Securing your app with signed headers

This page describes how to add an additional layer of security to your apps with signed headers. When configured, pomerium uses JSON Web Tokens (JWT) to make sure that a given request was handled by pomerium and the request to your app is authorized.

Prerequisites

To secure your app with signed headers, you'll need the following:

  • An application you want users to connect to.
  • A JWT library that supports the ES256 algorithm.

Rationale

Signed headers provide secondary security in case someone bypasses mTLS and network segmentation. This protects your app from the following kind of risks:

  • Pomerium is accidentally disabled
  • Misconfigured firewalls
  • Mutually-authenticated TLS
  • Access from within the project.

To properly secure your app, you must use signed headers for all app types.

Verification

To secure your app with JWT, cryptographically verify the header, payload, and signature of the JWT. The JWT is in the HTTP request header x-pomerium-iap-jwt-assertion. If an attacker bypasses pomerium, they can forge the unsigned identity headers, x-pomerium-authenticated-user-{email,id,groups}. JWT provides a more secure alternative.

Note that pomerium it strips the x-pomerium-* headers provided by the client when the request goes through the serving infrastructure.

Verify that the JWT's header conforms to the following constraints:

JWT description
exp Expiration time in seconds since the UNIX epoch. Allow 1 minute for skew.
iat Issued-at time in seconds since the UNIX epoch. Allow 1 minute for skew.
aud The client's final domain e.g. httpbin.corp.example.com.
iss Issuer must be pomerium-proxy.
sub Subject is the user's id. Can be used instead of the x-pomerium-authenticated-user-id header.
email Email is the user's email. Can be used instead of the x-pomerium-authenticated-user-email header.
groups Groups is the user's groups. Can be used instead of the x-pomerium-authenticated-user-groups header.

Manual verification

Though you will very likely be verifying signed-headers programmatically in your application's middleware, and using a third-party JWT library, if you are new to JWT it may be helpful to show what manual verification looks like. The following guide assumes you are using the provided docker-compose.yml as a base and httpbin. Httpbin gives us a convenient way of inspecting client headers.

  1. Provide pomerium with a base64 encoded Elliptic Curve (NIST P-256 aka secp256r1 aka prime256v1) Private Key. In production, you'd likely want to get these from your KMS.
# see ./scripts/generate_self_signed_signing_key.sh
openssl ecparam  -genkey  -name prime256v1  -noout  -out ec_private.pem
openssl req  -x509  -new  -key ec_private.pem  -days 1000000  -out ec_public.pem  -subj "/CN=unused"
# careful! this will output your private key in terminal
cat ec_private.pem | base64

Copy the base64 encoded value of your private key to pomerium-proxy's environmental configuration variable SIGNING_KEY.

SIGNING_KEY=ZxqyyIPPX0oWrrOwsxXgl0hHnTx3mBVhQ2kvW1YB4MM=
  1. Reload pomerium-proxy. Navigate to httpbin (by default, https://httpbin.corp.${YOUR-DOMAIN}.com), and login as usual. Click request inspection. Select `/headers'. Click try it out and then execute. You should see something like the following.

httpbin displaying jwt headers

  1. X-Pomerium-Jwt-Assertion is the signature value. It's less scary than it looks and basically just a compressed, json blob as described above. Navigate to jwt.io which provides a helpful GUI to manually verify JWT values.

  2. Paste the value of X-Pomerium-Jwt-Assertion header token into the Encoded form. You should notice that the decoded values look much more familiar.

httpbin displaying decoded jwt

  1. Finally, we want to cryptographically verify the validity of the token. To do this, we will need the signer's public key. You can simply copy and past the output of cat ec_public.pem.

httpbin displaying verified jwt

Viola! Hopefully walking through a manual verification has helped give you a better feel for how signed JWT tokens are used as a secondary validation mechanism in pomerium.

In an actual client, you'll want to ensure that all the other claims values are valid (like expiration, issuer, audience and so on) in the context of your application. You'll also want to make sure you have a safe and reliable mechanism for distributing pomerium-proxy's public signing key to client apps (typically, a key management service).

Automatic verification

In the future, we will be adding example client implementations for:

  • Python
  • Go
  • Java
  • C#
  • PHP