From: Marc Slemko
Subject: Mozilla Cookie Exploit
A while ago I discovered a bug in Mozilla that lets you steal cookies for
any domain by convincing the browser to load a specially formatted URL; I
have been too busy to get around to making the details known earlier, so
here they are. This is similar to holes that have been found, both by
myself and by others, previous in IE. Details available at
http://alive.znep.com/~marcs/security/mozillacookie/ and are also included
below. Update to Netscape 6.2.1 or Mozilla 0.9.7 for a fix. Using open
source products doesn't magically make you invulnerable to security
problems like those that plague Microsoft.
Mozilla Cookie Exploit
Last Modified: $Date: 2002/01/22 05:06:04 $
$Revision: 1.6 $
Table of Contents
Cookies are often used to identify and authenticate users to a
website. If an attacker can steal a user's cookies, then they can
impersonate that user. The completeness of the impersonation and the
actions the attacker can perform as that user depend on how the
particular site uses the cookies.
This bug in Mozilla allows an attacker to, if he can convince the
user's browser to load a given URL, steal their cookies for any given
domain. It does not require that active scripting is enabled in the
browser, and can be done with something as simple as an image tag,
allowing for hassle free use in HTML email, web based email services,
As expected, this bug is also present in Netscape 6.1. Upgrade to
Netscape 6.2.1 or Mozilla 0.9.7 or higher, which fix this bug.
The take-away message is that, due to implementation bugs in browser
and in web applications, cookies can be stolen. It is critical that
any application that depends on cookies does so with an understanding
of this fact, and takes appropriate measures to limit the damage that
can be done using stolen cookies.
* Current Status Summary: (last updated Mon Jan 21 20:48:17 PST
2002) I finally got around to making this vulnerability public.
* mid-Jan 2002: Netscape put up a note on their site saying that
there was a security hole that they fixed.
* Sometime between when I reported this bug to Netscape and when I
made it public: This bug was fixed with the release of Netscape
6.2.1 and Mozilla 0.9.7.
* November 15, 2001: I reported this bug to Netscape via their
security bug submission form. I had trouble finding a documented
method for submitting security bugs to mozilla.org, but eventually
figured out that email@example.com existed. In any case, both
submissions found their way to the same contact at Netscape.
Cookies are the mechanism used by most websites to identify and
authenticate a user. If you can steal someone's cookies, you can trick
the server into thinking you are them. Exactly what this gains you
depends on the application and how it is designed. It may gain you
very little, or it may gain you a whole lot (eg. Microsoft Passport
to Trouble). For more information about cookies, see The Unofficial
Cookies are set with a specific hostname or a domain, so that they are
only sent to that host or domain, with an exception or two that I
won't go into here. They can also be set with a specific path, or with
the secure flag, which means they will only be sent if the connection
is a SSL connection. Normally, this should mean that only the server
that set the cookie, or others it is operating in cooperation with
(eg. in the same domain) can read it.
Mozilla has a bug that lets you bypass this protection and steal
cookies for any domain. This is quite similar to bugs found in
Microsoft Internet Explorer in the past, such as this one and
this one. As has been shown time and time again, there are many
security flaws in many Microsoft products. Sadly, they are far from
being alone. There is almost certainly no web browser out there that
is functional enough to browse a significant percent of current
popular websites and that does not have similar security holes.
The details are very trivial. Loading a URL such as:
...will cause Mozilla to connect to the hostname specified before the
"%00", but send the cookies to the server based on the entire
hostname. The "%00" is the URL encoded version of the null character,
used in C to terminate strings.
This exploit can be used to steal cookies with a specific path set,
and can be used to steal cookies with the secure flag set, by using
the specific path and SSL in the request URL. Note, however, that
cookies set for a specific hostname (eg. "www.passport.com") can not
be stolen using this method, but only cookies set for an entire domain
This bug was first tested on Netscape 6.1 on Windows 2000 and Mozilla
0.9.5 build 2001111503 and 0.9.5 build 20011012 on Linux. It is
expected that all Netscape 6.x and Mozilla versions prior to the
recently released fixed versions are vulnerable.
An example exploit is available. Very straightforward.
$Id: index.html,v 1.6 2002/01/22 05:06:04 marcs Exp marcs $