switch to fetching the token endpoint from issuer (#119)

Switches to fetching the token endpoint url from the issuers well-known
openid-configuration file which aligns with other auth implementations.

Signed-off-by: Mike Mason <mimason@equinix.com>

oauth2x/auth.go

@@ -2,7 +2,12 @@ package oauth2x
 
 import (
 	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
 	"net/http"
+	"net/url"
+	"time"
 
 	"github.com/spf13/pflag"
 	"github.com/spf13/viper"
@@ -13,15 +18,29 @@ import (
 	"golang.org/x/oauth2/clientcredentials"
 )
 
+var (
+	tokenEndpointClient = &http.Client{
+		Timeout: 5 * time.Second, // nolint:gomnd // clear and unexported
+	}
+
+	// ErrTokenEndpointMissing is returned when the issuers .well-known/openid-configuration is missing the token_endpoint key.
+	ErrTokenEndpointMissing = errors.New("token endpoint missing from issuer well-known openid-configuration")
+)
+
 // NewClientCredentialsTokenSrc returns an oauth2 client credentials token source
-func NewClientCredentialsTokenSrc(ctx context.Context, cfg Config) oauth2.TokenSource {
+func NewClientCredentialsTokenSrc(ctx context.Context, cfg Config) (oauth2.TokenSource, error) {
+	tokenEndpoint, err := fetchIssuerTokenEndpoint(ctx, cfg.Issuer)
+	if err != nil {
+		return nil, err
+	}
+
 	ccCfg := clientcredentials.Config{
 		ClientID:     cfg.ID,
 		ClientSecret: cfg.Secret,
-		TokenURL:     cfg.TokenURL,
+		TokenURL:     tokenEndpoint,
 	}
 
-	return ccCfg.TokenSource(ctx)
+	return ccCfg.TokenSource(ctx), nil
 }
 
 // NewClient returns a http client using requested token source
@@ -32,9 +51,9 @@ func NewClient(ctx context.Context, tokenSrc oauth2.TokenSource) *http.Client {
 // Config handles reading in all the config values available
 // for setting up an oauth2 configuration
 type Config struct {
-	ID       string `mapstructure:"id"`
-	Secret   string `mapstructure:"secret"`
-	TokenURL string `mapstructure:"token_url"`
+	ID     string `mapstructure:"id"`
+	Secret string `mapstructure:"secret"`
+	Issuer string `mapstructure:"issuer"`
 }
 
 // MustViperFlags adds oidc oauth2 client credentials config to the provided flagset and binds to viper
@@ -45,6 +64,36 @@ func MustViperFlags(v *viper.Viper, flags *pflag.FlagSet) {
 	flags.String("oidc-client-secret", "", "oidc client secret")
 	viperx.MustBindFlag(v, "oidc.client.secret", flags.Lookup("oidc-client-secret"))
 
-	flags.String("oidc-client-token-url", "", "oidc token url")
-	viperx.MustBindFlag(v, "oidc.client.token_url", flags.Lookup("oidc-client-token-url"))
+	flags.String("oidc-client-issuer", "", "oidc issuer")
+	viperx.MustBindFlag(v, "oidc.client.issuer", flags.Lookup("oidc-client-issuer"))
+}
+
+func fetchIssuerTokenEndpoint(ctx context.Context, issuer string) (string, error) {
+	uri, err := url.JoinPath(issuer, ".well-known", "openid-configuration")
+	if err != nil {
+		return "", fmt.Errorf("invalid issuer: %w", err)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, uri, nil)
+	if err != nil {
+		return "", err
+	}
+
+	res, err := tokenEndpointClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer res.Body.Close() //nolint:errcheck // no need to check
+
+	var m map[string]interface{}
+	if err := json.NewDecoder(res.Body).Decode(&m); err != nil {
+		return "", err
+	}
+
+	tokenEndpoint, ok := m["token_endpoint"]
+	if !ok {
+		return "", ErrTokenEndpointMissing
+	}
+
+	return tokenEndpoint.(string), nil
 }