grpcutil: additional JWT validation (#5303)

Add additional validation to the grpcutil.RequireSignedJWT method. Log
any validation error, instead of returning error details in the gRPC
status message.

pkg/grpcutil/options.go

@@ -3,10 +3,12 @@ package grpcutil
 import (
 	"context"
 	"encoding/base64"
+	"fmt"
 	"time"
 
 	"github.com/go-jose/go-jose/v3"
 	"github.com/go-jose/go-jose/v3/jwt"
+	"github.com/pomerium/pomerium/internal/log"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
@@ -92,22 +94,30 @@ func RequireSignedJWT(ctx context.Context, key []byte) error {
 			return status.Error(codes.Unauthenticated, "unauthenticated")
 		}
 
-		tok, err := jwt.ParseSigned(rawjwt)
+		if err := validateJWT(rawjwt, key); err != nil {
-		if err != nil {
+			log.Ctx(ctx).Debug().Err(err).Msg("rejected gRPC request due to invalid JWT")
-			return status.Errorf(codes.Unauthenticated, "invalid JWT: %v", err)
+			return status.Error(codes.Unauthenticated, "invalid JWT")
-		}
-
-		var claims struct {
-			Expiry *jwt.NumericDate `json:"exp,omitempty"`
-		}
-		err = tok.Claims(key, &claims)
-		if err != nil {
-			return status.Errorf(codes.Unauthenticated, "invalid JWT: %v", err)
-		}
-
-		if claims.Expiry == nil || time.Now().After(claims.Expiry.Time()) {
-			return status.Errorf(codes.Unauthenticated, "expired JWT: %v", err)
 		}
 	}
 	return nil
 }
+
+func validateJWT(rawjwt string, key []byte) error {
+	tok, err := jwt.ParseSigned(rawjwt)
+	if err != nil {
+		return err
+	}
+
+	var claims map[string]*jwt.NumericDate
+	err = tok.Claims(key, &claims)
+	if err != nil {
+		return err
+	} else if len(claims) != 1 || claims["exp"] == nil {
+		return fmt.Errorf("expected exactly one claim (exp)")
+	}
+
+	if t := claims["exp"].Time(); time.Now().After(t) {
+		return fmt.Errorf("JWT expired at %s", t.Format(time.DateTime))
+	}
+	return nil
+}

pkg/grpcutil/options_test.go

@@ -9,7 +9,10 @@ import (
 	"testing"
 	"time"
 
+	"github.com/go-jose/go-jose/v3"
+	"github.com/go-jose/go-jose/v3/jwt"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/reflection"
@@ -100,3 +103,73 @@ func TestSignedJWT(t *testing.T) {
 		assert.Equal(t, codes.OK, status.Code(err))
 	})
 }
+
+func TestValidateJWT(t *testing.T) {
+	sign := func(t *testing.T, key []byte, claims any) string {
+		t.Helper()
+		signer, err := jose.NewSigner(jose.SigningKey{Algorithm: jose.HS256, Key: key},
+			(&jose.SignerOptions{}).WithType("JWT"))
+		require.NoError(t, err)
+		s, err := jwt.Signed(signer).Claims(claims).CompactSerialize()
+		require.NoError(t, err)
+		return s
+	}
+
+	key := cryptutil.NewKey()
+
+	t.Run("unexpected_format", func(t *testing.T) {
+		err := validateJWT("not a jwt", key)
+		assert.Error(t, err)
+	})
+	t.Run("unexpected_claim_type", func(t *testing.T) {
+		rawjwt := sign(t, key, jwt.Claims{
+			Subject: "subject",
+			Expiry:  jwt.NewNumericDate(time.Now().Add(time.Hour)),
+		})
+		err := validateJWT(rawjwt, key)
+		assert.Error(t, err)
+	})
+	t.Run("unexpected_claim_name", func(t *testing.T) {
+		rawjwt := sign(t, key, jwt.Claims{
+			IssuedAt: jwt.NewNumericDate(time.Now()),
+			Expiry:   jwt.NewNumericDate(time.Now().Add(time.Hour)),
+		})
+		err := validateJWT(rawjwt, key)
+		assert.ErrorContains(t, err, "expected exactly one claim (exp)")
+	})
+	t.Run("no_claims", func(t *testing.T) {
+		rawjwt := sign(t, key, jwt.Claims{})
+		err := validateJWT(rawjwt, key)
+		assert.ErrorContains(t, err, "expected exactly one claim (exp)")
+	})
+	t.Run("unexpected_expiry_type", func(t *testing.T) {
+		rawjwt := sign(t, key, map[string]any{
+			"exp": "foo",
+		})
+		err := validateJWT(rawjwt, key)
+		assert.ErrorContains(t, err, "expected number value")
+	})
+	t.Run("expired", func(t *testing.T) {
+		rawjwt := sign(t, key, jwt.Claims{
+			Expiry: jwt.NewNumericDate(time.Now().Add(-time.Minute)),
+		})
+		err := validateJWT(rawjwt, key)
+		assert.ErrorContains(t, err, "JWT expired")
+	})
+	t.Run("wrong_key", func(t *testing.T) {
+		otherKey := cryptutil.NewKey()
+		rawjwt := sign(t, otherKey, jwt.Claims{
+			Expiry: jwt.NewNumericDate(time.Now().Add(time.Hour)),
+		})
+
+		err := validateJWT(rawjwt, key)
+		assert.Error(t, err)
+	})
+	t.Run("ok", func(t *testing.T) {
+		rawjwt := sign(t, key, jwt.Claims{
+			Expiry: jwt.NewNumericDate(time.Now().Add(time.Hour)),
+		})
+		err := validateJWT(rawjwt, key)
+		assert.NoError(t, err)
+	})
+}