WordPress All-In-One Security / Firewall 4.1.2 CAPTCHA Bypass

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Multiple vulnerabilities in All In One WP Security & Firewall plugin  
login CAPTCHA  
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Sipke Mellema, July 2016  
  
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Abstract  
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The login CAPTCHA provided by the All In One WP Security & Firewall  
plugin can be circumvented in multiple ways, allowing an attacker to  
automate login attempts when the CAPTCHA is enabled.  
  
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OVE ID  
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OVE-20160719-0001  
  
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Tested versions  
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These issues were successfully tested on the All In One WP Security &  
Firewall WordPress Plugin version 4.1.2.  
  
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Fix  
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The first two findings are resolved in the All In One WP Security &  
Firewall plugin version 4.1.3.  
  
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Details  
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https://sumofpwn.nl/advisory/2016/multiple_vulnerabilities_in_all_in_one_wp_security___firewall_plugin_login_captcha.html  
  
Details finding 1: Complete bypass of CAPTCHA answer validation  
  
When the login CAPTCHA is enabled, the plugin will check if the user provided a CAPTCHA answer. If so, the answer will be checked for validity. However, the code does not account for the case where no CAPTCHA answer is provided. If no answer is sent, the login will continue as normal, even though the CAPTCHA setting is enabled.  
  
The vulnerable code is located in wp-security-user-login.php:  
  
//Check if captcha enabled  
if ($aio_wp_security->configs->get_value('aiowps_enable_login_captcha') == '1')  
{  
if (array_key_exists('aiowps-captcha-answer', $_POST)) //If the login form with captcha was submitted then do some processing  
{  
[.. captcha logic ..]  
}  
[.. missing else statement ..]  
}  
  
Details finding 2: CAPTCHA answer forgery  
  
  
The CAPTCHA answers leak the secret key used to create valid answers. By extracting the secret keys it's possible to forge valid CAPTCHA answers.  
  
The vulnerable code is located at /classes/wp-security-captcha.php:  
  
//Let's encode correct answer  
$captcha_secret_string = $aio_wp_security->configs->get_value('aiowps_captcha_secret_key');  
$current_time = time();  
$enc_result = base64_encode($current_time.$captcha_secret_string.$result);  
$equation_string .= '<input type="hidden" name="aiowps-captcha-string-info" id="aiowps-captcha-string-info" value="'.$enc_result.'" />';  
$equation_string .= '<input type="hidden" name="aiowps-captcha-temp-string" id="aiowps-captcha-temp-string" value="'.$current_time.'" />';  
$equation_string .= '<input type="text" size="2" id="aiowps-captcha-answer" name="aiowps-captcha-answer" value="" />';  
return $equation_string;  
  
The CAPTCHA form adds three fields to the login form:  
aiowps-captcha-string-info - A timestamp, a secret CAPTCHA key and the valid answer, base64 encoded  
aiowps-captcha-temp-string - The timestamp  
aiowps-captcha-answer - Answer to be filled in by the user  
For validating the correct answer, aiowps-captcha-string-info is checked against the timestamp provided by the user, combined with the answer provided by the user and the secret CAPTCHA key (base64 encoded).  
  
By decoding the value for aiowps-captcha-string-info, a user can extract the secret key and create valid answers.  
Details finding 3: CAPTCHA answer replay attack  
  
  
The CAPTCHA mechanism (which is described above) is created in such a way that the CAPTCHA answer never expires. A valid answer can be re-used, allowing automated login attempts.  
Details finding 4: Easy automatable CAPTCHA solving  
  
  
Math questions created by the login CAPTCHA are not obfuscated in any way. The math questions (such as "five + 2") can easily be parsed by a program to generate valid answers.  
Proofs of concepts  
  
1. Enable the login CAPTCHA and remove the aiowps-captcha-answer parameter from the POST request. The login will succeed as normal.  
  
2. Base64 decode the hidden field aiowps-captcha-string-info to obtain the CAPTCHA secret and a valid answer.  
  
3. Send two login attempt with the same (valid) aiowps-captcha-string-info, aiowps-captcha-temp-string and aiowps-captcha-answer parameters. The login attempt will be accepted.  
  
4. A programmer can use the array from the number_word_mapping method to evaluate the questions created by the CAPTCHA.  
  
  
  
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Summer of Pwnage (https://sumofpwn.nl) is a Dutch community project. Its  
goal is to contribute to the security of popular, widely used OSS  
projects in a fun and educational way.  
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