Cookies Continued...pt2

So our first step is cookie collection. In here we're trying to figure out to answer some basic questions such as :

• "how many cookies does the application create?"
• The way I did this is by first clearing all the cookies in my browser, and then visiting the application. Then, I check the cookie list.  Doing this tells me about all the cookies created when accessing the application.

• "which parts of the application generates a cookie?"
• Go to different parts of the web application and check the browser to see if any cookies are created.

• "which parts of the application causes a modification of the cookie?".
• Same as above - when we're messing around with the application, see if the status of any of the cookies has changed.

Our next step is to analyze the cookie. For example, look at the session ID of multiple cookies. Are they just linearly made? Are they random? Does duplicates exists?  We can also examine the value of the cookie. Often times, developers make the mistake of storing actual sensitive data in the cookie. What their actually suppose to do is store a generic value and reference the real data using the generic value. Some developers store real data in cookies and just encode various methods like Hex, Base64, or MD5. What they fail to understand is that these strings can easily be decoded and the real data can be seen by unwanted eyes.

If  tokens are in fact encoded, they could contain multiple sets of data in one long string. For example, if the token is 32 bit string, the first 16 bits could represent something static like a username or IP address and the next 16 bits could by dynamic. If this second 16 bit string changes incrementally, this could represent a date or time for each access.  One way to verify this is access the site at the same time using different browsers and using the same username/IP address. In that case, the cookies from each browser should be almost the same. 

Session Management Testing & Cookies

Session Management is how web application maintains and controls all user-interaction within the application. From authentication and logging off  - sessions are used to control all interactions between clients & servers. Client-server sessions are identified through a unique token often referred to as a Session ID or Cookie. Just like everything else on the web, sessions can be exploited and one such way is by using the created session by an authorized to our advantage.

If session IDs/Cookies are created in a predictable way, we can easily duplicate a valid cookies to "hijack the sessions of legitimate users".

So, first why are cookies used? Their used because they are an easy way to keep track of a client through multiple requests. It's how companies keep details about a certain client.  Cookies will last until they expire or they are destroyed (by server or client). OWASP describes the main advantages of as giving "a state to a stateless protocol like HTTP".  They store vital information for websites and thus can be a target for hackers.  The list below describes the patter for "cookie hacking".

1. Collect information about the cookie. Make sure to get a good sample

2. Perform cookie reverse engineering - in other words, try to figure out the cookie generation algorithm.

3. Manipulate cookies - once we figure out the algorithm, we can try to forge a valid cookie to attack the server.

Since the topic of Cookies and sessions is broad, I want to limit the size of my posts - I will continue on in the next post.

Auth. Testing - Testing for Logout & Browser cache Management

This test ensures that once a user "logs out" - they are actually logged out and it is not possible to reuse the user's session.  Secondly, we check to see if there is a timeout feature where after X amount of idle activity - the user is automatically logged out. The application must work in a way where once logged out, all session tokens (cookies) are either erased or made unusable.  Usually, we just want to make it unusable by rejecting the use of the cookie on a server level.

To properly assess this type of exploit, we must first find out what happens after a user logs out. Are session cookies erased? Are they made unusable? The easiest way to do this is by logging out, and then clicking the browser's back button. Are we still authenticated? In that case the cookies are still being used and can be exploited. If after clicking the back button we can still see visited pages but not access any new pages - it means that the private information was cached. The page allowed the browser to cache the content by "not setting the cache-control header".

It is important to have some type of server side code that prevents the reusing of old cookies.  Since cookies exists client side and can be edited by users, simply changing the values of the cookies to make it unusable is bad practice. We need to completely disallow an inactive cookie from doing any type of authentication.

How to Edit Cookies in Chrome.