# Cookie Tossing

{{#include ../../banners/hacktricks-training.md}}

### Description

If an attacker can **control a subdomain or the domain of a company or finds an 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 subdomains.**

> [!CAUTION]
> Therefore, **an attacker is going to be able to set to the domain and subdomains a specific cookie doing something like** `document.cookie="session=1234; Path=/app/login; domain=.example.com"`

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

- **Fixate the cookie of the victim to the attacker's account** so if the user doesn't notice, **he will perform the actions in the attacker's 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...)
  - An example of this [can be found here](https://snyk.io/articles/hijacking-oauth-flows-via-cookie-tossing/) where the attacker set his cookie in specific sections a victim will use to authorize **access to his git repos but from the attackers account** as he will be setting his cookies in the needed endpoints.
- If the **cookie doesn't change after login**, the attacker may just **fixate a cookie (session-fixation)**, wait until the victim logs in and then **use that cookie to log in as the victim**.
  - Sometimes, even if the session cookies changes, the attacker use the previous one and he will receive the new one also.
- 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).
  - Just like setting the value, the attacker could also get an unauthenticated cookie generated by the server, get the CSRF token from it and use it.

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

> [!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**.

### Protection Bypass

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**.

{{#ref}}
cookie-jar-overflow.md
{{#endref}}

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 Tossing attack may also be used to perform a **Cookie Bomb** attack:

{{#ref}}
cookie-bomb.md
{{#endref}}

### 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/)
- [**Cookie Crumbles: Unveiling Web Session Integrity Vulnerabilities**](https://www.youtube.com/watch?v=F_wAzF4a7Xg)

{{#include ../../banners/hacktricks-training.md}}