cookies and session

http://www.flickr.com/photos/torkildr/3462607995/Cookies and sessions

Monday, October

13, [email protected], [email protected] 1

HTTP is “stateless”

• By default, web servers are “forgetful”

• As far as they can tell, every request comes from a

totally new and different browser

• (Not exactly true. We'll discuss persistent connections in

the context of performance.)

Monday, October


Pros of stateless servers

• Chief benefit: Potential for replication

• Improved performance: A sysadmin can fire up N copies

of a website (on N machines) and any machine can serve

each request.

• Improved reliability: If a machine crashes, then another

can be started up in its place, and no data gets lost in the

process.

Monday, October


Cons of stateless servers

• Chief problem: Keeping track of which requests "go

together"

• Security challenge: If a user submits username & password

in http request X, then tries to access resources in http

request Y, how does the server know that request Y is from

somebody who already logged in?

• By the time that request Y comes in, the server will already

have forgotten that request X ever occurred.

• And on a replicated system, request Y might be served by a

different machine than request X.

Monday, October


Cookies to the rescue!

• Reminder:

• Cookie = a piece of data that is automatically copied

between the client and server

• Cookies can be set by the client (as in the last unit) or

by the server.

Monday, October


A simple way to use cookies for

login…

• When user sends a valid username & password in

request X, the server replies with a cookie containing

the username & password

• When user subsequently makes request Y, the

browser sends along the cookie.

• Sounds appealing: user only needs to log in once

• Serious security hole: anybody who gets his hands on the

user's computer can see cookies

Monday, October


Using just cookies for login

Monday, October


Browser Server

Type username& password Send username

& password

Cookie = usernm&pwd

Authenticate

Click a linkor whatever Request page

(send cookie)

Send back page

Warning

This design contains a

serious security hole.

A more secure way of cookies+login

• When user sends a valid username & password in

request X, the server replies with a cookie containing

a secret that the client couldn't possibly have guessed.

• When user subsequently makes request Y, the

browser sends along the cookie.

• Since the client couldn't have guessed this value without

logging in, the server knows that the user did in fact

previously log in.

Monday, October


Using cookies for login

Monday, October


Browser ServerFilesystem or

Database

Type username& password Send username

& password

Store a random numbervalid only for next 10 minutesCookie = the

random #

Authenticate

Click a linkor whatever Request page

(send cookie) Check if the number is right;if so, give another 10 minutes

Send back page

Session = state stored across

requests

• This is what we call a "session"

• Session is basically an add-on to the basic http functionality of a website• So that the website can remember information across

requests.

• You can store lots of stuff in session• Numbers, strings, stringified objects, …

Monday, October


Pros of sessions

• Stores information between requests

• Much more secure than the simple cookie-based

approach I showed you

• A bad person would need to steal the random number

(cookie) within 10 minutes of its creation

Monday, October


Cons of sessions

• Requires your web server to have write-access to some sort of storage medium• File system, database, …, if you want replication

• Otherwise just use memory (lost on server crash)

• Requires user to access site every few minutes• Though you can configure longer or shorter times

• This is a tradeoff between usability & security.

• EECS servers currently are set to 24 minutes.

Monday, October


Simple example of using session

<?php

session_start();

if (isset($_SESSION['numhits']))

$_SESSION['numhits'] = $_SESSION['numhits']+ 1;

else

$_SESSION['numhits'] = 1;

echo "You hit my server ".$_SESSION['numhits']." times.";

?>

Monday, October


Authentication

(Using hardcoded username&pwd

for now)<?php session_start(); /* login.php */

if (array_key_exists("username", $_REQUEST)

&& array_key_exists("password", $_REQUEST)) {

/* here is where we would check the username and password */

$_SESSION['uid'] = 1;

echo '<a href="inventory.php">View Inventory</a>';

} else {

?>

<form action="login.php" method="POST">

<div>Username: <input type="text" name="username"></div>

<div>Password: <input type="password" name="password"></div>

<div><input type="submit" value="OK"></div>

</form>

<?php

}

?>

Monday, October


<?php session_start(); /* inventory.php */

if (isset($_SESSION['uid']))

echo "This is where we would show inventory.";

else

echo "You need to <a href='login.php'>Log in</a>";

?>

You can set cookies without session

<?php

$nhits = isset($_COOKIE['numhits']) ? $_COOKIE['numhits'] : 0;

$nhits = $nhits + 1;

setcookie('numhits', $nhits, time()+86400*365);

/* expires in 365 days */

echo "You hit my server ".$nhits." times.";

?>

Monday, October


Summarizing cookies vs sessions

• Cookies

• Little bits of data that are stored on client but also copied automatically to the server

• Useful for storing little bits of data on the client, but they are visible to everybody

• So don't store sensitive data in cookies

• Sessions

• Data is stored on the server (e.g., filesystem), keyed by a random number

• The random number is sent as a cookie to the browser

• And the random number expires after a little whileMonday, October


When to use cookies vs sessions

• Use cookies when

• You need to save a small amount of data between requests,

and it doesn't need to be kept secret

• Use sessions when

• You need to save a larger amount of data between requests,

or when the data needs to be secret

Monday, October


Examples of information not to

store in unencrypted cookies

• Passwords

• Credit card numbers

• Social security numbers

• Student ID numbers

• Birthdates

• List of diseases the user has contracted

• Anything that must be kept secret

Monday, October


Yet another caveat

• After all of those warnings, you still can save secret

data in cookies, IF IT IS ENCRYPTED

• You will see how to do this later in the term

• But we don't really use encrypted cookies much

because it can cause usability problems.

Monday, October


cookies and session

Technology

http request y

http request x

server cookies

username passwordwhen

server replies

valid username password

different machine

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