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Weak CAPTCHA

Description​

The system uses an insecure CAPTCHA implementation that allows the usage of optical recognition tools to automatically pass it.

Impact​

Bypass security mechanisms to perform automated attacks.

Recommendation​

Ensure that captchas response is at servers level and is not exposed on the HTML web pages code, also, ensure that captcha can not be read by an OCR.

Threat​

Anonymous attacker from the Internet.

Expected Remediation Time​

⌚ 30 minutes.

Score​

Default score using CVSS 3.1. It may change depending on the context of the vulnerability.

Base​

  • Attack vector: N
  • Attack complexity: L
  • Privileges required: N
  • User interaction: N
  • Scope: U
  • Confidentiality: L
  • Integrity: N
  • Availability: N

Temporal​

  • Exploit code madurity: X
  • Remediation level: X
  • Report confidence: X

Result​

  • Vector string: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N/E:X/RL:X/RC:X
  • Score:
    • Base: 5.3
    • Temporal: 5.3
  • Severity:
    • Base: Medium
    • Temporal: Medium

Code Examples​

Compliant code​

The 3rd party captcha service is securely configured on the server side

<?php
if( isset( $_POST[ 'Change' ] ) ) {
  // Check Anti-CSRF token
  checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );
  // Hide the CAPTCHA form
  $hide_form = true;
  // Get input
  $pass_new  = $_POST[ 'password_new' ];
  $pass_new  = stripslashes( $pass_new );
  $pass_new  = md5( $pass_new );
  $pass_conf = $_POST[ 'password_conf' ];
  $pass_conf = stripslashes( $pass_conf );
  $pass_conf = md5( $pass_conf );
  $pass_curr = $_POST[ 'password_current' ];
  $pass_curr = stripslashes( $pass_curr );
  $pass_curr = md5( $pass_curr );
  // Check CAPTCHA from 3rd party
  $resp = recaptcha_check_answer(
    $_DVWA[ 'recaptcha_private_key' ],
    $_POST['g-recaptcha-response']
  );
  // Did the CAPTCHA fail?
  if( !$resp ) {
    // What happens when the CAPTCHA was entered incorrectly
    $html .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>";
    $hide_form = false;
  }
  else {
    // Check that the current password is correct
    $data = $db->prepare( 'SELECT password FROM users WHERE user = (:user) AND password = (:password) LIMIT 1;' );
    $data->bindParam( ':user', dvwaCurrentUser(), PDO::PARAM_STR );
    $data->bindParam( ':password', $pass_curr, PDO::PARAM_STR );
    $data->execute();
    // Check passwords and update database
    if( ( $pass_new == $pass_conf) && ( $data->rowCount() == 1 ) ) {
      // Update the database
      $data = $db->prepare( 'UPDATE users SET password = (:password) WHERE user = (:user);' );
      $data->bindParam( ':password', $pass_new, PDO::PARAM_STR );
      $data->bindParam( ':user', dvwaCurrentUser(), PDO::PARAM_STR );
      $data->execute();// Feedback for the end user - success!
      $html .= "<pre>Password Changed.</pre>";
    }
    else {
      // Feedback for the end user - failed!
      $html .= "<pre>There was a mistake</pre>";
      $hide_form = false;
    }
  }
}
// Generate Anti-CSRF token
generateSessionToken();
?>

Non compliant code​

There is an insecure configuration of the captcha functionality

<?php
if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '1' ) ) {
  // Hide the CAPTCHA form
  $hide_form = true;
  // Get input
  $pass_new  = $_POST[ 'password_new' ];
  $pass_conf = $_POST[ 'password_conf' ];
  // Check CAPTCHA from 3rd party
  $resp = recaptcha_check_answer(
    $_DVWA[ 'recaptcha_private_key'],
    $_POST['g-recaptcha-response']
  );
  // Did the CAPTCHA fail?
  if( !$resp ) {
    $html     .= "<pre><br />The CAPTCHA was incorrect. Please try again.</pre>";
    $hide_form = false;
    return;
  }
  else {
    //The step value can be modified on the client side and the captcha can be bypassed
    if( isset( $_POST[ 'Change' ] ) && ( $_POST[ 'step' ] == '2' ) ) {
    // Hide the CAPTCHA form
    $hide_form = true;// Get input
    $pass_new  = $_POST[ 'password_new' ];
    $pass_conf = $_POST[ 'password_conf' ];
    // Code to check if both password match
  }
}
?>

Requirements​

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