63 lines
4.0 KiB
Markdown
63 lines
4.0 KiB
Markdown
# Cookie Tossing
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### Description
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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.
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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.**
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{% hint style="danger" %}
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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"`
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{% endhint %}
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This can be really dangerous as the attacker may be able to:
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* **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...)
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* 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**.
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* 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).
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### Cookie Order
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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.
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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;`
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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.
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{% hint style="warning" %}
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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**.
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{% endhint %}
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### Protection Bypass
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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**.
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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**.
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{% content-ref url="cookie-jar-overflow.md" %}
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[cookie-jar-overflow.md](cookie-jar-overflow.md)
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{% endcontent-ref %}
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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.
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### Cookie Bomb
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A Cookie Tossin attack may be used also to perform **Cookie Bomb** attack:
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{% content-ref url="cookie-bomb.md" %}
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[cookie-bomb.md](cookie-bomb.md)
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{% endcontent-ref %}
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### Defense**s**
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#### **Use the prefix `__Host` in the cookie name**
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* 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 /
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* **This prevents subdomains from forcing a cookie to the apex domain since these cookies can be seen as "domain-locked"**
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### References
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* [**@blueminimal**](https://twitter.com/blueminimal)
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* [**https://speakerdeck.com/filedescriptor/the-cookie-monster-in-your-browsers**](https://speakerdeck.com/filedescriptor/the-cookie-monster-in-your-browsers)
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* [**https://github.blog/2013-04-09-yummy-cookies-across-domains/**](https://github.blog/2013-04-09-yummy-cookies-across-domains/)
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