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Remote Code Execution (RCE)

Introduction

This article covers cases of possible direct RCE on WordPress. This includes improper usage of functions inside of the plugin/theme which can be used to directly execute code or command on the server.

Learn how to secure your code against RCE

Useful Functions

Several functions could be useful to identify a possible RCE vulnerability:

Dynamic Function Call

PHP also supports a dynamic function call where we can execute a function from a string or variable. For example :

$action_type = $_GET["action"];
$input = $_GET["input"];


echo $action_type($input);

We can just simply supply the action parameter with arbitrary function such as system and put our shell command on the input parameter.

Example Cases

Below is an example of vulnerable code:

function image_render_callback($atts) {
  $atts = shortcode_atts( array(
    'sanitize' => 'esc_attr',
    'src'=>'',
        'text'=>''
  ), $atts);


    $chosen_callback = "esc_attr";
    $sanitize_callback = array("trim", "esc_attr", "esc_html", "sanitize_text_field");
    if(!in_array($atts["sanitize"], $sanitize_callback)){
        $chosen_callback = $atts["sanitize"];
    }


    if ( ! empty( $chosen_callback ) && is_callable( $chosen_callback ) ) {
        $text = call_user_func( $chosen_callback, $atts["text"] );
    }


    return sprintf("<img src='%s'>%s</img>", esc_attr($atts["src"]), esc_html($text));


}


add_shortcode("imagerender", "image_render_callback");

To exploit this, the Contributor+ role user simply needs to create a drafted post with the below content to trigger RCE via call_user_func function:

[imagerender src="https://patchstack.com" sanitize="system" text="cat /etc/passwd"]

Below are some of the findings related to RCE:

Contributors

rafiem