ET WEB_SPECIFIC_APPS Draytek WebServlet filename Parameter Directory Traversal Attempt (CVE-2021-20124)

alert http any any -> $HOME_NET any (
    msg:"ET WEB_SPECIFIC_APPS Draytek WebServlet filename Parameter Directory Traversal Attempt (CVE-2021-20124)";
    flow:established,to_server;
    http.method;
    content:"GET";
    http.uri;
    content:"/ACSServer/WebServlet|3f|"; startswith; fast_pattern;
    content:"act|3d|getMapImg_acs2";
    content:"filename|3d|";
    pcre:"/^[^\x26]*?(?:(?:\x2e|%2[Ee]){1,2}(?:\x2f|\x5c|%5[Cc]|%2[Ff]){1,}){2,}/R";
    reference:url,www.tenable.com/security/research/tra-2021-42;
    reference:cve,2021-20124;
    classtype:attempted-admin;
    sid:2071565; rev:1;
    metadata:affected_product DrayTek, attack_target Networking_Equipment, tls_state plaintext, created_at 2026_08_18, cve CVE_2021_20124, deployment Perimeter, deployment Internal, performance_impact Low, confidence High, signature_severity Major, tag Exploit, tag CISA_KEV, updated_at 2026_08_18, mitre_tactic_id TA0001, mitre_tactic_name Initial_Access, mitre_technique_id T1190, mitre_technique_name Exploit_Public_Facing_Application;
    target:dest_ip;
)