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 lista första sista föregående nästa 
Text 5582, 192 rader
Skriven 1999-08-11 06:47:01 av Eddy L O Jansson (2:206/233.0)
   Kommentar till text 5577 av Henrik Schröder (2:201/128.9)
Ärende: Kakmonstret.
====================

HS> För vad? Vad är så hemskt som kan hända om du tar emot kakor? Vad
HS> kan kakor göra som du inte tycker om?


                                      "Cookies are inherently evil.
                                       Just say no to cookies."
                                                  - Randal Schwartz

The mechanism for 3rd party cookies has been criticized because
of its implications for user privacy.  With the cooperation of
many websites, 3rd party cookies are used to monitor the surfing
habits of users.  However repugnant this may seem, it stems
from the legitimate function of HTTP, namely the willingness
of your browser to accept IMG tags as "instructions" of where
to connect.  But consider the security implications of a malicious
abuse of this mechanism.

While I am unwilling to go as far as Randal about cookies in general,
I am beginning to conclude that "3rd party" cookies really are evil.
Unfortunately, you cannot just say 'no' to 3rd party cookies.  In
both Netscape Communicator and MS Internet Explorer, there is no way
to turn off all 3rd party cookie activity (unless you disable cookies
entirely).  Communicator will let you refuse to *accept* these
cookies but will not control their divulgence.

IMHO, the unauthorized divulgence of 3rd party cookies makes up the
other half of their "evil" equation.  This is particularly true when
a cookie is used as a kind of weak authentication token.  (At
least one E-commerce site will let you charge to a user's credit
card by merely presenting the correct persistent cookie.  I won't
give their name publicly because, hey, it's a jungle out there ;-)

The active attacks presented below show that an arbitrary HTTP cookie
of the attacker's choosing can literally be *demanded* from a browser
any time its user surfs.  Under certain circumstances, HTTPS cookies
can be stolen.  These secure cookies are at best, only as secure as
the weakest mode of SSL *ever* used by the browser. This may be very
different from the mode of SSL enabled when the user *intends* to
send a secure cookie.

Stealing HTTP Cookies
---------------------

A single web page may make connections to several sites in order to
retrieve all of its graphical hypertext media.  Thus, typing a single
URL

        http://www.acme.com/

may spawn many connections to graphics.acme.com, each resulting from
IMG tags of the form

        <IMG src="http://graphics.acme.com/foo.jpg">.

The graphics.acme.com site could make use of cookies in order to
provide dynamic images tailored to the user's preferences.  This
example has every appearance of being legitimate, but the implicit
trust placed by the browser in the HTTP response could be
unwarranted.  A malicious adversary could actively modify the HTML to
include a false IMG tag such as

        <IMG src="http://e-commerce.widgetstore.com/hrule.jpg">

forcing the unsuspecting browser to send its Widget Store cookie out
into the Internet.  The attacker -- acting as an "intruder in the
middle" -- steals the the cookie of choice from user's browser.  With

   TARGET_URL = http://e-commerce.widgetstore.com/hrule.jpg,

and

   server = any legitimate server (e.g. home.netscape.com),

the main stages of this attack are depicted below:

            GET ...
   browser  --------------------------------------> intruder

            GET ...
   intruder --------------------------------------> server

            <html>...</html>
   server   --------------------------------------> intruder

            <html>...<img src=TARGET_URL></html>
   intruder --------------------------------------> browser

            Cookie: foo=bar; ...
   browser  --------------------------------------> intruder
            (intended for TARGET_URL)

            forged TARGET_URL resource
   intruder --------------------------------------> browser


Note that it is not necessary for the intruder to stand
in between the browser and the server corresponding
to TARGET_URL.  That is just the easiest way for the intruder
to go undetected.


Stealing HTTPS Cookies
----------------------

By definition, HTTPS cookies are never sent without SSL protection.
However, variants of our attack to steal HTTP cookies could be
designed to exploit SSL weaknesses.

Suppose your Widget Store E-Commerce cookie is secure and its server
supports 128-bit encryption.  On the other hand, suppose that Al's
Shitty Mortgage Company supports only 40-bit encryption and only SSL
version 2.  You don't like 40-bit SSLv2, but you are willing to drop
your guard temporarily in order to connect to Shitty Mortgage and get
Al's latest interest rate.

Throughout this section, the target cookie is the Widget Store
E-Commerce cookie with SSL URL

   TARGET_URL = https://e-commerce.widgetstore.com/hrule.jpg

Also take,

   server     = any http server (e.g. Netscape home)
   target     = e-commerce.widgetstore.com

We assume that the small horizontal rule image (common to many servers)
is available from Widget Store and can be added to any HTML without
being noticed.  The following attack combines our HTTP cookie
stealing with the well-known ciphersuite rollback attack (see [WS96])
in which an SSLv2 session is forced into 40-bit mode.  The attacker
actively acquires the desired cookie encrypted with an unknown 40-bit
key.  After a couple hours of exhaustive search, the plaintext cookie
is recovered.

   intruder: Wait for browser to drop to 40-bit SSLv2 and
             connect to server  (listen to Al's traffic).

            GET ...
   browser  --------------------------------------> intruder

            GET ...
   intruder --------------------------------------> server

            <html>...</html>
   server   --------------------------------------> intruder

            <html>...<img src=TARGET_URL></html>
   intruder --------------------------------------> browser

            Ciphersuite rollback attack: browser
            & target establish 40-bit session key, k.
   browser  <-------------> intruder <------------> target

            {Cookie: foo=bar; ...}_k
   browser  --------------------------------------> intruder

            {Cookie: foo=bar; ...}_k
   intruder --------------------------------------> target

            {hrule.jpg}_k
   target   --------------------------------------> intruder

            {hrule.jpg}_k
   intruder --------------------------------------> browser


Note that because the intruder cannot easily determine the 40-bit
session key in real time, she must remain in the loop and wait to
conduct a brute-force search off-line.

References
----------

[CKY1] Persistent State HTTP Cookies, Netscape Communications,
       URL: http://www.netscape.com/newsref/std/cookie_spec.html

[CKY2] D. Kristol, L. Montulli, HTTP State Management Mechanism,
       RFC 2109, 1997.

[W3C]  World Wide Web Consortium Security FAQ,
       URL:  http://www.w3.org/Security/Faq/.

[WS96] D. Wagner, B. Schneier, "Analysis of the SSL 3.0 Protocol",
       1996, URL: http://www.counterpane.com/ssl.html.

John Pliam
pliam@ima.umn.edu
http://www.ima.umn.edu/~pliam

---
 * Origin:  (2:206/233)