSL session with that IP address and correctly checked all three
certificate conditions (*) - indeed, if any of them weren't true, a warning
would pop up.
In step 4), Navigator looked up the IP address for (this
time from HOSTS file, but it could easily have been from the same DNS server)
and found again Now, since there was already one SSL session
open with that IP address, Navigator *INCORRECTLY* decided to use that
session instead of establishing another one.


This exploit will show how the flaw could be used to gather user's secret

Assume there is a web bookstore at Users go to (via normal HTTP connection), browse the
books and add them to their virtual shopping baskets. At the check-out,
they are directed to a secure order form (e.g. where they enter their
personal and credit card information which is then submitted (again via
secure HTTPS connection) to the server. This is a typical web e-commerce
Assume that IP address of is

The attacker sets up his own web server with IP address and
installs on it a valid SSL certificate for host (he could
have purchased this certificate from e.g. Verisign if he owns the domain; he could have stolen the certificate or he could have broken
into a web server with a certificate already installed).
The attacker makes this web server function as a gateway to - meaning that all requests are forwarded to, so virtually this server "looks and feels" exactly like
the real There is just one difference: the page before
the order form (e.g.
contains a small (1x1) image originating from
(secure HTTPS connection).

Then, the attacker "poisons" a heavily used DNS server so that it will return for requests about (normally it returns

What happens then?

All users of that DNS server who will try to visit (via normal HTTP) will connect to instead of but will not notice anything because everything will look
just the way it should. They will browse the books and add them to their
shopping baskets and at check-out, they will be presented with the order form
But the previous HTML page containing the hyperlink to the order form will
also contain a small (1x1) image with source
Navigator will successfully download this image and for that it will
establish a SSL session with This session then stays open.
When the order form is accessed, Navigator tries to establish another SSL
session, this time to Since DNS server claims this
server has the same IP address as (, Navigator
will use the existing SSL session with and will not check the
The result: Navigator is displaying a SECURE ORDER FORM that it believes to
be originating from the genuine server while in fact
it is originating from the fake one. No warning about an invalid certificate
is issued to the user so he also believes to be safe.
When user submits his secret information, it goes to (through) the attacker's
server where it is collected for massive abuse.
For users to notice the foul play they would have to look at the certificate
properties while on a "secure" page
The properties would show that the certificate used was issued for host
Also, monitoring network traffic would show that the server is not at where it should be but rather at

It is a very rare practice to check any of these when nothing suspect is


It should be noted that in the previous exploit, if the users tried to
access over secure (HTTPS) connection from
the very start, Navigator would issue a warning. It is imperative for the
exploit to work that some time *before* the first secure connection to a successful secure connection is made to That's why a valid certificate must be installed

Also, it should be noted that Navigator's SSL sessions don't last forever.
We haven't been able to predict the duration of these sessions
(it seems to be depending on many things like inactivity time, total time
etc.) and we also haven't investigated the possible effects of SSL
session resuming.


Netscape has (even prior to our notification - see the Acknowledgments
section) provided a Navigator Add-on called Personal Security Manager (PSM),
freely downloadable at:

Installation of PSM, as far as we have tested it, corrects the identified

Netscape Communicator (v4.73) currently includes the fix for this
vulnerability. It is available for download at:


Navigator/Communicator users who can't or don't want to install PSM can use
a "manual" method to make sure they are not under attack:

When visiting an SSL-protected site, double click on the lock icon (bottom
left corner) or the key icon (in older browsers) and see whether the
certificate used for the connection is really issued for the correct
hostname. E.g. If you visit, make sure the
certificate used is issued for and not for some other


It is important to emphasize that the flaw presented completely compromises
SSL's ability to provide strong server authentication and therefore poses
a serious threat to Navigator users relying on its SSL protection.

Users of web services

Netscape Navigator/Communicator users who are also users of any critical web
services employing Secure Sockets Layer (SSL) protection to provide secrecy
and integrity of browser-server communication are strongly advised to
install Personal Security Manager or upgrade to Communicator 4.73 and thus
disable this vulnerability.

