'Check if the file exists.
:param web_shell_url: The URL to check'

def _check_if_exists(self, web_shell_url):

    try:
        response = self._uri_opener.GET(web_shell_url, cache=True)
    except BaseFrameworkException:
        om.out.debug(('Failed    to    GET    webshell:' + web_shell_url))
    else:
        signature = self._match_signature(response)
        if (signature is None):
            return
        desc = u'An    HTTP    response    matching    the    web    backdoor    signature    "%s"    was    found    at:    "%s";    this    could    indicate    that    the    server    has    been    compromised.'
        desc %= (signature, response.get_url())
        _severity = (severity.HIGH if (len(signature) > 8) else severity.MEDIUM)
        v = Vuln(u'Potential    web    backdoor', desc, _severity, response.id, self.get_name())
        v.set_url(response.get_url())
        kb.kb.append(self, 'backdoors', v)
        om.out.vulnerability(v.get_desc(), severity=v.get_severity())
        fr = FuzzableRequest.from_http_response(response)
        self.output_queue.put(fr)
