How Can We Prevent PHP Session Fixation and Hijacking?

PHP Session Fixation and Hijacking: Prevention and Mitigation

Session Fixation

Session fixation occurs when an attacker deliberately sets the session identifier for a user. This weakens the security of the session as attackers can use the predefined identifier to impersonate the user. To prevent session fixation:

1. Disable Session Transmitting in URLs: Set session.use_trans_sid to 0 in your php.ini.
2. Restrict Cookies for Session Storage: Set session.use_only_cookies to 1 in php.ini.
3. Regenerate Session ID: Call session_regenerate_id(true) whenever the session's status changes (e.g., after login).

Session Hijacking

Session hijacking is the act of obtaining a valid session identifier and using it to send requests as the original user. While preventing session hijacking directly is not possible, several measures can make it more difficult:

1. Strong Hashing: Set session.hash_function to a strong algorithm like SHA256 or SHA512 in php.ini.
2. Increase Hash Bits: Set session.hash_bits_per_character to 5 to make guessing session IDs more challenging.
3. Entropy Inclusion: Add entropy to the session ID by setting session.entropy_file to /dev/urandom and session.entropy_length to a suitable number.
4. Custom Session Name: Change the session name from the default PHPSESSID using session_name().
5. Rotation: Rotate the session ID periodically to reduce an attacker's session duration.
6. User Agent Check: Include the user's browser agent ($_SERVER['HTTP_USER_AGENT']) in the session and check it on subsequent requests.
7. IP Address Tracking: Store the user's IP address ($_SERVER['REMOTE_ADDR']) in the session and check it against subsequent requests.
8. Token Comparison: Generate a token for both the session and the browser side. Increment and compare the token on each request.

Session Regeneration

Regenerating the session ID using session_regenerate_id(true) also invalidates the old session data. Therefore, this action is sufficient when a session status change occurs.

Thorough Session Destruction

When ending a session, use destroySession() rather than session_destroy() to thoroughly remove all traces from both the browser and server:

function destroySession() {
    $params = session_get_cookie_params();
    setcookie(session_name(), '', time() - 42000,
        $params['path'], $params['domain'],
        $params['secure'], $params['httponly']
    );
    session_destroy();
}

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