fix: validate JWT token on alias look ahead (#114)

- in order to ensure proper validation for alias look ahead the provided
 JWT token must match the role's configuration.

backend.go

@@ -3,6 +3,7 @@ package kubeauth
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"strings"
 	"sync"
@@ -106,6 +107,17 @@ func (b *kubeAuthBackend) config(ctx context.Context, s logical.Storage) (*kubeC
 	return conf, nil
 }
 
+func (b *kubeAuthBackend) loadConfig(ctx context.Context, s logical.Storage) (*kubeConfig, error) {
+	config, err := b.config(ctx, s)
+	if err != nil {
+		return nil, err
+	}
+	if config == nil {
+		return nil, errors.New("could not load backend configuration")
+	}
+	return config, nil
+}
+
 // role takes a storage backend and the name and returns the role's storage
 // entry
 func (b *kubeAuthBackend) role(ctx context.Context, s logical.Storage, name string) (*roleStorageEntry, error) {

path_login.go

@@ -87,13 +87,10 @@ func (b *kubeAuthBackend) pathLogin(ctx context.Context, req *logical.Request, d
 		}
 	}
 
-	config, err := b.config(ctx, req.Storage)
+	config, err := b.loadConfig(ctx, req.Storage)
 	if err != nil {
 		return nil, err
 	}
-	if config == nil {
-		return nil, errors.New("could not load backend configuration")
-	}
 
 	serviceAccount, err := b.parseAndValidateJWT(jwtStr, role, config)
 	if err != nil {
@@ -171,6 +168,7 @@ func (b *kubeAuthBackend) getAliasName(role *roleStorageEntry, serviceAccount *s
 
 // aliasLookahead returns the alias object with the SA UID from the JWT
 // Claims.
+// Only JWTs matching the specified role's configuration will be accepted as valid.
 func (b *kubeAuthBackend) aliasLookahead(ctx context.Context, req *logical.Request, data *framework.FieldData) (*logical.Response, error) {
 	roleName, resp := b.getFieldValueStr(data, "role")
 	if resp != nil {
@@ -190,15 +188,14 @@ func (b *kubeAuthBackend) aliasLookahead(ctx context.Context, req *logical.Reque
 		return logical.ErrorResponse(fmt.Sprintf("invalid role name %q", roleName)), nil
 	}
 
-	// Parse into JWT
-	parsedJWT, err := jws.ParseJWT([]byte(jwtStr))
+	config, err := b.loadConfig(ctx, req.Storage)
 	if err != nil {
 		return nil, err
 	}
 
-	// Decode claims into a service account object
-	sa := &serviceAccount{}
-	err = mapstructure.Decode(parsedJWT.Claims(), sa)
+	// validation of the JWT against the provided role ensures alias look ahead requests
+	// are authentic.
+	sa, err := b.parseAndValidateJWT(jwtStr, role, config)
 	if err != nil {
 		return nil, err
 	}

path_login_test.go

@@ -610,6 +610,12 @@ func TestAliasLookAhead(t *testing.T) {
 			config:  defaultTestBackendConfig(),
 			wantErr: errors.New("missing jwt"),
 		},
+		"invalid_jwt": {
+			role:    "plugin-test",
+			config:  defaultTestBackendConfig(),
+			jwt:     jwtBadServiceAccount,
+			wantErr: errors.New("service account name not authorized"),
+		},
 		"serviceaccount_uid": {
 			role: "plugin-test",
 			jwt:  jwtData,