Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.1074
diff -u -p -r1.1074 common.inc
--- includes/common.inc	3 Jan 2010 11:04:58 -0000	1.1074
+++ includes/common.inc	3 Jan 2010 21:32:40 -0000
@@ -4373,7 +4373,7 @@ function drupal_clear_js_cache() {
  */
 function drupal_json_encode($var) {
   // json_encode() does not escape <, > and &, so we do it with str_replace().
-  return str_replace(array('<', '>', '&'), array('\x3c', '\x3e', '\x26'), json_encode($var));
+  return str_replace(array('<', '>', '&'), array('\u003c', '\u003e', '\u0026'), json_encode($var));
 }
 
 /**
@@ -4382,8 +4382,7 @@ function drupal_json_encode($var) {
  * @see drupal_json_encode()
  */
 function drupal_json_decode($var) {
-  // json_decode() does not unescape <, > and &, so we do it with str_replace().
-  return json_decode(str_replace(array('\x3c', '\x3e', '\x26'), array('<', '>', '&'), $var), TRUE);
+  return json_decode($var, TRUE);
 }
 
 /**
Index: modules/simpletest/tests/common.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/common.test,v
retrieving revision 1.97
diff -u -p -r1.97 common.test
--- modules/simpletest/tests/common.test	15 Dec 2009 05:22:05 -0000	1.97
+++ modules/simpletest/tests/common.test	3 Jan 2010 21:25:38 -0000
@@ -1741,6 +1741,7 @@ class DrupalJSONTest extends DrupalUnitT
     }
     // Characters that must be escaped.
     $html_unsafe = array('<', '>', '&');
+    $html_unsafe_escaped = array('\u003c', '\u003e', '\u0026');
 
     // Verify there aren't character encoding problems with the source string.
     $this->assertIdentical(strlen($str), 128, t('A string with the full ASCII table has the correct length.'));
@@ -1763,6 +1764,10 @@ class DrupalJSONTest extends DrupalUnitT
     foreach ($html_unsafe as $char) {
       $this->assertTrue(strpos($json, $char) === FALSE, t('A JSON encoded string does not contain @s.', array('@s' => $char)));
     }
+    // Verify that JSON encoding escapes the HTML unsafe characters
+    foreach ($html_unsafe_escaped as $char) {
+      $this->assertTrue(strpos($json, $char) > 0, t('A JSON encoded string contains @s.', array('@s' => $char)));
+    }
     $json_decoded = drupal_json_decode($json);
     $this->assertNotIdentical($source, $json, t('An array encoded in JSON is not identical to the source.'));
     $this->assertIdentical($source, $json_decoded, t('Encoding structured data to JSON and decoding back results in the original data.'));