alert tcp $EXTERNAL_NET any -> $HOME_NET [23,2323,3323,4323] (msg:"ET HUNTING SUSPICIOUS busybox shell"; flow:to_server,established; content:"shell"; fast_pattern:only; pcre:"/\bshell\b/"; flowbits:isset,ET.telnet.busybox; threshold:type limit, count 1, track by_src, seconds 30; reference:url,lists.emergingthreats.net/pipermail/emerging-sigs/2016-August/027524.html; classtype:attempted-admin; sid:2023017; rev:3; metadata:attack_target Server, created_at 2016_08_08, deployment Datacenter, performance_impact Low, confidence Medium, signature_severity Major, updated_at 2019_07_26;)