OpenResty OAuth Proxy Plugin

The OAuth Proxy is an extension to your API gateway that you need when your single page applications (SPAs) call OAuth-secured APIs using the Token Handler pattern. The SPA sends an encrypted proxy cookie that transports an access token. The OAuth Proxy plugin decrypts the cookie to extract the access token.

When the OAuth Proxy work completes, your OpenResty API gateway routes the request to the target API, which uses the access token to implement its OAuth security.

Download the OAuth Proxy

You get the OAuth Proxy by downloading a ZIP file from the Curity Developer Portal, with a filename of the following form. This zip file contains a Lua plugin that you deploy to your gateway and a README file with detailed instructions.

curity-token-handler-proxy-openresty-<version>.zip

Deploy the OAuth Proxy

You must deploy the OAuth Proxy to a gateway that runs in your SPA's backend for frontend domain. If a deployment uses the following URLs, the OAuth Proxy executes in a gateway at https://bff.product.example:

• SPA Web Origin: https://www.product.example
• Backend for Frontend Base URL: https://bff.product.example
• Target API Base URL: https://api.example.com

To deploy the OAuth Proxy and its dependencies, use the luarocks file provided. One way to deploy the files is to create a custom Docker image:

FROM openresty/openresty:jammy

COPY plugin/* \
    /tmp/oauth-proxy/
RUN cd /tmp/oauth-proxy && luarocks make oauth-proxy-*.rockspec \
    && cd / && rm -r /tmp/oauth-proxy

The luarocks command deploys Lua files to the Lua plugins location, which is a path similar to the following:

/usr/local/openresty/luajit/share/lua/5.1

When you deploy the OpenResty gateway you typically also provide API routes that reference the plugins. For an example, study the following Docker Compose file, where routes are supplied in a default.conf file:

services:
  api-gateway:
    image: custom_openresty:1.0.0
    hostname: apigateway-internal
    ports:
      - 80:3001
    volumes:
      - ./apigateway/openresty/nginx.conf:/usr/local/openresty/nginx/conf/nginx.conf
      - ./apigateway/openresty/default.conf:/etc/nginx/conf.d/default.conf

Prepare Decryption

You need an asymmetric keypair as part of your overall token handler deployment. To create one, follow the instructions in the Configure Cryptographic Keys section of the Create a Token Handler tutorial. This results in a PKCS#8 file with an encrypted private key, and also a private key password.

Configure API Routes

You must configure the OAuth Proxy for all API routes that the SPA calls. Also, add a route such as /oauthuserinfo if the SPA calls the authorization server's OpenID Connect userinfo endpoint.

First, you must configure a CORS plugin of your choice, to instruct the browser to allow requests with cookies from the SPA's web origin, and deny access to untrusted origins. Then, configure the OAuth Proxy plugin. You must provide the cookie decryption key for the OAuth Proxy, using the content of the PKCS#8 file, and the PKCS#8 file's password. An example configuraton is shown here:

server {
    server_name bff.product.example;
    listen      3001;

    location /api/ {

        rewrite_by_lua_block {
            
            local cors = require 'cors'
            cors.run({
                origins = {'http://www.product.example'},
                credentials = true,
                max_age = 86400
            })
            
            local oauth_proxy = require 'oauth-proxy'
            oauth_proxy.run({
                cookie_key = '-----BEGIN ENCRYPTED PRIVATE KEY-----\n...',
                cookie_key_pass = 'Password1'
            })
        }

        proxy_pass http://api-internal:3001/;
    }
}

The cookie_key and cookie_key_pass values can be populated in various ways. For example, at deployment time you can use a tool such as envsubst to produce the final configuration file. Alternatively, you can reference environment variables:

local oauth_proxy = require 'oauth-proxy'
oauth_proxy.run({
    cookie_key = os.getenv('TH_COOKIE_KEY'),
    cookie_key_pass = os.getenv('TH_COOKIE_KEY_PASS')
})

If you use environment variables, they must be referenced first in your main nginx.conf file:

env TH_COOKIE_KEY;
env TH_COOKIE_KEY_PASS;

Additional configuration settings are also available. For example, if the OAuth Agent uses a different prefix for cookie names than the default value th-, you should configure a cookie_prefix in the OAuth Proxy that matches the value from the OAuth Agent. You can find full information on configuration settings in the README file within your downloaded ZIP file, including some advanced options for caching.

OAuth Proxy Output

At runtime, the OAuth Proxy processes cookies in incoming requests from SPAs. On success it rewrites the HTTP request, setting the access token in the HTTP authorization header. The target API then receives a JWT access token in the standard way and can use it for its authorization.

If the OAuth Proxy experiences an invalid cookie error, it returns an error response to the SPA with a 401 HTTP status code. For other types of error, the OAuth Proxy returns a 500 HTTP status code.

Example Deployment

You can follow the Token Handler Deployment Example to fully understand how the OAuth Proxy integrates into an SPA and API solution. The tutorial deploys an example Single Page Application that calls an example API through the OAuth Proxy. You can run the end-to-end example on a development computer with various configurations.

Conclusion

The OAuth Proxy is part of your API gateway. It enables your SPAs to use the correct browser security and also make direct calls to API URLs. The OAuth Proxy plugin deals with the cookie complexity so that you do not need to implement it in your API code.