WordPress Core 5.2.2 Suffers – ‘post previews’ XSS Cross Site Scripting Vulnerability

Currently WordPress Core Version 5.2.2 suffers from cross site scripting vulnerability located within “post previews”. Flaw is fixed by upgrading to WordPress version

# 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
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

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