diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index 9f1bcf129ffc24b42f2ae48502e517d11545836e..3a63aea19f26ae402c62c0e6a2217ff4d113f832 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -1,4 +1,8 @@
 
+Drupal 7.27, 2014-04-16
+----------------------
+- Fixed security issues (information disclosure). See SA-CORE-2014-002.
+
 Drupal 7.26, 2014-01-15
 ----------------------
 - Fixed security issues (multiple vulnerabilities). See SA-CORE-2014-001.
diff --git a/includes/ajax.inc b/includes/ajax.inc
index ab0111cedec8ed375f61768ecf8906ac08777ae4..8446bf8914e5107cbdb8802904ad78f88bb0be76 100644
--- a/includes/ajax.inc
+++ b/includes/ajax.inc
@@ -308,10 +308,11 @@ function ajax_render($commands = array()) {
  * pulls the form info from $_POST.
  *
  * @return
- *   An array containing the $form and $form_state. Use the list() function
- *   to break these apart:
+ *   An array containing the $form, $form_state, $form_id, $form_build_id and an
+ *   initial list of Ajax $commands. Use the list() function to break these
+ *   apart:
  *   @code
- *     list($form, $form_state, $form_id, $form_build_id) = ajax_get_form();
+ *     list($form, $form_state, $form_id, $form_build_id, $commands) = ajax_get_form();
  *   @endcode
  */
 function ajax_get_form() {
@@ -331,6 +332,17 @@ function ajax_get_form() {
     drupal_exit();
   }
 
+  // When a page level cache is enabled, the form-build id might have been
+  // replaced from within form_get_cache. If this is the case, it is also
+  // necessary to update it in the browser by issuing an appropriate Ajax
+  // command.
+  $commands = array();
+  if (isset($form['#build_id_old']) && $form['#build_id_old'] != $form['#build_id']) {
+    // If the form build ID has changed, issue an Ajax command to update it.
+    $commands[] = ajax_command_update_build_id($form);
+    $form_build_id = $form['#build_id'];
+  }
+
   // Since some of the submit handlers are run, redirects need to be disabled.
   $form_state['no_redirect'] = TRUE;
 
@@ -345,7 +357,7 @@ function ajax_get_form() {
   $form_state['input'] = $_POST;
   $form_id = $form['#form_id'];
 
-  return array($form, $form_state, $form_id, $form_build_id);
+  return array($form, $form_state, $form_id, $form_build_id, $commands);
 }
 
 /**
@@ -366,7 +378,7 @@ function ajax_get_form() {
  * @see system_menu()
  */
 function ajax_form_callback() {
-  list($form, $form_state) = ajax_get_form();
+  list($form, $form_state, $form_id, $form_build_id, $commands) = ajax_get_form();
   drupal_process_form($form['#form_id'], $form, $form_state);
 
   // We need to return the part of the form (or some other content) that needs
@@ -379,7 +391,19 @@ function ajax_form_callback() {
     $callback = $form_state['triggering_element']['#ajax']['callback'];
   }
   if (!empty($callback) && function_exists($callback)) {
-    return $callback($form, $form_state);
+    $result = $callback($form, $form_state);
+
+    if (!(is_array($result) && isset($result['#type']) && $result['#type'] == 'ajax')) {
+      // Turn the response into a #type=ajax array if it isn't one already.
+      $result = array(
+        '#type' => 'ajax',
+        '#commands' => ajax_prepare_response($result),
+      );
+    }
+
+    $result['#commands'] = array_merge($commands, $result['#commands']);
+
+    return $result;
   }
 }
 
@@ -1210,3 +1234,26 @@ function ajax_command_restripe($selector) {
     'selector' => $selector,
   );
 }
+
+/**
+ * Creates a Drupal Ajax 'update_build_id' command.
+ *
+ * This command updates the value of a hidden form_build_id input element on a
+ * form. It requires the form passed in to have keys for both the old build ID
+ * in #build_id_old and the new build ID in #build_id.
+ *
+ * The primary use case for this Ajax command is to serve a new build ID to a
+ * form served from the cache to an anonymous user, preventing one anonymous
+ * user from accessing the form state of another anonymous users on Ajax enabled
+ * forms.
+ *
+ * @param $form
+ *   The form array representing the form whose build ID should be updated.
+ */
+function ajax_command_update_build_id($form) {
+  return array(
+    'command' => 'updateBuildId',
+    'old' => $form['#build_id_old'],
+    'new' => $form['#build_id'],
+  );
+}
diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index e5de2d1806023b1161ab88945c5b69ad48a08144..4cc39142eec29a53a50a160c04aa83e43db21d76 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -8,7 +8,7 @@
 /**
  * The current system version.
  */
-define('VERSION', '7.26');
+define('VERSION', '7.27');
 
