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If the **cookie** is using some **Base encoding** (like Base64) or similar you may be able to **decode it**, **change** the **content** and **impersonate** arbitrary users.
The attacker gets a cookie from a web page and sends a link to the victim to **login using the very same cookie**. If the cookie is not changed when a user logs in, this could be useful because the attacker could be able to impersonate the user through a cookie.
The attacker sends his own session to the victim. The victim will see that he is already logged in and will suppose that he is inside his account but **the actions will be performed inside the attacker's account**.
* Check the "**remember me**" option if it exists to see how it works. If it exists and could be vulnerable, always use the cookie of **remember me** without any other cookie.
* Check if the previous cookie works even after you change the password.
If the cookie remains the same (or almost) when you log in, this probably means that the cookie is related to some field of your account (probably the username). Then you can:
* Try to create a lot of **accounts** with usernames very **similar** and try to **guess** how the algorithm is working.
* Try to **bruteforce the username**. If the cookie saves only as an authentication method for your username, then you can create an account with username "**Bmin**" and **bruteforce** every single **bit** of your cookie because one of the cookies that you will try will the one belonging to "**admin**".
If the attack has been successfully performed, then you could try to encrypt a string of your choice. For example, if you would want to **encrypt****user=administrator**
This execution will give you the cookie correctly encrypted and encoded with the string **user=administrator** inside.
**CBC-MAC**
Maybe a cookie could have some value and could be signed using CBC. Then, the integrity of the value is the signature created by using CBC with the same value. As it is recommended to use as IV a null vector, this type of integrity checking could be vulnerable.
Create a user called for example "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" and check if there is any pattern in the cookie (as ECB encrypts with the same key every block, the same encrypted bytes could appear if the username is encrypted).
There should be a pattern (with the size of a used block). So, knowing how are a bunch of "a" encrypted you can create a username: "a"\*(size of the block)+"admin". Then, you could delete the encrypted pattern of a block of "a" from the cookie. And you will have the cookie of the username "admin".
The `Domain` attribute specifies **which hosts can receive a cookie**. If unspecified, the attribute **defaults** to the **same host** that set the cookie, _**excluding subdomains**_. **If `Domain` is****specified**, then **subdomains are always included**. Therefore, specifying `Domain` is less restrictive than omitting it. However, it can be helpful when subdomains need to share information about a user.
For example, if you set `Domain=mozilla.org`, cookies are available on subdomains like `developer.mozilla.org`. But if you don't, the cookie won't be sent to subdomains.
The `Path` attribute indicates a **URL path that must exist in the requested URL to send the `Cookie` header**. The `%x2F` ("/") character is considered a directory separator, and subdirectories match as well.
Table from [Invicti](https://www.netsparker.com/blog/web-security/same-site-cookie-attribute-prevent-cross-site-request-forgery/) and slightly modified.\
**\*Notice that from Chrome80 (feb/2019) the default behaviour of a cookie without a cookie samesite** **attribute will be lax** ([https://www.troyhunt.com/promiscuous-cookies-and-their-impending-death-via-the-samesite-policy/](https://www.troyhunt.com/promiscuous-cookies-and-their-impending-death-via-the-samesite-policy/)).\
Notice that temporary, after applying this change, the **cookies without a SameSite****policy** in Chrome will be **treated as None** during the **first 2 minutes and then as Lax for top-level cross-site POST request.**
* If the page is **sending the cookies as the response** of a requests (for example in a **PHPinfo** page), it's possible to abuse the XSS to send a request to this page and **steal the cookies** from the response (check an example in [https://hackcommander.github.io/pentesting-article-1/)](https://hackcommander.github.io/pentesting-article-1/)
* This could be Bypassed with **TRACE****HTTP** requests as the response from the server (if this HTTP method is available) will reflect the cookies sent. This technique is called **Cross-Site Tracking**.
* This technique is avoided by **modern browsers by not permitting sending a TRACE** request from JS. However, some bypasses to this have been found in specific software like sending `\r\nTRACE` instead of `TRACE` to IE6.0 SP2.
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