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## Description
If an attacker can **control a subdomain of 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.**
{% hint style="danger" %}
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"`
{% endhint %}
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...)
* 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 **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.
{% 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
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 a **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/)
Support HackTricks and get benefits!
- Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
- Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
- Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
- **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.**
- **Share your hacking tricks by submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**