fixing audience check, using email in default credentials if it exist…

…s, using iam config email, if populated

auth/gcp/iam.go

@@ -45,9 +45,13 @@ func NewDefaultIAMVerifier(ctx context.Context, cfg IAMConfig, clientFunc func(c
 		return nil, err
 	}
 
-	eml, err := GetDefaultEmail(ctx, "", clientFunc(ctx))
-	if err != nil {
-		return nil, errors.Wrap(err, "unable to get default email")
+	eml := cfg.ServiceAccountEmail
+	// only fall back if one isn't injected
+	if eml == "" {
+		eml, err = GetDefaultEmail(ctx, "", clientFunc(ctx))
+		if err != nil {
+			return nil, errors.Wrap(err, "unable to get default email")
+		}
 	}
 
 	return auth.NewVerifier(ks,
@@ -79,7 +83,7 @@ func IAMVerifyFunc(vf func(ctx context.Context, cs IAMClaimSet) bool) auth.Verif
 
 // ValidIAMClaims ensures the token audience issuers matches expectations.
 func ValidIAMClaims(cs IAMClaimSet, audience string) bool {
-	return cs.Aud != audience
+	return cs.Aud == audience
 }
 
 // VerifyIAMEmails is an auth.VerifyFunc that ensures IAMClaimSets are valid

auth/gcp/metadata.go

@@ -2,16 +2,32 @@ package gcp
 
 import (
 	"context"
+	"encoding/json"
 	"io/ioutil"
 	"net/http"
 
 	"github.com/pkg/errors"
+	"golang.org/x/oauth2/google"
+	iam "google.golang.org/api/iam/v1"
 )
 
 // GetDefaultEmail is a helper method for users on GCE or the 2nd generation GAE
 // environment.
 func GetDefaultEmail(ctx context.Context, addr string, hc *http.Client) (string, error) {
-	email, err := metadataGet(ctx, addr, hc, "instance/service-accounts/default/email")
+	creds, err := findDefaultCredentials(ctx, iam.CloudPlatformScope)
+	if err != nil {
+		return "", errors.Wrap(err, "unable to find credentials to sign JWT")
+	}
+
+	email, err := getEmailFromCredentials(creds)
+	if err != nil {
+		return "", errors.Wrap(err, "unable to get email from given credentials")
+	}
+	if email != "" {
+		return email, nil
+	}
+
+	email, err = metadataGet(ctx, addr, hc, "instance/service-accounts/default/email")
 	return email, errors.Wrap(err, "unable to get default email from metadata")
 }
 
@@ -39,3 +55,19 @@ func metadataGet(ctx context.Context, addr string, hc *http.Client, suffix strin
 	tkn, err := ioutil.ReadAll(resp.Body)
 	return string(tkn), errors.Wrap(err, "unable to read metadata response")
 }
+
+var findDefaultCredentials = google.FindDefaultCredentials
+
+func getEmailFromCredentials(creds *google.Credentials) (string, error) {
+	if len(creds.JSON) == 0 {
+		return "", nil
+	}
+
+	var data map[string]string
+	err := json.Unmarshal(creds.JSON, &data)
+	if err != nil {
+		return "", errors.Wrap(err, "unable to parse credentials")
+	}
+
+	return data["client_email"], nil
+}