ET MALWARE Possible PHP.MAILER WebShell Register Shutdown Function Request Inbound

alert http $EXTERNAL_NET any -> any any (
    msg:"ET MALWARE Possible PHP.MAILER WebShell Register Shutdown Function Request Inbound";
    flow:established,to_server;
    http.method;
    content:"POST";
    http.uri;
    content:".php"; endswith;
    http.request_body;
    content:"r=register_shutdown_function"; startswith; fast_pattern;
    content:"&d="; distance:0;
    content:"&s="; distance:0;
    content:"&c="; distance:0;
    reference:url,blog.trendmicro.com/trendlabs-security-intelligence/spam-campaign-abuses-php-functions-for-persistence-uses-compromised-devices-for-evasion-and-intrusion/;
    classtype:trojan-activity;
    sid:2027970; rev:3;
    metadata:attack_target Server, created_at 2019_09_09, deployment Perimeter, performance_impact Low, confidence Medium, signature_severity Major, updated_at 2020_09_17;
)