Main examples of such critical web services are:

- web banking systems (especially the ones using passwords for
  authentication - even one-time passwords),
- web stores (especially the ones accepting credit card data) and
- other web-based e-commerce systems.

Providers of web services

Providers of critical web services employing Secure Sockets Layer (SSL)
protection to provide secrecy and integrity of browser-server communication
should advise their users to install Personal Security Manager or upgrade to
Communicator 4.73 and thus disable this vulnerability.

Since this vulnerability allows for the type of attack that can completely
bypass the real/original web server, there are no technical countermeasures
which providers of web services could deploy at their sites.

Web services using client SSL certificates for user authentication

This vulnerability does NOT allow the attacker to steal client's SSL key
and thus execute the man-in-the-middle attack on web services using client
SSL certificates for user authentication. It still does, however, allow
the attacker to place a fake server (an exact copy) and collect other
information users provide (including the data in their client SSL


Tests were performed on:

Communicator 4.72 - affected
Communicator 4.61 - affected
Navigator 4.07 - affected


We would like to acknowledge Netscape (specifically Mr. Bob Lord and Mr.
Kevin Murray) for prompt and professional response to our notification of
the identified vulnerability and their help in understanding the flaw and
"polishing" this report.

We would also like to acknowledge Mr. Matthias Suencksen of Germany, who
has discovered some aspects of this vulnerability before we did (back in
May 1999).


Netscape has issued a Security Note about this vulnerability under a title
"The Acros-Suencksen SSL Vulnerability" at:


For further details about this issue please contact:

Mr. Mitja Kolsek

ACROS, d.o.o.
Stantetova 4
SI - 2000 Maribor, Slovenia

phone: +386 41 720 908

PGP Key available at PGP.COM's key server.
PGP Fingerprint: A655 F61C 5103 F561  6D30 AAB2 2DD1 562A


This report was sent to:

- BugTraq mailing list
- NTBugTraq mailing list
- Win2KSecAdvice mailing list
- ACROS client mailing list


The information in this report is purely informational and meant only for
the purpose of education and protection. ACROS, d.o.o. shall in no event be
liable for any damage whatsoever, direct or implied, arising from use or
spread of this information.
All identifiers (hostnames, IP addresses, company names, individual names
etc.) used in examples and exploits are used only for explanatory purposes
and have no connection with any real host, company or individual. In no
event should it be assumed that use of these names means specific hosts,
companies or individuals are vulnerable to any attacks nor does it mean that
they consent to being used in any vulnerability tests.
The use of information in this report is entirely at user's risk.


(c) 2000 ACROS, d.o.o., Slovenia. Forwarding and publishing of this document
is permitted providing all information between marks "[BEGIN-ACROS-REPORT]"
and "[END-ACROS-REPORT]" remains unchanged.


II. Impact

Attackers can trick users into disclosing information (potentially including credit card numbers, personal data, or other sensitive information) intended for a legitimate web site, even if that web site uses SSL to authenticate and secure transactions.

III. Solution

Install an update from your vendor.

Appendix A lists information from vendors about updates.

If you are a DNS administrator, maintain the integrity of your DNS server

One way to exploit this vulnerability, described above, relies on the ability of the attacker to compromise DNS information. If you are a DNS administrator, making sure your DNS server is up-to-date and free of known vulnerabilities reduces the ability of an intruder to execute this type of attack. Administrators of BIND DNS servers are encouraged to read

Validate certificates at each use

Despite the existence of this flaw, it is still possible to guard against attempted attacks by validating certificates manually each time you connect to an SSL-secured web site. Doing so will substantially reduce the ability of an attacker to use flaws in the DNS system to bypass SSL-authentication.

Appendix A. Vendor Information


Information about this problem is available at


None of our products are affected by this vulnerability.

The CERT Coordination Center thanks the ACROS Security Team of Slovenia (Contact:, for the bulk of the text in this advisory.

Shawn Hernan was the primary author of the CERT/CC portions of this document.

Copyright 2000 Carnegie Mellon University; portions Copyright 2000 ACROS, d.o.o., Slovenia.

Revision History

May 12, 2000:  Initial release
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