Web applications typically use custom login forms for user authentication, while usually diverse visually often share underlying mechanisms.

Login forms usually appear as simple boxes requesting a username and password but it begins the process of a complex client-side and server-side technologies, which at their core login cores are essentially HTML forms embedded within a webpage. Typically including input fields like <input> for capturing username and password, along with a <button> for submitting usually type="submit" to initiate the authentication process.

<form action="/login" method="post">
  <label for="username">Username:</label>
  <input type="text" id="username" name="username"><br><br>
  <label for="password">Password:</label>
  <input type="password" id="password" name="password"><br><br>
  <input type="submit" value="Submit">
</form>

_{Most login form structures}

This form, when submitted, sends a POST request to the /login endpoint on the server, including the entered username and password as form data.

POST /login HTTP/1.1
Host: www.example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 29

username=john&password=secret123

_{HTTP POST Request Example}

The POST method indicates the HTTP Methods and Codes which indicates to the web server when received that data has been sent to create or update a resource on the URL endpoint /login.

The header also includes a Content-Type header which specifies how the data is encoded in the request body.
The Content-Length header which indicates the size of the data.
Finally, the Request Body which contains the username and password encoded as key-value pairs.

Quote

When a user interacts with a login form, their browser handles the initial processing. The browser captures the entered credentials, often employing JavaScript for client-side validation or input sanitization. Upon submission, the browser constructs an HTTP POST request. This request encapsulates the form data—including the username and password—within its body, often encoded as application/x-www-form-urlencoded or multipart/form-data

http-post-form¶

Hydra's http-post-form service is specifically designed to target login forms, enabling automation of POST requests dynamically inserting the payloads for each username and password combination into the request body.

Leveraging Hydra's capabilities allow attackers to efficiently test numerous credential combinations against a login form.

The general structure of a Hydra command using http-post-form is as follows:

TheMalevolent1@htb[/htb]$ hydra [options] target http-post-form "path:params:condition_string"

Understanding the Condition String¶

Hydra's http-post-form module relies on success and failure condition strings.

Exercises¶

Q: After successfully brute-forcing, and then logging into the target, what is the full flag you find?
A:

Next Section

Understanding Login Forms¶

Basic Login Form Example¶

http-post-form¶

Understanding the Condition String¶

Exercises¶