WAF (OSS) ModSecurity Plugin example

Last updated: 3 minutes read.

Use Case

Traditionally, a Web Application Firewall (WAF) would be the first layer requests would hit, before reaching the API gateway. This is not possible if the Gateway has to terminate SSL, for things such as mTLS.

So what do you do if you still want to run your requests through a WAF to automatically scan for malicious action? We incorporate a WAF as part of the request lifecycle by using Tyk’s plugin architecture.


  • Already running Tyk - Community Edition or Pro
  • Docker, to run the WAF


This is NOT a production ready plugin because

  • The JavaScript plugin creates a new connection with the WAF for every request
  • The request is not sent over SSL
  • The WAF is only sent the query params for inspection.

For higher performance, the plugin could be written in Golang, and a connection pool would be opened and maintained over SSL

Install Steps

1. Turn JSVM on your tyk.conf at the root level:

Turn on JSVM interpreter to allow Tyk to run JavaScript plugins.

"enable_jsvm": true

2. Place the JavaScript plugin on Tyk file system

Copy the JS Plugin as a local .js file to the Gateway’s file system.

From the Gateway root, this will download the plugin called waf.js into the middleware directory:

curl https://raw.githubusercontent.com/TykTechnologies/custom-plugins/master/plugins/js-pre-post-waf/waf.js | cat > middleware/waf.js

(Instructions) If you are running Tyk in Docker, you can get into Tyk Gateway with docker exec

$ docker ps | grep gateway
670039a3e0b8        tykio/tyk-gateway:latest           "./entrypoint.sh"        14 minutes ago      Up 14 minutes>8080/tcp             tyk-demo_tyk-gateway_1

## copy container name or ID 
$ docker exec -it 670039a3e0b8 bash

## Now SSH'd into Tyk Gateway container and can perform curl
root@670039a3e0b8:/opt/tyk-gateway# ls

apps	   entrypoint.sh   install  middleware	templates  tyk-gateway.pid  tyk.conf.example
coprocess  event_handlers  js	    policies	tyk	   tyk.conf	    utils

## Download the plugin
root@670039a3e0b8:/opt/tyk-gateway# curl https://raw.githubusercontent.com/TykTechnologies/custom-plugins/master/plugins/js-pre-post-waf/waf.js | cat > middleware/waf.js

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1125  100  1125    0     0   3906      0 --:--:-- --:--:-- --:--:--  3975

waf.js source

3. Import API definition into Tyk

Copy the following Tyk API definition and import it into your environment.

API Definition JSON

Here’s the important section which adds the plugin to the request lifecycle for this API:

"custom_middleware": {
      "pre": [
          "name": "Waf",
          "path": "./middleware/waf.js"
How to Import?

Tyk Pro

Tyk CE

4. Run WAF ModSecurity Using Docker

First run ModSecurity with the popular Core RuleSet in Docker

$ docker run -ti -p 80:80 -e PARANOIA=1 --rm owasp/modsecurity-crs:v3.0

Open a second terminal and curl it

$ curl localhost

hello world

We should see the request show in the WAF server: - - [30/Jun/2020:00:56:42 +0000] "GET / HTTP/1.1" 200 12

Now try a dirty payload:

$ curl 'localhost/?param="><script>alert(1);</script>'

<title>403 Forbidden</title>
<p>You don't have permission to access /
on this server.<br />

Our WAF catches the response and returns a 403.

Now we try through Tyk.

## Clean requests, should get response from upstream's IP endpoint
$ curl localhost:8080/waf/ip

  "origin": ","

## WAF will detect malicious payload and instruct Tyk to deny
$ curl 'localhost:8080/waf/ip?param="><script>alert(1);</script>
    "error": "Bad request!"