Brute-forcing a stay-logged-in cookie

Description

This lab allows users to stay logged in even after they close their browser session. The cookie used to provide this functionality is vulnerable to brute-forcing.

Reproduction and proof of concept

1. With Burp running, log in with wiener:peter with the Stay logged in option selected. Notice that this sets a stay-logged-in cookie.

username=wiener&password=peter&stay-logged-in=on

2. Examine this cookie in the Inspector panel and notice that it is Base64-encoded. Its decoded value is wiener:51dc30ddc473d43a6011e9ebba6ca770. Study the length and character set of this string and notice that it could be an MD5 hash. Given that the plaintext is your username, you can make an educated guess that this may be a hash of your password. Hash your password using MD5 to confirm that this is the case. We now know that the cookie is constructed as follows:

base64(username+':'+md5HashOfPassword)

3. Log out of your account.

4. Send the most recent GET /my-account request to Burp Intruder.

5. In Burp Intruder, add a payload position to the stay-logged-in cookie and add your own password as a single payload.

6. Under Payload processing, add the following rules in order. These rules will be applied sequentially to each payload before the request is submitted.

• Hash: MD5

• Add prefix: wiener:

• Encode: Base64-encode

7. As the Update email button is only displayed when you access the /my-account page in an authenticated state, we can use the presence or absence of this button to determine whether we have successfully brute-forced the cookie. On the Options tab, add a grep match rule to flag any responses containing the string Update email. Start the attack.

8. Notice that the generated payload was used to successfully load your own account page. This confirms that the payload processing rules work as expected, and you were able to construct a valid cookie for your own account.

9. Make the following adjustments and then repeat this attack:

• Remove your own password from the payload list and add the list of candidate passwords instead.

• Change the Add prefix rule to add carlos: instead of wiener:.

10. When the attack is finished, the lab will be solved. Notice that only one request returned a response containing Update email. The payload from this request is the valid stay-logged-in cookie for Carlos’s account.

Exploitability

An attacker needs to brute-force Carlos’s cookie to gain access to his My account page.