Authentication Bypass via Parameter Modification

• Authentication mechanisms can be flawed if they rely on HTTP parameters, leading to vulnerabilities that allow for authentication and authorization bypasses, including privilege escalation.

Parameter Modification Vulnerability:

• This vulnerability is often related to authorization issues, such as Insecure Direct Object Reference (IDOR), which can allow unauthorized access to resources.

Example Scenario:

• Consider a web application where a user logs in with credentials for htb-stdnt and is redirected to:

  /admin.php?user_id=123

• Upon accessing this URL, the user notices limited privileges and data visibility.

Investigating the user_id Parameter:

• Removing the user_id parameter from the request results in a redirect to the login screen (/index.php), indicating that the parameter is crucial for authentication.

• The valid session (indicated by the PHPSESSID cookie) does not prevent the redirect, suggesting that the application relies on the user_id parameter for access control.

Exploiting the Vulnerability:

• By directly accessing the URL with the user_id parameter:

  /admin.php?user_id=123

• The attacker can bypass authentication checks.

• Since the parameter is named user_id, it likely corresponds to the ID of the user accessing the page. If the attacker can guess or brute-force the user ID of an administrator, they can gain access to administrative privileges.

Brute-Forcing User IDs:

• Techniques from the Brute-Force Attacks section can be applied to discover valid administrator user IDs, allowing the attacker to specify the admin's ID in the user_id parameter and access protected information.

Final Remarks:

• More advanced vulnerabilities can also lead to authentication bypass, including:

  • Type Juggling

  • Injection Vulnerabilities

  • Logic Bugs

Conclusion:

• Always validate and secure parameters used in authentication and authorization processes to prevent unauthorized access and privilege escalation. Regularly review and test for parameter manipulation vulnerabilities in web applications.