Logstash Filter for Modsecurity logging

This is a filter plugin for Logstash to parse ModSecurity audit log files. Tested in conjunction with OWASP Core Rule Set. This plugin will parse the fairly incomprehensible ModSecurity 'auditlog' format using a Logstash Filter to facilitate Modsecurity event storage, analysis and correlation using Elastic.

Use case

This plugin for Logstash can be used to parse Modsecurity audit log files and push them to an Elasticsearch cluster. Once the data has been indexed the user can visualize, analyze and investigate ModSecurity events with tools like Kibana. Centralizing and indexing ModSecurity alerts also facilitates the capability to easily identify false-positives and adjust the ModSecurity configuration to increase detection accuracy. Real-time monitoring and alerting can be achived with tools such as Grafana and ElastAlert, providing the user real-time treat awareness.

Settings:

split_alerts
Split all alerts into separate fields (eg. alert_1, alert_2). If set to false all alerts will be combined in a single field named 'alerts' which will contain an array of alert objects. Object arrays are not (yet) properly supported by Kibana. Keep in mind that having a lot of messages per alert will result in exceeding the 'index.mapping.total_fiels.limit'. To prevent exceeding this limit, all alerts with more than 6 messages will have the remaining messages combined in a separate msgOther field. As of June 2017, there is an unofficial plugin that supports object arrays in Kibana available on Github.

store_request (ModSecurity Section I) & store_response (ModSecurity Section E)
Setting this value to true will push the entire request, if present in the auditlog, to the output. It is advisable to disable indexing on _request_body

request_headers (ModSecurity Section B) & request_headers (ModSecurity Section F)
Case insensitive list of header keys that that should be extracted if present. Will be stored as a separate field in the output with 'request_' as prefix (eg. request_cookie).

Usage

logstash.yml example

filter {
   modsec {
       split_alerts => boolean (default true)
       store_request => boolean (default false)
       store_response => boolean (default false)
       request_headers => csv header keys (default "host,content-length,user-agent,cookie,content-type,origin,refferer")
       response_headers => csv header keys (default "content-length")
   }
}

Developing, building, testing and installing

Code

• To get started, you'll need JRuby with the Bundler gem installed.

• Install dependencies

bundle install

Test

• Update your dependencies

bundle install

• Run tests

bundle exec rspec spec

Run in an installed Logstash

• Build your plugin gem

gem build logstash-filter-modsec.gemspec

• Install the plugin from the Logstash home

# Logstash 2.3 and higher
bin/logstash-plugin install logstash-filter-modsec-0.1.0.gem

• Start Logstash and proceed to test the plugin