 /**
  * Core API compatibility.
diff --git a/includes/form.inc b/includes/form.inc
index 4e467bab3af09d87a53fdab69843e32ee11a0f88..fd80e09bb3b36e026f3b65eef7ad0272dbd2dc63 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -168,6 +168,12 @@ function drupal_get_form($form_id) {
  *       processed.
  *     - base_form_id: Identification for a base form, as declared in a
  *       hook_forms() implementation.
+ *     - immutable: If this flag is set to TRUE, a new form build id is
+ *       generated when the form is loaded from the cache. If it is subsequently
+ *       saved to the cache again, it will have another cache id and therefore
+ *       the original form and form-state will remain unaltered. This is
+ *       important when page caching is enabled in order to prevent form state
+ *       from leaking between anonymous users.
  *   - rebuild_info: Internal. Similar to 'build_info', but pertaining to
  *     drupal_rebuild_form().
  *   - rebuild: Normally, after the entire form processing is completed and
@@ -459,16 +465,24 @@ function drupal_rebuild_form($form_id, &$form_state, $old_form = NULL) {
   $form = drupal_retrieve_form($form_id, $form_state);
 
   // If only parts of the form will be returned to the browser (e.g., Ajax or
-  // RIA clients), re-use the old #build_id to not require client-side code to
-  // manually update the hidden 'build_id' input element.
+  // RIA clients), or if the form already had a new build ID regenerated when it
+  // was retrieved from the form cache, reuse the existing #build_id.
   // Otherwise, a new #build_id is generated, to not clobber the previous
   // build's data in the form cache; also allowing the user to go back to an
   // earlier build, make changes, and re-submit.
   // @see drupal_prepare_form()
-  if (isset($old_form['#build_id']) && !empty($form_state['rebuild_info']['copy']['#build_id'])) {
+  $enforce_old_build_id = isset($old_form['#build_id']) && !empty($form_state['rebuild_info']['copy']['#build_id']);
+  $old_form_is_mutable_copy = isset($old_form['#build_id_old']);
+  if ($enforce_old_build_id || $old_form_is_mutable_copy) {
     $form['#build_id'] = $old_form['#build_id'];
+    if ($old_form_is_mutable_copy) {
+      $form['#build_id_old'] = $old_form['#build_id_old'];
+    }
   }
   else {
+    if (isset($old_form['#build_id'])) {
+      $form['#build_id_old'] = $old_form['#build_id'];
+    }
     $form['#build_id'] = 'form-' . drupal_random_key();
   }
 
@@ -523,6 +537,15 @@ function form_get_cache($form_build_id, &$form_state) {
           }
         }
       }
+      // Generate a new #build_id if the cached form was rendered on a cacheable
+      // page.
+      if (!empty($form_state['build_info']['immutable'])) {
+        $form['#build_id_old'] = $form['#build_id'];
+        $form['#build_id'] = 'form-' . drupal_random_key();
+        $form['form_build_id']['#value'] = $form['#build_id'];
+        $form['form_build_id']['#id'] = $form['#build_id'];
+        unset($form_state['build_info']['immutable']);
+      }
       return $form;
     }
   }
@@ -535,15 +558,28 @@ function form_set_cache($form_build_id, $form, $form_state) {
   // 6 hours cache life time for forms should be plenty.
   $expire = 21600;
 
