Insecure Deserialization

"Insecure Deserialization is a vulnerability which occurs when untrusted data is used to abuse the logic of an application" (Acunetix., 2017)

This definition is still quite broad to say the least. Simply, insecure deserialization is replacing data processed by an application with malicious code; allowing anything from DoS (Denial of Service) to RCE (Remote Code Execution) that the attacker can use to gain a foothold in a pentesting scenario.

Specifically, this malicious code leverages the legitimate serialization and deserialization process used by web applications. We'll be explaining this process and why it is so commonplace in modern web applications.

OWASP rank this vulnerability as 8 out of 10 because of the following reasons:

- Low exploitability. This vulnerability is often a case-by-case basis - there is no reliable tool/framework for it. Because of its nature, attackers need to have a good understanding of the inner-workings of the ToE.

- The exploit is only as dangerous as the attacker's skill permits, more so, the value of the data that is exposed. For example, someone who can only cause a DoS will make the application unavailable. The business impact of this will vary on the infrastructure - some organisations will recover just fine, others, however, will not.

What's Vulnerable?

In summary, ultimately, any application that stores or fetches data where there are no validations or integrity checks in place for the data queried or retained. A few examples of applications of this nature are:

- E-Commerce Sites - Forums - API's - Application Runtimes (Tomcat, Jenkins, Jboss, etc) Deploy


A prominent element of object-oriented programming (OOP), objects are made up of two things:

  • State

  • Behavior

Simply, objects allow you to create similar lines of code without having to do the leg-work of writing the same lines of code again. For example, a lamp would be a good object. Lamps can have different types of bulbs, this would be their state, as well as being either on/off - their behavior!

Rather than having to accommodate every type of bulb and whether or not that specific lamp is on or off, you can use methods to simply alter the state and behavior of the lamp.

if a dog was sleeping, would this be: a behavior


A Tourist approaches you in the street asking for directions. They're looking for a local landmark and got lost. Unfortunately, English isn't their strong point and nor do you speak their dialect either. What do you do? You draw a map of the route to the landmark because pictures cross language barriers, they were able to find the landmark. Nice! You've just serialised some information, where the tourist then deserialised it to find the landmark. Continued Serialisation is the process of converting objects used in programming into simpler, compatible formatting for transmitting between systems or networks for further processing or storage. Alternatively, deserialisation is the reverse of this; converting serialised information into their complex form - an object that the application will understand. What does this mean? Say you have a password of "password123" from a program that needs to be stored in a database on another system. To travel across a network this string/output needs to be converted to binary. Of course, the password needs to be stored as "password123" and not its binary notation. Once this reaches the database, it is converted or deserialised back into "password123" so it can be stored.

How can we leverage this? Simply, insecure deserialization occurs when data from an untrusted party (I.e. a hacker) gets executed because there is no filtering or input validation; the system assumes that the data is trustworthy and will execute it no holds barred.


Cookies are an essential tool for modern websites to function. Tiny pieces of data, these are created by a website and stored on the user's computer.

Websites use these cookies to store user-specific behaviors like items in their shopping cart or session IDs.

Whilst plaintext credentials is a vulnerability in itself, it is not insecure deserialization as we have not sent any serialized data to be executed!

Cookies are not permanent storage solutions like databases. Some cookies such as session ID's will clear when the browser is closed, others, however, last considerably longer. This is determined by the "Expiry" timer that is set when the cookie is created.

Some cookies have additional attributes, a small list of these are below:




Cookie Name

The Name of the Cookie to be set


Cookie Value

Value, this can be anything plaintext or encoded


Secure Only

If set, this cookie will only be set over HTTPS connections



Set a timestamp where the cookie will be removed from the browser



The cookie will only be sent if the specified URL is within the request


Creating Cookies

Cookies can be set in various website programming languages. For example, Javascript, PHP or Python to name a few. The following web application is developed using Python's Flask, so it is fitting to use it as an example.

dateTime =
timestamp = str(dateTime)
resp.set_cookie("registrationTimestamp", timestamp)

Setting cookies in Flask is rather trivial. Simply, this snippet gets the current date and time, stores it within the variable "timestamp" and then stores the date and time in a cookie named "registrationTimestamp".

This is what it will look like in the browser.

  • If a cookie had the path of , what would the URL that the user has to visit be?


  • What is the acronym for the web technology that Secure cookies work over?

    • HTTPS

Cookies Practical

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