Caddy WAF

A Web Application Firewall middleware for the Caddy web server, written in Go.

• Module ID: http.handlers.waf — registered in Caddy's package registry, so the module is selectable on the download page
• Go module path: github.com/fabriziosalmi/caddy-waf
• Current version: v0.4.16 (see caddywaf.go — const wafVersionDefault)
• License: AGPL-3.0 — note this is a copyleft licence; check it suits your deployment before integrating

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Overview

caddy-waf is an HTTP handler middleware that inspects requests and responses across four well-defined phases, applies a regular-expression rule set with anomaly scoring, enforces IP/DNS/ASN/country blacklists and whitelists, performs token-bucket-style rate limiting, and exposes a JSON metrics endpoint.

The middleware is implemented as a single Caddy module registered under the ID http.handlers.waf. It can be configured through the Caddyfile or directly via JSON.

Capabilities

Engineering notes

• Linear-time regex: rules are compiled by Go's regexp (RE2). No catastrophic backtracking.
• Wait-free counters: per-rule hit counts use atomic.Int64 stored in a sync.Map.
• Bounded body reads: request bodies are read through io.LimitReader (max_request_body_size, default 10 MiB) and restored with io.MultiReader so downstream handlers still see the full body.
• Bounded response buffering: the response body is only held in memory when a Phase 4 rule exists to inspect it, and never past max_response_body_size (default 10 MiB). Beyond that — or as soon as the upstream flushes — the WAF releases what it holds and streams the rest, so memory never scales with the response size.
• Zero-copy body string: the body is exposed to rule matching via unsafe.String to avoid an allocation per request.
• Circuit breaker for GeoIP: geoip_fail_open controls whether a GeoIP lookup failure blocks the request or allows it through.
• Panic recovery: ServeHTTP installs a deferred recovery that returns 500 Internal Server Error on panic.

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Quick start

The fastest path to a working build:

curl -fsSL -H "Pragma: no-cache" \
  https://raw.githubusercontent.com/fabriziosalmi/caddy-waf/refs/heads/main/install.sh | bash

The script ensures Go and xcaddy are installed, clones the repository, downloads the GeoLite2 database, builds Caddy with the caddy-waf module, and starts the server.

A representative provisioning log:

INFO  Provisioning WAF middleware     {"log_level":"info","log_path":"debug.json","log_json":true,"anomaly_threshold":5}
INFO  http.handlers.waf  Tor exit nodes updated  {"count":1093}
INFO  WAF middleware version  {"version":"v0.4.16"}
INFO  Rate limit configuration  {"requests":100,"window":10,"cleanup_interval":300,"paths":["/api/v1/.*"],"match_all_paths":false}
WARN  GeoIP database not found. Country blacklisting/whitelisting will be disabled  {"path":"GeoLite2-Country.mmdb"}
INFO  IP blacklist loaded     {"path":"ip_blacklist.txt","valid_entries":223770,"invalid_entries":0,"total_lines":223770}
INFO  DNS blacklist loaded    {"path":"dns_blacklist.txt","valid_entries":854479,"total_lines":854479}
INFO  WAF rules loaded successfully   {"total_rules":33,"rule_counts":"Phase 1: 17 rules, Phase 2: 16 rules, Phase 3: 0 rules, Phase 4: 0 rules, "}
INFO  WAF middleware provisioned successfully

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Installation

Requirements

• Go 1.26.0 or newer (go.mod declares go 1.26.0, propagated from caddy/v2 which requires it)
• Caddy v2.11.x or newer (current build uses github.com/caddyserver/caddy/v2 v2.11.4)
• xcaddy for building Caddy with plugins

Method 1 — Build with xcaddy (recommended)

go install github.com/caddyserver/xcaddy/cmd/xcaddy@latest
xcaddy build --with github.com/fabriziosalmi/caddy-waf
./caddy list-modules | grep waf   # expect: http.handlers.waf

Method 2 — Quick script

curl -fsSL -H "Pragma: no-cache" \
  https://raw.githubusercontent.com/fabriziosalmi/caddy-waf/refs/heads/main/install.sh | bash

Method 3 — Build from source

git clone https://github.com/fabriziosalmi/caddy-waf.git
cd caddy-waf
go mod tidy
wget https://git.io/GeoLite2-Country.mmdb           # optional, only for GeoIP
xcaddy build --with github.com/fabriziosalmi/caddy-waf=./
./caddy fmt --overwrite
./caddy run

Method 4 — caddy add-package

[!IMPORTANT] Prefer xcaddy or the container image. Caddy's maintainers have proposed moving add-package, remove-package and upgrade out of Caddy's core to discourage their use (caddyserver/caddy#7010): the commands call Caddy's shared build server, and using them in CI/CD is an anti-pattern. Raised for this project in #138 by a Caddy maintainer.

