alert http $EXTERNAL_NET any -> $HOME_NET any (msg:"ET EXPLOIT libpng tRNS overflow attempt"; flow:established,to_client; file_data; content:"|89|PNG|0D 0A 1A 0A|"; content:!"PLTE"; content:"tRNS"; distance:0; byte_test:4,>,256,-8,relative,big; reference:cve,CAN-2004-0597; classtype:attempted-admin; sid:2001058; rev:10; metadata:created_at 2010_07_30, confidence Medium, signature_severity Major, updated_at 2019_07_26;)