Post request with host = “>alert(‘qualys_xss_joomla_2.5.3’)

On my web server Yesterday I have caught a POST request with following text for a host

"><script>alert('qualys_xss_joomla_2.5.3')</script> 

in full it was like this

headers {   ...   "host": "%22%3e%3cscript%3ealert('qualys_xss_joomla_2.5.3')%3c%2fscript%3e",   "x-forwarded-port": "443",   "x-forwarded-proto": "https",   ... } httpMethod POST 

Is it a hacking attack? What does it aimed for?