+  // Ensure that the form build_id embedded in the form structure is the same as
+  // the one passed in as a parameter. This is an additional safety measure to
+  // prevent legacy code operating directly with form_get_cache and
+  // form_set_cache from accidentally overwriting immutable form state.
+  if ($form['#build_id'] != $form_build_id) {
+    watchdog('form', 'Form build-id mismatch detected while attempting to store a form in the cache.', array(), WATCHDOG_ERROR);
+    return;
+  }
+
   // Cache form structure.
   if (isset($form)) {
     if ($GLOBALS['user']->uid) {
       $form['#cache_token'] = drupal_get_token();
     }
+    unset($form['#build_id_old']);
     cache_set('form_' . $form_build_id, $form, 'cache_form', REQUEST_TIME + $expire);
   }
 
   // Cache form state.
+  if (variable_get('cache', 0) && drupal_page_is_cacheable()) {
+    $form_state['build_info']['immutable'] = TRUE;
+  }
   if ($data = array_diff_key($form_state, array_flip(form_state_keys_no_cache()))) {
     cache_set('form_state_' . $form_build_id, $data, 'cache_form', REQUEST_TIME + $expire);
   }
diff --git a/misc/ajax.js b/misc/ajax.js
index 90c3bb8bbef3e306cfa2b176920a9eff5adf18b0..63dd65fdde10d970ac73be02719755655751892e 100644
--- a/misc/ajax.js
+++ b/misc/ajax.js
@@ -616,6 +616,13 @@ Drupal.ajax.prototype.commands = {
       .removeClass('odd even')
       .filter(':even').addClass('odd').end()
       .filter(':odd').addClass('even');
+  },
+
+  /**
+   * Command to update a form's build ID.
+   */
+  updateBuildId: function(ajax, response, status) {
+    $('input[name="form_build_id"][value="' + response.old + '"]').val(response.new);
   }
 };
 
diff --git a/modules/file/file.module b/modules/file/file.module
index 3d351fa2c73936783fbd046f250e9b956d359485..5a635fd7521b3c60335c4bd4c45b536f76f40d0b 100644
--- a/modules/file/file.module
+++ b/modules/file/file.module
@@ -246,7 +246,7 @@ function file_ajax_upload() {
     return array('#type' => 'ajax', '#commands' => $commands);
   }
 
-  list($form, $form_state) = ajax_get_form();
+  list($form, $form_state, $form_id, $form_build_id, $commands) = ajax_get_form();
 
   if (!$form) {
     // Invalid form_build_id.
@@ -284,7 +284,6 @@ function file_ajax_upload() {
   $js = drupal_add_js();
   $settings = call_user_func_array('array_merge_recursive', $js['settings']['data']);
 
-  $commands = array();
   $commands[] = ajax_command_replace(NULL, $output, $settings);
   return array('#type' => 'ajax', '#commands' => $commands);
 }
diff --git a/modules/simpletest/drupal_web_test_case.php b/modules/simpletest/drupal_web_test_case.php
index 6d0e59a4a917cfff9b694f144a160e204c98cd85..d71b1e1e6d158a80c1870d7fcb68e93f50446f85 100644
--- a/modules/simpletest/drupal_web_test_case.php
+++ b/modules/simpletest/drupal_web_test_case.php
@@ -2269,6 +2269,13 @@ protected function drupalPostAJAX($path, $edit, $triggering_element, $ajax_path
             }
             break;
 
+          case 'updateBuildId':
+            $buildId = $xpath->query('//input[@name="form_build_id" and @value="' . $command['old'] . '"]')->item(0);
+            if ($buildId) {
+              $buildId->setAttribute('value', $command['new']);
+            }
+            break;
+
           // @todo Add suitable implementations for these commands in order to
           //   have full test coverage of what ajax.js can do.
           case 'remove':
diff --git a/modules/simpletest/tests/ajax.test b/modules/simpletest/tests/ajax.test
index 664d52042af6dc51095aaf2e332f9bb766bd47e2..a0c7be8a2472ea2871d2ff049f9e94dada15a663 100644
--- a/modules/simpletest/tests/ajax.test
+++ b/modules/simpletest/tests/ajax.test
@@ -497,6 +497,85 @@ class AJAXMultiFormTestCase extends AJAXTestCase {
   }
 }
 
