alert tcp $EXTERNAL_NET any -> $HOME_NET 21 (msg:"ET FTP FTP RETR command attempt without login"; flow:established,to_server; flowbits:isnotset,ET.ftp.user.login; content:!"USER"; depth:4; content:"RETR"; nocase; reference:url,www.nsftools.com/tips/RawFTP.htm; classtype:attempted-recon; sid:2010736; rev:2; metadata:created_at 2010_07_30, confidence Medium, signature_severity Informational, updated_at 2019_07_26;)