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;)