Penetration Testing, Website Hacking

Beginner Guide to Understand Cookies and Session Management

From Wikipedia and w3schools


A cookie is a small piece of data sent by a server to a browser and stored on the user’s computer while the user is browsing. Cookies are produced and shared between the browser and the server using the HTTP Header.

It Allows server store and retrieves data from the client, It Stored in a file on the client side and maximum size of cookie that can be stored is limited up to 4K in any web browser. Cookies have a short time period because they have expiry date and time as soon as the browser closed.

example when you visit YouTube and search for Bollywood songs, this gets noted in your browsing history, the next time you open YouTube on your browser, the cookies read your browsing history and you will be shown Bollywood songs on your YouTube homepage

Creating cookie

The setcookie() function is used for the cookie to be sent along with the rest of the HTTP headers.

When a developer creates a cookie, with the function setcookie, he must specify at least three arguments. These arguments are setcookie (name, value, expiration);

Image Source: Google

Cookie Attributes

  1. Name: Specifies the name of the cookie
  2. Value: Specifies the value of the cookie
  3. Secure: Specifies whether or not the cookie should only be transmitted over a secure HTTPS connection. TRUE indicates that the cookie will only be set if a secure connection exists. Default is FALSE
  4. Domain: Specifies the domain name of the cookie. To make the cookie available on all subdomains of, set the domain to “”. Setting it to will make the cookie only available in the www subdomain
  5. Path: Specifies the server path of the cookie. If set to “/”, the cookie will be available within the entire domain. If set to “/php/”, the cookie will only be available within the php directory and all sub-directories of php. The default value is the current directory that the cookie is being set in
  6. HTTPOnly: If set to TRUE the cookie will be accessible only through the HTTP protocol (the cookie will not be accessible by scripting languages). This setting can help to reduce identity theft through XSS attacks. Default is FALSE
  7. Expires: Specifies when the cookie expires. The value: time ()+86400*30, will set the cookie to expire in 30 days. If this parameter is omitted or set to 0, the cookie will expire at the end of the session (when the browser closes). Default is 0

Necessity of Cookies

Cookies can be used for various purposes –

  • Identifying Unique Visitors.
  • Http is a stateless protocol; cookies permit us to track the state of the application using small files stored on the user’s computer.
  • Recording the time each user spends on a website.

Type of cookies

Session Cookie

This type of cookies dies when the browser is closed because they are stored in the browser’s memory. They’re used for e-commerce websites so the user can continue browsing without losing what he put in his cart. If the user visits the website again after closing the browser these cookies will not be available. It is safer because no developer other than the browser can access them.

Persistent Cookie

These cookies do not depend on the browser session because they are stored in a file of browser computer. If the user closes the browser and then access the website again then these cookies will still be available. The lifetime of these cookies are specified in cookies itself (as expiration time). They are less secure.

Third Party Cookie

A cookie set by a domain name that is not the domain name that appears in the browser address bar these cookies is mainly used for tracking user browsing patterns and/or finding the Advertisement recommendations for the user.

Secure Cookie

A secure cookie can only be transmitted over an encrypted connection.  A cookie is made secure by adding the secure flag to the cookie. Browsers which support the secure flag will only send cookies with the secure flag when the request is going to an HTTPS page.

HTTP Only Cookie

It informs the browser that this particular cookie should only be accessed by the server. Any attempt to access the cookie from the client script is strictly prohibited. This is important security protection for session cookies.

Zombies Cookie

A zombie cookie is an HTTP cookie that is recreated after deletion. Cookies are recreated from backups stored outside the web browser’s dedicated cookie storage.


PHP session: when any user made any changes in a web application like the sign in or out, the server does not know who that person on the system is. To shoot this problem PHP session introduce which store user information to be used across several web pages.

Session variables hold information about one single user and are exist to all pages in one application.

Example: login ID user name and password.

Session ID

PHP code generates a unique identification in the form of hash for that specific session which is a random string of 32 hexadecimal numbers such as 5f7dok65iif989fwrmn88er47gk834 is known as PHPsessionID.

session ID or token is a unique number which is used to identify a user that has logged into a website. The session ID is stored inside the server, it is assigned to a specific user for the duration of that user’s visit (session). The session ID can be stored as a cookie, form field, or URL.


