Logging with OWASP CRS, Part 1: What Your WAF Is Telling You

Tue, 03 Nov 2026 09:00:00 -0300

A web application firewall produces exactly one thing you can hold in your hands: logs. Blocking is a side effect of a rule matching; the log line is the record that it happened, why, and what the request looked like. If nobody reads those logs, the WAF is a very expensive way to return 403s.

This is the first of a three part series on logging with OWASP CRS:

Part 1 (this post): what ModSecurity and CRS write to disk, how to read it, what to look for, and the two small additions we recommend before you ship a single line to a log platform.
Part 2: ModSecurity, with the logs shipped to the Elastic Stack (Logstash, Elasticsearch, Kibana) and a ready made dashboard.
Part 3: Coraza, with the logs shipped to Grafana and Loki through Alloy, and the same dashboard.

Parts 2 and 3 come with Docker Compose stacks you can run locally in a few minutes. Each post has its files attached, and a zip with all of them at the end. Both use the CRS project’s own container images and the same CRS release. Part 2 runs owasp/modsecurity-crs, NGINX with ModSecurity v3 (libmodsecurity). Part 3 runs coraza-crs, Caddy with Coraza. The examples in this post come from ModSecurity v3; most of it applies to ModSecurity v2 on Apache and to Coraza as well, and where the engines differ, we say so.

The Top 5 Ways CRS Can Help You Fight the OWASP Top 10

Tue, 21 Nov 2017 16:36:31 +0100

The new edition OWASP Top Ten list mentions ModSecurity and the OWASP ModSecurity Core Rule Set for the first time.

Let me explain you what the Core Rule Set does and how it can help you protect your services from these risks.

The CRS - short for OWASP ModSecurity Core Rule Set - is a set of generic attack detection rules. They are meant for use with ModSecurity or compatible web application firewalls. The CRS aims to protect web applications from a wide range of attacks with a minimum of false alerts. The Core Rule Set is thus meant as a 1st line of defense against web application attacks as described by the OWASP Top Ten.

Logging on CRS Project

Logging with OWASP CRS, Part 1: What Your WAF Is Telling You

The Top 5 Ways CRS Can Help You Fight the OWASP Top 10