NGINX njs Request Inspection for CVE2021-44228

As the Log4Shell Vulnerability is still hard to mitigate and a couple of users have asked us if NGINX will be able to have something that will prevent requests from coming through the proxy layer we have just created a small njs script / configuration that will scan the URI, all incoming headers as well as the POST body for know strings.

Disclaimer

This configuration is not officially supported by NGINX and F5. Please track issues in this repository.

Prerequisite

NGINX njs module (> 0.4.0) Download and Installation Instructions here

Installation

Download the cve.js file and place it into your NGINX Configuration directory (/etc/nginx/conf.d/, /etc/nginx/) and load it using js_import.

js_import cve from /etc/nginx/conf.d/cve.js

Enabling the Header / URI request scanning in for all locations in your server block.

    if ( $isJNDI = "1" ) {  return 404 "Not Found!\n"; }

Example Configuration

Header and URI Variables

js_import cve from conf.d/cve.js;
js_set $isJNDI cve.inspect;

server {
    listen 8090;
    ...
    if ( $isJNDI = "1" ) {  return 404 "Not Found!\n"; }

    location / {
	return 200 "OK\n";
	...
    }
}

Post-Body Scanning

The configuration to scan the POST-Body data are a little bit more complex.

First, NGINX needs an mirror location to be able to inspect the whole post body. More Information. Create a location and add it to the server block. Please note, POST body scanning works only on location level.

    location /_scannBodyJNDI {
        internal;
        return 204;
    }

Second, we can hook into the scanning process. Add a new js_set directive to the configuration

js_import cve from cve202144228/cve.js;
js_set $isJNDI cve.inspect;
#add this
js_set $bodyScanned cve.postBodyInspect;

Reconfigure your already existing location block

   location /your-location/ {
       set $upstream "http://127.0.0.1:8099";  # Your Upstream-Definition. This can be a host OR an `upstream` defition.
       mirror /_scannBodyJNDI;
       client_body_in_single_buffer on;        # Minimize memory copy operations on request body
       client_body_buffer_size      128k;      # Largest body to keep in memory (before writing to file)
       client_max_body_size         128k;
    
       proxy_pass $bodyScanned; #Your new upstraem has to be set to this variable!
   }

Last add a error-proxy server configuration for all bad requests

server {
    listen 8999;

    location / {
        return 404 "Not Found!\n";
    }
}

If the Port 8999 is not available on your instance choose another one and change that in the server configuration in the cve.js file

function postBodyInspect(r) {;
    if (r.method === "POST") {
        try {
            if (checkIOCStrings(r, r.variables.request_body)) {
	        return "http://127.0.0.1:CHANGEME/";
	    } else {
	        return r.variables.upstream;
	    }
        } catch(e) {
            r.error(`POST Body inspection failed!`);
        }
    }
}