Now let’s have a look over this picture and see what this picture says:

In the given picture we can clearly see there are three components inside it: HTTP Client, HTTP server and Database (holding session ID).

Step1: the client sends a request to the server via POST or GET.

Step2: session Id created on the web server. Server saves session ID into the database and using set-cookie function send session ID to the client browser as a response.

Step3: a cookie with session ID stored on client browser is sent back to the server where server matches it from the database and sends a response as HTTP 200 OK.

Image Source:

Session hijacking

As we know different users have unique session ID when an attacker sniff the session via man-in-middle attack or via XSS and steal session ID or session token this is called session hijacking. When an attacker sends the stealing session ID to the web server, server match that ID from database stored session ID. If they both matched to each other then the server reply with HTTP 200 OK and attacker get successfully access without submitting proper Identification.

Example 1: Using the session cookies issued to the user by the server.

For example, on any website an official user logged-in, and the server has generated a session cookie SESSION-TOKEN for that user. If the SESSION-TOKEN is the cookie which recognized the session of that user, the attacker can steal the SESSION-TOKEN cookie value to login as the legitimate user. The attacker can perform a cross-site scripting or other technique to steal the cookie from the victim’s browser.

Let’s suppose the attacker steals the cookie PHPSESSID=user-raj-logged-in-2341785645. Now, he can use the cookie with the following request to post a status (HACKED!!!!!!) in the victim’s home page:

POST /home/post_status.php HTTP/1.1
Cookie: PHPSESSID=user-raj-logged-in-2341785645
Status= HACKED!!!!!&Submit=submit

The attacker uses the cookie subjected to the authorized user, and gains control on the user’s session.

Example 2: Guessing the cookie values of users if a complicated algorithm is not used for the cookie generation.

For example, consider a website uses an algorithm to generate cookies for the users. If the user name is “raj”, then the cookie generated for the user could be “LOGINID=-772017- qszbik”. In this case, the algorithm used to generate the cookie can be as follows:

The first part of the cookie is the date i.e. 7/7/2017 and the second part is the arrangement of the previous and next alphabet letter for each letter of the username “John” (i.e., the previous letter for r is “q” and the following letter is “s”). If the attacker is able to break the algorithm, he might estimate the cookie of users and hack their session.




If the attacker decides to hack the session of admin, he can make a cookie as LOGINID =772017- zbcelnhjmo, login to raj’s session and post a status on his account.

Cookie:LOGINID =772017- zbcelnhjmo
Todays_status=I am hacked!!!!!!&Submit=submit
Session hijacking tutorial

For this tutorial, I have targeted DVWA, here cookie name is dvwa Session.

Note: session ID for this page will change every time when we will close the browser.

Now capture the browser request using burp suite.

From the given image we can see the cookie holds PHPSESSID P38kq30vi6arr0b321p2uv86k0; now send this intercepted data into a repeater to observe its response.

In response you can see the highlighted data show set-cookie: dvwaSession =1 more over HTTP 200 OK response from server-side.

According to the developer each time a new sessionID will generate by server each time, but attacker sniff this session ID P38kq30vi6arr0b321p2uv86k0 for unauthorized login.

Next time we receive another session id when data is intercepted through burp suite i.e. PHPSESSID= gutnu601knp4qsrgfdb4ad0te3, again send this intercepted data into a repeater to observe its response.

But before we perceive its response, replace new PHPSESSID from old PHPSESSID.

From the given image you can observe we have replaced the SESSION ID and then generate its response in which set-cookie: dvwaSession =6 and HTTP 200 OK response from server-side.  

Now change the value inside intercepted data and then forward this request to the server.

Session Vs cookies

Session Cookies
Data are stored on Server Data are stored in Client’s Browser
Sessions Data are more secure because they never travel on every HTTPRequest Travel with each and Every HTTP request
You can store Objects (Store Large Amount of Data) You can store strings type (Max File Size 4 kb)
Session Cannot be used for Future Reference Cookies are mostly used for future reference

Author: Aarti Singh is a Researcher and Technical Writer at Hacking Articles an Information Security Consultant Social Media Lover and Gadgets. Contact here