Overview

ModSecurity WAF agent for Sentinel reverse proxy. Provides full OWASP Core Rule Set (CRS) support via libmodsecurity bindings with 800+ detection rules.

Note: This agent requires libmodsecurity installed on your system. For a lightweight, zero-dependency alternative with basic detection rules, see WAF agent.

Features

Full OWASP CRS Support: 800+ detection rules out of the box
SecLang Compatibility: Load any ModSecurity rules
Request Body Inspection: JSON, form data, XML, and all content types
Response Body Inspection: Detect data leakage (opt-in)
Block or Detect-Only Mode: Monitor before blocking
Path Exclusions: Skip inspection for trusted paths
Paranoia Levels: 1-4, balance security vs. false positives

Prerequisites

This agent requires libmodsecurity >= 3.0.13:

macOS:

brew install modsecurity

Ubuntu/Debian:

apt install libmodsecurity-dev

Installation

Using Cargo

cargo install sentinel-agent-modsec

Configuration

Command Line

sentinel-modsec-agent \
  --socket /var/run/sentinel/modsec.sock \
  --rules /etc/modsecurity/crs/crs-setup.conf \
  --rules "/etc/modsecurity/crs/rules/*.conf"

Environment Variables

Option	Env Var	Description	Default
`--socket`	`AGENT_SOCKET`	Unix socket path	`/tmp/sentinel-modsec.sock`
`--rules`	`MODSEC_RULES`	Paths to rule files (supports glob patterns like `*.conf`)	-
`--block-mode`	`MODSEC_BLOCK_MODE`	Block (true) or detect-only (false)	`true`
`--exclude-paths`	`MODSEC_EXCLUDE_PATHS`	Paths to exclude (comma-separated)	-
`--body-inspection`	`MODSEC_BODY_INSPECTION`	Enable request body inspection	`true`
`--max-body-size`	`MODSEC_MAX_BODY_SIZE`	Maximum body size to inspect	`1048576` (1MB)
`--response-inspection`	`MODSEC_RESPONSE_INSPECTION`	Enable response body inspection	`false`
`--verbose`, `-v`	`MODSEC_VERBOSE`	Enable debug logging	`false`

Sentinel Configuration

agent "modsec" {
    socket "/var/run/sentinel/modsec.sock"
    timeout 100ms
    events ["request_headers" "request_body_chunk" "response_body_chunk"]
}

route {
    match { path-prefix "/" }
    agents ["modsec"]
    upstream "backend"
}

OWASP CRS Setup

Download CRS

# Clone the CRS repository
sudo mkdir -p /etc/modsecurity
sudo git clone https://github.com/coreruleset/coreruleset /etc/modsecurity/crs

# Copy example configuration
sudo cp /etc/modsecurity/crs/crs-setup.conf.example /etc/modsecurity/crs/crs-setup.conf

Run with CRS

sentinel-modsec-agent \
  --socket /var/run/sentinel/modsec.sock \
  --rules /etc/modsecurity/crs/crs-setup.conf \
  --rules "/etc/modsecurity/crs/rules/*.conf"

Paranoia Levels

Level	Description	Use Case
1	Standard protection, minimal false positives	Production - most applications
2	Elevated protection, some false positives	Security-sensitive apps
3	High protection, moderate false positives	Staging/testing, or with tuning
4	Maximum protection, high false positives	Security research

Configure in /etc/modsecurity/crs/crs-setup.conf:

SecAction "id:900000,phase:1,pass,t:none,nolog,setvar:tx.blocking_paranoia_level=1"

Response Headers

Header	Description
`X-WAF-Blocked`	`true` if request was blocked
`X-WAF-Message`	ModSecurity message
`X-WAF-Detected`	Detection message (detect-only mode)

CRS Rule Categories

File	Protection
REQUEST-913-*	Scanner detection
REQUEST-920-*	Protocol enforcement
REQUEST-930-*	Local file inclusion (LFI)
REQUEST-931-*	Remote file inclusion (RFI)
REQUEST-932-*	Remote code execution (RCE)
REQUEST-941-*	Cross-site scripting (XSS)
REQUEST-942-*	SQL injection
REQUEST-943-*	Session fixation
REQUEST-944-*	Java attacks
RESPONSE-950-*	Data leakage

Comparison with WAF Agent

Feature	ModSecurity	WAF
Detection Rules	800+ CRS rules	~20 regex rules
SecLang Support	Yes	No
Custom Rules	Yes	No
Dependencies	libmodsecurity (C)	Pure Rust
Binary Size	~50MB	~5MB
Installation	Requires libmodsecurity	`cargo install`

Use ModSecurity when:

You need full OWASP CRS compatibility
You have existing ModSecurity/SecLang rules
You require comprehensive protection

Use WAF agent when:

You want zero-dependency deployment
You need low latency and minimal resources
Basic attack detection is sufficient

Agent	Integration
WAF	Lightweight, pure Rust alternative
AI Gateway	AI-specific security controls
Rate Limiter	Combine with rate limiting