Go JSON Fetcher and JWT Validator

A go package for fetching JSON Web Key Sets (JWKS) from an authroization server and validating JSON Web Tokens (JWTs) using these keys. It includes HTTP middleware for integration with web services.

Features

• JWKS Fetching: Retrieves JWKS from a specified discovery URL (e.g. Microsoft Entra ID discovery endpoint).
• Automatic Key Rotation: Periodically fetches the JWKS endpoint to ensure keys are up-to-date.
• JWT Validation:
  • Verifies the JWT signature using the public key corresponding to the kid (Key ID) in the token header.
  • Validates the alg (algorithm) used for signing against a configurable list.
  • Validates the aud (audience) claim against a configurable list of allowed audiences.
  • Does not validate iss (issuer) or use.
• HTTP Middleware: Provides standard Go http.Handler middleware to protect endpoints.
• RSA Support: Currently supports JWTs signed with RSA algorithms.

Installation

go get github.com/intility/go-jwks

Example

package main

import (
	"context"
	"fmt"
	"log/slog"
	"net/http"
	"time"

	jwks "github.com/intility/go-jwks"

	"github.com/golang-jwt/jwt/v5"
)

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel() 

	// Provide the base URL of your identity provider. The package will construct the discovery URL.
	// Optionally set the refresh interval (defaults to 24 hours).
	jwksOpts := &jwks.JWKSFetcherOpts{
		BaseURL: "https://login.microsoftonline.com/YOUR_TENANT_ID",
		FetchInterval: 12 * time.Hour,
	}

	fetcher, err := jwks.NewJWKSFetcher(jwksOpts)
	if err != nil {
		slog.Error("failed to create fetcher", "error", err)
	}

  // Start fetching JWKS
	fetcher.Start(ctx)

	// Configure JWT Validator
	audiences := []string{
		"api://YOUR_API_CLIENT_ID", 
	}
	// Specify allowed signing algorithms
	validMethods := []string{
		jwt.SigningMethodRS256.Alg(), 
	}

	// Create the JWT Validator instance
	validator := jwks.NewJWTValidator(fetcher, audiences, validMethods)

	// Create the HTTP Middleware
	jwtMiddleware := jwks.JWTMiddleware(validator)

	mux := http.NewServeMux()

	// Apply the middleware to protected routes
	mux.Handle("/api/protected/ping", jwtMiddleware(http.HandlerFunc(pingHandler)))

	slog.Info("Server starting on :8080")
	if err := http.ListenAndServe(":8080", mux); err != nil {
		slog.Error("Server failed", "error", err)
	}
}

func pingHandler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "application/json")
	fmt.Fprintf(w, `{"message": "pong"}`)
}

Key fetching/synchronization

In the oauth2 protocol, the client will receive an access token signed by an authorization server. This token can then be included in the request header sent to the server. When the server receives this request, it needs to verify its signature using the public key from the authorization server. These keys are rotated often, and such the server must reach out the authorization server periodically to refresh its local key store. This synchronization is handled by the JWTFetcher running in the background.

JWT validation

To start validating JWTs, create a JWTValidator instance with the NewJWTValidator function. This object holds the in-memory store of JWKS from the fetcher, allowed audiences and valid singing methnods specified by the user.

Passing this validator to the JWTMiddleware function returns a http.HandlerFunc middleware ready to authenticate incoming requests. The middleware expects a "Authorization: Bearer <token>" jwt header.

TODO

Add support for EC (Elliptic Curve) key types (kty: "EC") and algorithms.