Research and Development

What are cookies and why are they important?

A cookie is a small piece of data sent from a web site and stored in a user’s web browser and is subsequently includes with all authenticated requests that belong to that session. Some cookies contain the user session data in a web site, which is vital. Others cookies are used for tracking long-term records of an individuals browsing history and preferences such as their preferred language. Sometimes they are also used for tracking and monitoring a user’s activities across different web sites.

Due to the fact that HTTP is a stateless protocol, the web site needs a way to authenticate the user in each request. Every time the user visits a new page within a web site, the browser sends the users cookie back to the server, allowing the server to serve the correct data to that individual user, which is tracked using a session ID. Cookies therefore play an integral part in ensuring persistence of data used across multiple HTTP requests throughout the time a user visits a web site.

What does a cookie look like?

Set-Cookie: __cfduid=d8a3ae94f81234321; expires=Mon, 23-Dec-2019 23:50:00 GMT; path=/;; HttpOnly

The cookie below is an example of a common cookie generated for WordPress. Here we break down each part of the cookie and explain what it is used for:

  • Set-Cookie – the web server asks the browser to save the cookie with this command
  • __cfduid=d8a3ae94f81234321;: This is the cookie itself. At the left of the equals symbol is the name of the cookie and to the right is its value
  • expires=Mon, 23-Dec-2019 23:50:00 GMT; – this is the date and time when the cookie will expire
  • path=/;; – the cookie domain and path define the scope of the cookie. They tell the browser that cookies should only be sent back to the server for the given domain and path
  • HttpOnly – this attribute (without a value associated) tells the browser that JavaScript cannot be used to access the cookie, which must only be accessed through HTTP or HTTPS. Sometimes you will also see the attribute “Secure”, which prevents the cookie being sent over the unencrypted HTTP protocol (i.e. the cookie will only be transmitted over HTTPS)

What is the impact of having your cookies compromised?

A traditional and important role of a cookie is to store a users session ID, which is used to identify a user. If this type of cookie is stolen by a malicious user, they would be able to gain access to web site as the user for which the cookie belonged to (i.e. the malicoius user would have access to your account within the web site).

In the case of the tracking cookie, the malicious user would have access to your browsing history for the web site.

Another problem arises when sensitive data is stored in cookies, for example a username, and this is also a vector for server side exploitation if its contents are not properly validated, which can potentially lead to serious vulnerabilties such as SQL Injection or remote code execution.

What are the main cookie threats?

cookie monster image

Cookie Monster.

There are different attacking vectors in which obtaining and modifying cookies can occur, leading to session hijacking of an authenticated user session, or even SQL injection attacks against the server. These threats may take place when an attacker takes control of the web browser using Cross-site Scripting, or Spyware, in order to obtain a users SessionID cookie that can then be used by an attacker to impersonate the legitimate user, as shown in the following example:

Obtaining access to the cookie can be as easy as using the following JavaScript line:


Imagine that the web site has a search form that is vulnerable to Cross-site Scripting (Reflective Cross-site Scripting in this case).

An attacker could use the following payload to send the cookie to an external web site:


The final step would be to send the vulnerable link to an admin and wait for them to click on it. If the attacker uses an URL shortener, this allows for further obfuscation of the malicous URL, as the admin will be unable to see the content of the link they have been sent.

An attacker able to read files from a given user may also attempt to retrieve the cookies stored in files from a system. Furthermore some browsers store persistent cookies in a binary file that is easily readable with existing public tools.

Security weaknesses may also reside server side when cookies are modified, if input validation routines are not adequately implemented. The example below shows how to bypass the authentication process:

//In /core/user.php: (cs cart vulnerability)

if (fn_get_cookie(AREA_NAME . '_user_id')) {
 $udata = db_get_row("SELECT user_id, user_type, tax_exempt, last_login, membership_status, membership_id FROM $db_tables[users]
 WHERE user_id='".fn_get_cookies(AREA_NAME . '_user_id')."' AND password='".fn_get_cookie(AREA_NAME . '_password')."'");
 fn_define('LOGGED_VIA_COOKIE', true);


//Cookie: cs_cookies[customer_user_id]=1'/*;

For their role, cookies are really important and may be used in different attacks.

Now that you are more aware of the dangers, it would be wise to ensure steps are taken to deploy web site cookies safely and securely. Look out for the second part of this post!

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