Liferay Portal 7.1 CE GA=3 / SimpleCaptcha API - Cross-Site Scripting

# Exploit Title: Liferay Portal < 7.1 CE GA4 / SimpleCaptcha API XSS
# Date: 04/06/2019
# Exploit Author: Valerio Brussani (@val_brux)
# Website: www.valbrux.it
# Vendor Homepage: https://www.liferay.com/
# Software Link: https://www.liferay.com/it/downloads-community
# Version: < 7.1 CE GA4
# Tested on: Liferay Portal 7.1 CE GA3
# CVE: CVE-2019-6588
# Reference1: https://dev.liferay.com/web/community-security-team/known-vulnerabilities/liferay-portal-71/-/asset_publisher/7v4O7y85hZMo/content/cst-7130-multiple-xss-vulnerabilities-in-7-1-ce-ga3
# Reference2: https://www.valbrux.it/blog/2019/06/04/cve-2019-6588-liferay-portal-7-1-ce-ga4-simplecaptcha-api-xss/


Introduction
In Liferay Portal before 7.1 CE GA4, an XSS vulnerability exists in the SimpleCaptcha API when custom code passes unsanitized input 
into the “url” parameter of the JSP taglib call <liferay-ui:captcha url=”<%= url %>” /> or <liferay-captcha:captcha url=”<%= url %>” />. 
A customized Liferay portlet which directly calls the Simple Captcha API without sanitizing the input could be susceptible to this vulnerability.
 
Poc
In a sample scenario of custom code calling the <liferay-ui:captcha url=”<%= url %>” /> JSP taglib, appending a payload like the following to the body parameters of a customized form:
 
&xxxx%22%3e%3cscript%3ealert(1)</script> 
 
The script is reflected in the src attribute of the <img> tag, responsible of fetching the next available captcha:
 
<img alt=”xxx” class=”xxxx” src=”xxxxxx“><script>alert(1)</script>=” />