+/**
+ * Test Ajax forms when page caching for anonymous users is turned on.
+ */
+class AJAXFormPageCacheTestCase extends AJAXTestCase {
+  protected $profile = 'testing';
+
+  public static function getInfo() {
+    return array(
+      'name' => 'AJAX forms on cached pages',
+      'description' => 'Tests that AJAX forms work properly for anonymous users on cached pages.',
+      'group' => 'AJAX',
+    );
+  }
+
+  public function setUp() {
+    parent::setUp();
+
+    variable_set('cache', TRUE);
+  }
+
+  /**
+   * Return the build id of the current form.
+   */
+  protected function getFormBuildId() {
+    $build_id_fields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($build_id_fields), 1, 'One form build id field on the page');
+    return (string) $build_id_fields[0]['value'];
+  }
+
+  /**
+   * Create a simple form, then POST to system/ajax to change to it.
+   */
+  public function testSimpleAJAXFormValue() {
+    $this->drupalGet('ajax_forms_test_get_form');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
+    $build_id_initial = $this->getFormBuildId();
+
+    $edit = array('select' => 'green');
+    $commands = $this->drupalPostAJAX(NULL, $edit, 'select');
+    $build_id_first_ajax = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_initial, $build_id_first_ajax, 'Build id is changed in the simpletest-DOM on first AJAX submission');
+    $expected = array(
+      'command' => 'updateBuildId',
+      'old' => $build_id_initial,
+      'new' => $build_id_first_ajax,
+    );
+    $this->assertCommand($commands, $expected, 'Build id change command issued on first AJAX submission');
+
+    $edit = array('select' => 'red');
+    $commands = $this->drupalPostAJAX(NULL, $edit, 'select');
+    $build_id_second_ajax = $this->getFormBuildId();
+    $this->assertEqual($build_id_first_ajax, $build_id_second_ajax, 'Build id remains the same on subsequent AJAX submissions');
+
+    // Repeat the test sequence but this time with a page loaded from the cache.
+    $this->drupalGet('ajax_forms_test_get_form');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
+    $build_id_from_cache_initial = $this->getFormBuildId();
+    $this->assertEqual($build_id_initial, $build_id_from_cache_initial, 'Build id is the same as on the first request');
+
+    $edit = array('select' => 'green');
+    $commands = $this->drupalPostAJAX(NULL, $edit, 'select');
+    $build_id_from_cache_first_ajax = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_from_cache_initial, $build_id_from_cache_first_ajax, 'Build id is changed in the simpletest-DOM on first AJAX submission');
+    $this->assertNotEqual($build_id_first_ajax, $build_id_from_cache_first_ajax, 'Build id from first user is not reused');
+    $expected = array(
+      'command' => 'updateBuildId',
+      'old' => $build_id_from_cache_initial,
+      'new' => $build_id_from_cache_first_ajax,
+    );
+    $this->assertCommand($commands, $expected, 'Build id change command issued on first AJAX submission');
+
+    $edit = array('select' => 'red');
+    $commands = $this->drupalPostAJAX(NULL, $edit, 'select');
+    $build_id_from_cache_second_ajax = $this->getFormBuildId();
+    $this->assertEqual($build_id_from_cache_first_ajax, $build_id_from_cache_second_ajax, 'Build id remains the same on subsequent AJAX submissions');
+  }
+}
+
+
 /**
  * Miscellaneous Ajax tests using ajax_test module.
  */
diff --git a/modules/simpletest/tests/form.test b/modules/simpletest/tests/form.test
index 1d430b582495944675cb7869a0f47ac1877248e8..f90b854c7d7c4a9a2ca94c3ef234c638a7e01641 100644
--- a/modules/simpletest/tests/form.test
+++ b/modules/simpletest/tests/form.test
@@ -1156,6 +1156,182 @@ class FormsFormStorageTestCase extends DrupalWebTestCase {
       $this->assertText('State persisted.');
     }
   }
+
+  /**
+   * Verify that the form build-id remains the same when validation errors
+   * occur on a mutable form.
+   */
+  function testMutableForm() {
+    // Request the form with 'cache' query parameter to enable form caching.
+    $this->drupalGet('form_test/form-storage', array('query' => array('cache' => 1)));
+    $buildIdFields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($buildIdFields), 1, 'One form build id field on the page');
+    $buildId = (string) $buildIdFields[0]['value'];
+
+    // Trigger validation error by submitting an empty title.
+    $edit = array('title' => '');
+    $this->drupalPost(NULL, $edit, 'Continue submit');
+
+    // Verify that the build-id did not change.
+    $this->assertFieldByName('form_build_id', $buildId, 'Build id remains the same when form validation fails');
+  }
+
+  /**
+   * Verifies that form build-id is regenerated when loading an immutable form
+   * from the cache.
+   */
+  function testImmutableForm() {
+    // Request the form with 'cache' query parameter to enable form caching.
+    $this->drupalGet('form_test/form-storage', array('query' => array('cache' => 1, 'immutable' => 1)));
+    $buildIdFields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($buildIdFields), 1, 'One form build id field on the page');
+    $buildId = (string) $buildIdFields[0]['value'];
+
+    // Trigger validation error by submitting an empty title.
+    $edit = array('title' => '');
+    $this->drupalPost(NULL, $edit, 'Continue submit');
+
+    // Verify that the build-id did change.
+    $this->assertNoFieldByName('form_build_id', $buildId, 'Build id changes when form validation fails');
+
+    // Retrieve the new build-id.
+    $buildIdFields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($buildIdFields), 1, 'One form build id field on the page');
+    $buildId = (string) $buildIdFields[0]['value'];
+
+    // Trigger validation error by again submitting an empty title.
+    $edit = array('title' => '');
+    $this->drupalPost(NULL, $edit, 'Continue submit');
+
+    // Verify that the build-id does not change the second time.
+    $this->assertFieldByName('form_build_id', $buildId, 'Build id remains the same when form validation fails subsequently');
+  }
+
+  /**
+   * Verify that existing contrib code cannot overwrite immutable form state.
