Parse CEF with logstash when you can't use the input codec

## Description

Sometimes you need to parse CEF with logstash but it is wrapped in some other format, such as JSON or syslog. In this case, you can't use the CEF codec in the input, so you're doomed to do it yourself since there is not currently a CEF codec for filter blocks. Try this logstash recipe, it works well. Just rmeember a comma will break it- so feel free to replace "," with a more rare character like "|" or something...

## Context

At this point, the message has been processed so its a full CEF message starting in cef_message. Syslog "headers" were already removed in a previous step.

## Stripped from production

filter {
    if "cef" in [tags] {
        mutate {
            # CEF:0 is pipe delimited, split into individual fields 
            split => ["cef_message", "|"]
            add_field => { "cef_version" => "%{cef_message[0]}" }
            add_field => { "cef_device_vendor" => "%{cef_message[1]}" }
            add_field => { "cef_device_product" => "%{cef_message[2]}" }
            add_field => { "cef_device_version" => "%{cef_message[3]}" }
            add_field => { "cef_sig_id" => "%{cef_message[4]}" }
            add_field => { "cef_sig_name" => "%{cef_message[5]}" }
            add_field => { "cef_sig_severity" => "%{cef_message[6]}" }
            add_field => { "cef_kv_message" => "%{cef_message[7]}" }
        }


        # These next two (mutate and kv) are where the CEF is parsed
        # The mutate is required to bypass a limitation, the kv is
        # pretty straightforward key/value pair matching. It splits
        # them into key/value pairs and pops them into what will later
        # be renamed a device specific namespace. For now it is just
        # called `event_container`

        # Workaround because `kv` doesn't support values that contain spaces
        # Transform and then use comma to separate instead of spaces. Then
        # make sure you act on commas instead of spaces in the kv
        mutate {
            gsub => ["cef_kv_message", "(\S+=)", ", \1"]
        }

        kv {
            source => "cef_kv_message"
            trim_value => " "
            trim_key => " "
            value_split => "="
            field_split => ","
            target => "event_container"
            add_tag => "cef_kv_success"
            # remove_field => ["tmp_message", "cef_kv_message", "message", "cef_message"]
            remove_field => ["cef_kv_message", "message", "cef_message"]
            remove_tag => ["kv_section"]
        }

    }
}