The command works today and the module remains registered, so this section stays accurate. Treat it as a convenience for one-off, hand-operated installs — not as the way to build or deploy caddy-waf.

The module is registered in Caddy's package registry, so an existing Caddy v2.7+ binary can pull it in without a Go toolchain:

caddy add-package github.com/fabriziosalmi/caddy-waf
caddy list-modules | grep waf   # expect: http.handlers.waf

It is also selectable on caddyserver.com/download. See docs/add-package-guide.md for version pinning, removal, and when to prefer xcaddy instead.

Method 5 — Docker

Images are published to GitHub Container Registry on every release tag, for linux/amd64 and linux/arm64:

docker pull ghcr.io/fabriziosalmi/caddy-waf:0.4.16
docker run --rm -p 8080:8080 ghcr.io/fabriziosalmi/caddy-waf:0.4.16

Tags are 0.4.16, 0.4 and latest — note there is no v prefix, unlike the Go module version. Pin an exact version in anything you deploy. See docs/docker.md for volumes, Compose, and hot reload.

---

Minimal Caddyfile

{
    auto_https off
    admin localhost:2019
}

:8080 {
    log {
        output stdout
        format console
        level INFO
    }

    route {
        waf {
            metrics_endpoint   /waf_metrics
            rule_file          rules.json
            ip_blacklist_file  ip_blacklist.txt
            dns_blacklist_file dns_blacklist.txt
        }

        @wafmetrics path /waf_metrics
        handle @wafmetrics {
            # Falls through to the WAF handler, which serves metrics as JSON.
        }

        handle {
            respond "Hello world!" 200
        }
    }
}

A fully annotated example is provided in Caddyfile and caddyfile.example.

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Documentation

fabriziosalmi.github.io/caddy-waf — the same pages as below, with full-text search and cross-linking. The Markdown under docs/ remains the source and stays readable on GitHub.

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Project layout

.
├── caddywaf.go            Module registration, lifecycle, file watchers, metrics endpoint
├── handler.go             ServeHTTP, phase dispatch, blocking decisions
├── config.go              Caddyfile directive parsing
├── rules.go               Rule loading, validation, regex caching, hit accounting
├── request.go             Target extraction (URI, headers, body, JSON paths, ...)
├── response.go            Response recorder, block writer
├── blacklist.go           IP and DNS blacklist loaders and lookups
├── ratelimiter.go         Per-IP / per-path sliding-window rate limiter
├── geoip.go               MaxMind country and ASN lookups, with optional cache
├── tor.go                 Periodic Tor exit-node list fetcher
├── logging.go             Async log worker, sensitive-data redaction
├── helpers.go             IP parsing helpers
├── debug_waf.go           Debug helpers for rule evaluation
├── assets.go              Embedded dashboard assets (build tag)
├── assets_stub.go         No-op asset provider for builds without the dashboard
├── types.go               Public types (Middleware, Rule, RateLimit, ...)
├── doc.go                 Package documentation
├── rules.json             Default rule set (used by Caddyfile)
├── rules/                 Modular rule files by category
├── ui/                    Dashboard, with third-party assets vendored same-origin
├── ip_blacklist.txt       Default IP blacklist
├── dns_blacklist.txt      Default DNS blacklist
├── tor_blacklist.txt      Tor exit-node cache (auto-managed)
├── docs/                  Documentation
└── *_test.go              Unit and integration tests

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Testing

make test            # go test -v ./...
make it              # go test -v ./... -tags=it (integration)
make lint            # golangci-lint run
make test-integration # runs test.py inside a python:3.9-slim container

The repository also ships Python suites covering offensive payloads (test.py), traffic generation (caddytest.py), and benchmarking (benchmark.py). See docs/testing.md and docs/caddytest.md.

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Commercial support & consulting

Running caddy-waf in production? I offer paid support, custom rule development, and security consulting — WAF tuning, hardening, TLS automation, and cloud detection & alerting. Reach out: [email protected].

Contributing

Pull requests are welcome. The project values:

1. Tests that exercise the change.
2. Rule contributions placed under rules/ using the documented JSON schema.
3. Documentation kept in sync with code (every directive change must be reflected in docs/configuration.md and the relevant topic page).

See CONTRIBUTING.md for the workflow and CODE_OF_CONDUCT.md.

Security

Please do not open a public issue for a vulnerability. Use private vulnerability reporting — it is enabled on this repository, so it works for anyone without special permissions — or email [email protected]. Full policy in SECURITY.md.

Only the latest release receives security fixes; there are no backport branches. Published advisories are listed under Security → Advisories.

Upgrade note: v0.3.3 and earlier are affected by GHSA-gfj3-cmff-q8wh, a high-severity unauthenticated denial of service (unbounded response buffering, CVSS 7.5). Fixed in v0.3.4.

License

AGPL-3.0. See LICENSE.