+   */
+  public function testImmutableFormLegacyProtection() {
+    $this->drupalGet('form_test/form-storage', array('query' => array('cache' => 1, 'immutable' => 1)));
+    $build_id_fields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($build_id_fields), 1, 'One form build id field on the page');
+    $build_id = (string) $build_id_fields[0]['value'];
+
+    // Try to poison the form cache.
+    $original = $this->drupalGetAJAX('form_test/form-storage-legacy/' . $build_id);
+    $this->assertEqual($original['form']['#build_id_old'], $build_id, 'Original build_id was recorded');
+    $this->assertNotEqual($original['form']['#build_id'], $build_id, 'New build_id was generated');
+
+    // Assert that a watchdog message was logged by form_set_cache.
+    $status = (bool) db_query_range('SELECT 1 FROM {watchdog} WHERE message = :message', 0, 1, array(':message' => 'Form build-id mismatch detected while attempting to store a form in the cache.'));
+    $this->assert($status, 'A watchdog message was logged by form_set_cache');
+
+    // Ensure that the form state was not poisoned by the preceeding call.
+    $original = $this->drupalGetAJAX('form_test/form-storage-legacy/' . $build_id);
+    $this->assertEqual($original['form']['#build_id_old'], $build_id, 'Original build_id was recorded');
+    $this->assertNotEqual($original['form']['#build_id'], $build_id, 'New build_id was generated');
+    $this->assert(empty($original['form']['#poisoned']), 'Original form structure was preserved');
+    $this->assert(empty($original['form_state']['poisoned']), 'Original form state was preserved');
+  }
+}
+
+/**
+ * Test the form storage when page caching for anonymous users is turned on.
+ */
+class FormsFormStoragePageCacheTestCase extends DrupalWebTestCase {
+  protected $profile = 'testing';
+
+  public static function getInfo() {
+    return array(
+      'name'  => 'Forms using form storage on cached pages',
+      'description'  => 'Tests a form using form storage and makes sure validation and caching works when page caching for anonymous users is turned on.',
+      'group' => 'Form API',
+    );
+  }
+
+  public function setUp() {
+    parent::setUp('form_test');
+
+    variable_set('cache', TRUE);
+  }
+
+  /**
+   * Return the build id of the current form.
+   */
+  protected function getFormBuildId() {
+    $build_id_fields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($build_id_fields), 1, 'One form build id field on the page');
+    return (string) $build_id_fields[0]['value'];
+  }
+
+  /**
+   * Build-id is regenerated when validating cached form.
+   */
+  public function testValidateFormStorageOnCachedPage() {
+    $this->drupalGet('form_test/form-storage-page-cache');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_initial = $this->getFormBuildId();
+
+    // Trigger validation error by submitting an empty title.
+    $edit = array('title' => '');
+    $this->drupalPost(NULL, $edit, 'Save');
+    $this->assertText($build_id_initial, 'Old build id on the page');
+    $build_id_first_validation = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_initial, $build_id_first_validation, 'Build id changes when form validation fails');
+
+    // Trigger validation error by again submitting an empty title.
+    $edit = array('title' => '');
+    $this->drupalPost(NULL, $edit, 'Save');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_second_validation = $this->getFormBuildId();
+    $this->assertEqual($build_id_first_validation, $build_id_second_validation, 'Build id remains the same when form validation fails subsequently');
+
+    // Repeat the test sequence but this time with a page loaded from the cache.
+    $this->drupalGet('form_test/form-storage-page-cache');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_from_cache_initial = $this->getFormBuildId();
+    $this->assertEqual($build_id_initial, $build_id_from_cache_initial, 'Build id is the same as on the first request');
+
+    // Trigger validation error by submitting an empty title.
+    $edit = array('title' => '');
+    $this->drupalPost(NULL, $edit, 'Save');
+    $this->assertText($build_id_initial, 'Old build id is initial build id');
+    $build_id_from_cache_first_validation = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_initial, $build_id_from_cache_first_validation, 'Build id changes when form validation fails');
+    $this->assertNotEqual($build_id_first_validation, $build_id_from_cache_first_validation, 'Build id from first user is not reused');
+
+    // Trigger validation error by again submitting an empty title.
+    $edit = array('title' => '');
+    $this->drupalPost(NULL, $edit, 'Save');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_from_cache_second_validation = $this->getFormBuildId();
+    $this->assertEqual($build_id_from_cache_first_validation, $build_id_from_cache_second_validation, 'Build id remains the same when form validation fails subsequently');
+  }
+
+  /**
+   * Build-id is regenerated when rebuilding cached form.
+   */
+  public function testRebuildFormStorageOnCachedPage() {
+    $this->drupalGet('form_test/form-storage-page-cache');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_initial = $this->getFormBuildId();
+
+    // Trigger rebuild, should regenerate build id.
+    $edit = array('title' => 'something');
+    $this->drupalPost(NULL, $edit, 'Rebuild');
+    $this->assertText($build_id_initial, 'Initial build id as old build id on the page');
+    $build_id_first_rebuild = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_initial, $build_id_first_rebuild, 'Build id changes on first rebuild.');
+
+    // Trigger subsequent rebuild, should regenerate the build id again.
+    $edit = array('title' => 'something');
+    $this->drupalPost(NULL, $edit, 'Rebuild');
+    $this->assertText($build_id_first_rebuild, 'First build id as old build id on the page');
+    $build_id_second_rebuild = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_first_rebuild, $build_id_second_rebuild, 'Build id changes on second rebuild.');
+  }
 }
 
