# Cookie Tossing

### Description

If an attacker is able to **control a subdomain of the domain of a company or finds a XSS in a subdomain** he will be able to perform this attack.

As it was indicated in the Cookies Hacking section, when a **cookie is set to a domain (specifying it) it will be used in the domain and in subdomains.**

{% hint style="danger" %}
Therefore, **an attacker is going to be able to set to the domain and subdomains an specific cookie doing something like **`document.cookie="session=1234; Path=/app/login; domain=.example.com"`
{% endhint %}

This can be really dangerous as the attacker may be able to:

* **Fixate the cookie of the victim to the attackers account **so if the user doesn't notice, **he will perform the actions in the attackers account **and the attacker may obtain some interesting information (check the history of the searches of the user in the platform, the victim may set his credit card in the account...)
* If the **cookie doesn't change after login**, the attacker may just **fixate a cookie**, wait until the victim logs in and then **use that cookie to log in as the victim**.
* If the **cookie is setting some initial value** (like in flask where the **cookie **may **set **the **CSRF token **of the session and this value will be maintained after the victim logs in), the **attacker may set this known value and then abuse it** (in that scenario, the attacker may then make the user perform a CSRF request as he knows the CSRF token).

### Cookie Order

When a browser receives two cookies with the same name **partially affecting the same scope** (domain, subdomains and path), the **browser will send both values of the cookie** when both are valid for the request.

Depending on who has **the most specific path **or which one is the **oldest one**, the browser will **set the value of the cookie first** and then the value of the other one like in: `Cookie: iduser=MoreSpecificAndOldestCookie; iduser=LessSpecific;`

Most **web sites will only use the first value**. Then, if an attacker wants to set a cookie it's better to set it before another one if set or set it with a more specific path.

{% hint style="warning" %}
Moreover, the capability to **set a cookie in a more specific path** is very interesting as you will be able to make the **victim work with his cookie except in the specific path where the malicious cookie set will be sent before**.
{% endhint %}

### Protection Bypass

A possible protection against this attack would be that the **web server won't accept requests with two cookies with the same name but two different values**.

To bypass the scenario where the attacker is setting a cookie after the victim was already given the cookie, the attacker could cause a **cookie overflow** and then, once the** legit cookie is deleted, set the malicious one**.

{% content-ref url="cookie-jar-overflow.md" %}
[cookie-jar-overflow.md](cookie-jar-overflow.md)
{% endcontent-ref %}

Another useful **bypass **could be to **URL encode the name of the cookie** as some protections check for 2 cookies with the same name in a request and then the server will decode the names of the cookies.

### Cookie Bomb

A Cookie Tossin attack may be used also to perform **Cookie Bomb** attack:

{% content-ref url="cookie-bomb.md" %}
[cookie-bomb.md](cookie-bomb.md)
{% endcontent-ref %}

### Defense**s**

#### **Use the prefix `__Host` in the cookie name**

* If a cookie name has this prefix, it **will only be accepted** in a Set-Cookie directive if it is marked Secure, was sent from a secure origin, does not include a Domain attribute, and has the Path attribute set to /
* **This prevents subdomains from forcing a cookie to the apex domain since these cookies can be seen as "domain-locked"**

### References

* ****[**@blueminimal**](https://twitter.com/blueminimal)****
* ****[**https://speakerdeck.com/filedescriptor/the-cookie-monster-in-your-browsers**](https://speakerdeck.com/filedescriptor/the-cookie-monster-in-your-browsers)****
* ****[**https://github.blog/2013-04-09-yummy-cookies-across-domains/**](https://github.blog/2013-04-09-yummy-cookies-across-domains/)****