[ LUGOS ] spretni Mariborcan(i)
Igor Furlan
ifurlan na zilog.com
Sob Maj 13 00:39:58 CEST 2000
-----Original Message-----
From: CERT Advisory [mailto:cert-advisory na cert.org]
Sent: Friday, May 12, 2000 1:12 PM
To: cert-advisory na cert.org
Subject: CERT Advisory CA-2000-05
-----BEGIN PGP SIGNED MESSAGE-----
CERT Advisory CA-2000-05
Netscape Navigator Improperly Validates SSL Sessions
Original release date: May 12, 2000
Source: ACROS, CERT/CC
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.
Overview
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.
=====[BEGIN-ACROS-REPORT]=====
========================================================================
=
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
SUMMARY
=======
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 -
EVEN IF THE COMMUNICATION IS PROTECTED BY SSL PROTOCOL.
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 www.e-bank.com, the certificate must be for
www.e-bank.com)
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 http://www.e-bank.com. Browser asks DNS
server for IP address of www.e-bank.com and gets 100.100.100.100.
Browser
then connects to 100.100.100.100 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 www.e-bank.com with attacker's IP
address
99.99.99.99. Attacker sets up a web server at 99.99.99.99 that web-wise
looks exactly like the original www.e-bank.com server. User trying to
connect to www.e-bank.com 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 100.100.100.100 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 www.e-bank.com at 99.99.99.99
rather
than to 100.100.100.100, attacker's server must provide a valid
certificate
for www.e-bank.com, which it can't unless the attacker has stolen the
secret
key and the certificate from the real server. Let's look at three
possibilities:
1) Attacker could issue a certificate for www.e-bank.com 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.
www.something.org). That wouldn't work since browser will accept only
valid certificates for www.e-bank.com.
It would seem that this problem of web-spoofing is successfully solved
with
SSL certificates.
PROBLEM
=======
There is a flaw in implementation of SSL certificate checks in Netscape
Navigator.
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.
DEMONSTRATION
=============
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
https://www.verisign.com.
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:
207.240.177.177 www.verisign.com
This will make the computer (and, consequently, the browser) think that
IP
address of www.verisign.com (which is actually 205.139.94.60) is in fact
207.240.177.177 (which is actually IP address of www.thawte.com).
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 https://www.thawte.com. It works as it
should -
Thawte's server provides a valid SSL certificate for its hostname
(www.thawte.com) and so the SSL session is established.
4) With the same instance of Navigator, go to https://www.verisign.com.
Now
watch the Thawte's homepage appear again WITHOUT ANY WARNINGS!
What happened here? In step 3), Navigator looked up the IP address for
www.thawte.com (from the DNS server) and found 207.240.177.177. 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 www.verisign.com
(this
time from HOSTS file, but it could easily have been from the same DNS
server)
and found again 207.240.177.177. 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.
EXPLOIT
=======
This exploit will show how the flaw could be used to gather user's
secret
information.
Assume there is a web bookstore at www.thebookstore.com. Users go to
http://www.thebookstore.com (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.
https://www.thebookstore.com/order_form.html) 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
concept.
Assume that IP address of www.thebookstore.com is 100.100.100.100.
The attacker sets up his own web server with IP address 99.99.99.99 and
installs on it a valid SSL certificate for host www.attacker.com (he
could
have purchased this certificate from e.g. Verisign if he owns the domain
attacker.com; 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
www.thebookstore.com - meaning that all requests are forwarded to
www.thebookstore.com, so virtually this server "looks and feels" exactly
like
the real www.thebookstore.com. There is just one difference: the page
before
the order form (e.g. http://www.thebookstore.com/basket.html)
contains a small (1x1) image originating from https://www.attacker.com
(secure HTTPS connection).
Then, the attacker "poisons" a heavily used DNS server so that it will
return
99.99.99.99 for requests about www.thebookstore.com (normally it returns
100.100.100.100).
What happens then?
All users of that DNS server who will try to visit (via normal HTTP)
http://www.thebookstore.com will connect to 99.99.99.99 instead of
100.100.100.100 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
https://www.thebookstore.com/order_form.html.
