ET INFO SUSPCIOUS Non-standard base64 charset used for encoding

alert http $EXTERNAL_NET any -> $HOME_NET any (
    msg:"ET INFO SUSPCIOUS Non-standard base64 charset used for encoding";
    flow:established,from_server;
    file_data;
    content:" & 15) << 4)"; fast_pattern;
    content:"(|22|";
    content:!"|22|"; within:65;
    content:"|22|"; distance:65; within:1;
    content:!"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"; distance:-66; within:62;
    content:!"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"; distance:-66; within:62;
    content:!"abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; distance:-66; within:62;
    content:!"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"; distance:-66; within:62;
    content:!"ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz"; distance:-66; within:62;
    content:!"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"; distance:-66; within:62;
    flowbits:set,et.exploitkitlanding;
    classtype:bad-unknown;
    sid:2017364; rev:7;
    metadata:created_at 2013_08_21, confidence High, signature_severity Informational, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2019_07_26;
)