alert tcp $EXTERNAL_NET any -> $SMTP_SERVERS 25 (msg:"GPL SMTP expn root"; flow:to_server,established; content:"expn"; nocase; content:"root"; fast_pattern; distance:0; nocase; pcre:"/^expn\s+root/smi"; reference:arachnids,31; reference:cve,1999-0531; reference:nessus,10249; classtype:attempted-recon; sid:2100660; rev:13; metadata:created_at 2010_09_23, cve CVE_1999_0531, signature_severity Unknown, updated_at 2019_07_26;)