But the previous HTML page containing the hyperlink to the order form
will
also contain a small (1x1) image with source
https://www.attacker.com/a.gif.
Navigator will successfully download this image and for that it will
establish a SSL session with www.attacker.com. This session then stays
open.
When the order form is accessed, Navigator tries to establish another
SSL
session, this time to www.thebookstore.com. Since DNS server claims this
server has the same IP address as www.attacker.com (99.99.99.99),
Navigator
will use the existing SSL session with 99.99.99.99 and will not check
the
certificate.
The result: Navigator is displaying a SECURE ORDER FORM that it believes
to
be originating from the genuine server www.thebookstore.com 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 https://www.thebookstore.com/...
The properties would show that the certificate used was issued for host
www.attacker.com.
Also, monitoring network traffic would show that the server is not at
100.100.100.100 where it should be but rather at 99.99.99.99.
It is a very rare practice to check any of these when nothing suspect is
happening.
Notes
- -----
It should be noted that in the previous exploit, if the users tried to
access https://www.thebookstore.com 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
https://www.thebookstore.com a successful secure connection is made to
https://www.attacker.com. That's why a valid certificate must be
installed
on www.attacker.com.
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.
SOLUTION
========
Netscape has (even prior to our notification - see the Acknowledgments
section) provided a Navigator Add-on called Personal Security Manager
(PSM),
freely downloadable at:
http://www.iplanet.com/downloads/download/detail_128_316.html
Installation of PSM, as far as we have tested it, corrects the
identified
flaw.
Netscape Communicator (v4.73) currently includes the fix for this
vulnerability. It is available for download at:
http://home.netscape.com/download/
WORKAROUND
==========
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 https://www.verisign.com, make sure the
certificate used is issued for www.verisign.com and not for some other
hostname.
ADVISORY
========
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
certificates).
TESTING RESULTS
===============
Tests were performed on:
Communicator 4.72 - affected
Communicator 4.61 - affected
Navigator 4.07 - affected
ACKNOWLEDGMENTS
===============
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).
REFERENCES
==========
Netscape has issued a Security Note about this vulnerability under a
title
"The Acros-Suencksen SSL Vulnerability" at:
http://home.netscape.com/security/notes/index.html
SUPPORT
=======
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
e-mail: mitja.kolsek na acros.si
PGP Key available at PGP.COM's key server.
PGP Fingerprint: A655 F61C 5103 F561 6D30 AAB2 2DD1 562A
DISTRIBUTION
============
This report was sent to:
- - BugTraq mailing list
- - NTBugTraq mailing list
- - Win2KSecAdvice mailing list
- - SI-CERT
- - ACROS client mailing list
DISCLAIMER
==========
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.
COPYRIGHT
=========
(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.
=====[END-ACROS-REPORT]=====
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
http://www.cert.org/advisories/CA-2000-03.html
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
iPlanet
Information about this problem is available at
http://home.netscape.com/security/notes/index.html
Microsoft
None of our products are affected by this vulnerability.
_________________________________________________________________
The CERT Coordination Center thanks the ACROS Security Team of
Slovenia (Contact: mitja.kolsek na acros.si), for the bulk of the text
in this advisory.
_________________________________________________________________
Shawn Hernan was the primary author of the CERT/CC portions of this
document.
______________________________________________________________________
This document is available from:
http://www.cert.org/advisories/CA-2000-05.html
______________________________________________________________________
CERT/CC Contact Information
Email: cert na cert.org
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
U.S.A.
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
We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from
http://www.cert.org/CERT_PGP.key
If you prefer to use DES, please call the CERT hotline for more
information.
Getting security information
CERT publications and other security information are available from
our web site
http://www.cert.org/
To be added to our mailing list for advisories and bulletins, send
email to cert-advisory-request na cert.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.
______________________________________________________________________
NO WARRANTY
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; portions Copyright 2000
ACROS, d.o.o., Slovenia.
Revision History
May 12, 2000: Initial release
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