Cookies

COEN 351 E-commerce Security

Client / Session Identification

HTTP Headers Client IP Address HTTP User Login FAT URLs Cookies

Client / Session Identification

HTTP Header fields: “From”

User’s email address, request. Could be used by all browsers, but are only

used for web-bots gathering data.

“User-Agent” User’s browser software, request.

“Referer” (Sic) Page user came from by following link

Client / Session Identification

HTTP Header fields: “Authorization”

User name and password “Client-ip” “X-Forwarded-For”

Client-ip “Cookie”

Client / Session Identification

User-Agent Gives the server information about

the browser.

Client / Session Identification

Client IP Address Not part of the HTTP header Available from the package Easily spoofed Changed by NATs and Proxies

Client / Session Identification HTTP login based on WWW-

Authenticate and Authorization headers.

1. Browser requests page with GET2. Server anwsers with: 401 Login Required,

WWW-authenticate: Basic realm=“joe”3. Browser resends GET request, adds

Authorization: Basic am98re454. Server fulfills request.5. Browser now will resend stored user-name

with every request.

Client / Session Identification Fat URL

Maintain state information in the URL Server generates a session id. Server adds session id to all URLs requested

from the hyperlink. Amazon.com uses this technique.

http://www.amazon.com/exec/obidos/subst/home/home.html/103-6082309-4209430

http://www.amazon.com/exec/obidos/ASIN/0439784549/ref=s9_ts_r/103-6082309-4209430

http://www.amazon.com/gp/cart/view.html/ref=ord_cart_shr/103-6082309-4209430

Cookies

Cookies: ASCI strings stored at the browser. Submitted with each request to a

target website.

Cookies

Cookies: Session cookies

Stored only for the duration of a web-session.

Persistent cookies Remain stored until they expire.

Cookies

Cookie-Jar Client-side state storage Netscape / Firefox store cookies in a

single text file called cookies.txt MS IE stores cookies in the cache.

Cookies

Server specifies optional domain. Cookie gets sent with all requests to

this domain. Server specifies optional expiration

date Server can specify “secure” option:

Cookie is only sent when using SSL.

Cookies

Version 0 cookies (Netscape cookies)Set-Cookie: name=value [;expires=date]

[;path=path] [;domain-name = value] [;secure]

Set-Cookie: customer=Mary; expires Wednesday, 09-September-2006 24:00:01 GMT; domain=“scu.edu”; path=/soe; secure

Cookies

Version 1 cookies (RFC 296) Less-used Provides a number of extensions

Cookies

Privacy risk Can be controlled by web-browser. Used to track consumer behavior.

Harder, but possible to track an individual user.

Cookies

Security Risk Users can change cookies before

continuing to browse. Counter-measure: strong encryption

Users could swap / steal cookies. E.g. when used for authentication

Session Hijacking

Cookies

Session Hijacking Counter measure:

Server needs to send a new cookie after every change in state and verify that a request comes with a valid cookie.

For example, by appending a MAC of session state to the cookie after each change of state.

Cookies Poor practices:

Poor encryption of cookies. Web-based email uses a cookie for

authentication. Cookie contains the user name encrypted by

XOR-ing with a secret string. Attacker can crack the cookie encryption by

creating fake accounts. Attacker can now craft a cookie useful for

authentication. Something similar happened to hotmail

and yahoo early on.

Cookies

Poor practices: Poor encryption of cookies.

Shopping cart encoded in cookie. Cookie contained shopping cart details in plain

text. Attacker changed prices of items.

Relying on cookie for authentication Cookie is sniffed from the net. Cookie is stolen by impersonating a web-

site.

Cookie Alternative: Web Bugs

Used to track viewers of web-sites. HTML page contains a request to

download a resource from a “counting” site.

The resource is so small that the viewer does not notice the download.

Counting site receives the request and adds IP address to its user database.

Cookie Alternative: Web Bugs Examples:

Found by Privacy Foundation on Intuit’s home page for Quicken.com several years ago.

<img src=“http://ad.doubleclick.net/ad/pixel./quicken/NEW” width=1 height=1 border=0>

<IMG WIDTH=1 HEIGHT=1 border=0

SRC=“http://media.preferences.com/ping?ML_SD=IntuitTE_Intuit_1x1_RunOfSite_Any&db_acfr=4B31-C2FB-10E2&event=reghome&group=register&time=1999.10.27.20.5 6.37”>

Cookie Alternative: Web Bugs

Can be embedded in any html code. User profiles written in html. Email messages.

But only when read with a client that can display HTML messages and with a computer connected to the internet.

Usenet messages.