JAuth

Let’s log in with the provided credentials and examine the cookie. It is a JWT.

To analyze its headers, I’ll use the tool JWT Debugger:

In order for JWT to pass for the task, we need to modify it.

NOTE

The JWT structure consists of three components:

• header
• payload
• signature (secret key required)

They are all separated by 2 dividers. .

Source

HMACSHA256(
    base64UrlEncode(header) + "." + base64UrlEncode(payload),
    secret
)

Let’s change user to admin :

A secret key is required for the signature, which is not mentioned in the condition, so we will choose that the encryption algorithm is not applied (none).

IMPORTANT

JWT always consists of 3 parts with 2 characters . , so even though there is no signature, we must add . at the end.

Now insert this token into the cookie field and refresh the page:

picoCTF{succ3ss_@u7h3nt1c@710n_3444eacf}