diff --git a/src/Composer/Json/JsonFile.php b/src/Composer/Json/JsonFile.php
index 07a6f922f..1b662fa91 100644
--- a/src/Composer/Json/JsonFile.php
+++ b/src/Composer/Json/JsonFile.php
@@ -185,7 +185,9 @@ class JsonFile
             self::validateSyntax($content, $this->path);
         }
 
+        $isComposerSchemaFile = false;
         if (null === $schemaFile) {
+            $isComposerSchemaFile = true;
             $schemaFile = __DIR__ . self::COMPOSER_SCHEMA_PATH;
         }
 
@@ -196,7 +198,10 @@ class JsonFile
 
         $schemaData = (object) array('$ref' => $schemaFile);
 
-        if ($schema !== self::LAX_SCHEMA) {
+        if ($schema === self::LAX_SCHEMA) {
+            $schemaData->additionalProperties = true;
+            $schemaData->required = array();
+        } elseif ($schema === self::STRICT_SCHEMA && $isComposerSchemaFile) {
             $schemaData->additionalProperties = false;
             $schemaData->required = array('name', 'description');
         }
@@ -204,8 +209,6 @@ class JsonFile
         $validator = new Validator();
         $validator->check($data, $schemaData);
 
-        // TODO add more validation like check version constraints and such, perhaps build that into the arrayloader?
-
         if (!$validator->isValid()) {
             $errors = array();
             foreach ((array) $validator->getErrors() as $error) {
diff --git a/tests/Composer/Test/Json/JsonFileTest.php b/tests/Composer/Test/Json/JsonFileTest.php
index eb0646ae4..d93b0414d 100644
--- a/tests/Composer/Test/Json/JsonFileTest.php
+++ b/tests/Composer/Test/Json/JsonFileTest.php
@@ -207,6 +207,38 @@ class JsonFileTest extends TestCase
         unlink($file);
     }
 
+    public function testCustomSchemaValidationLax()
+    {
+        $file = tempnam(sys_get_temp_dir(), 'c');
+        file_put_contents($file, '{ "custom": "property", "another custom": "property" }');
+
+        $schema = tempnam(sys_get_temp_dir(), 'c');
+        file_put_contents($schema, '{ "properties": { "custom": { "type": "string" }}}');
+
+        $json = new JsonFile($file);
+
+        $this->assertTrue($json->validateSchema(JsonFile::LAX_SCHEMA, $schema));
+
+        unlink($file);
+        unlink($schema);
+    }
+
+    public function testCustomSchemaValidationStrict()
+    {
+        $file = tempnam(sys_get_temp_dir(), 'c');
+        file_put_contents($file, '{ "custom": "property" }');
+
+        $schema = tempnam(sys_get_temp_dir(), 'c');
+        file_put_contents($schema, '{ "properties": { "custom": { "type": "string" }}}');
+
+        $json = new JsonFile($file);
+
+        $this->assertTrue($json->validateSchema(JsonFile::STRICT_SCHEMA, $schema));
+
+        unlink($file);
+        unlink($schema);
+    }
+
     public function testParseErrorDetectMissingCommaMultiline()
     {
         $json = '{