 /**
diff --git a/modules/simpletest/tests/form_test.module b/modules/simpletest/tests/form_test.module
index c7885d7a0f99ec643ef0ccfc5e1d65f08322b04c..602b4090dd7c6100eb495a600b69b8029e191b94 100644
--- a/modules/simpletest/tests/form_test.module
+++ b/modules/simpletest/tests/form_test.module
@@ -90,6 +90,21 @@ function form_test_menu() {
     'type' => MENU_CALLBACK,
   );
 
+  $items['form_test/form-storage-legacy'] = array(
+    'title' => 'Emulate legacy AHAH-style ajax callback',
+    'page callback' => 'form_test_storage_legacy_handler',
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
+
+  $items['form_test/form-storage-page-cache'] = array(
+    'title' => 'Form storage with page cache test',
+    'page callback' => 'drupal_get_form',
+    'page arguments' => array('form_test_storage_page_cache_form'),
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
+
   $items['form_test/wrapper-callback'] = array(
     'title' => 'Form wrapper callback test',
     'page callback' => 'form_test_wrapper_callback',
@@ -746,9 +761,36 @@ function form_test_storage_form($form, &$form_state) {
     $form_state['cache'] = TRUE;
   }
 
+  if (isset($_REQUEST['immutable'])) {
+    $form_state['build_info']['immutable'] = TRUE;
+  }
+
   return $form;
 }
 
+/**
+ * Emulate legacy AHAH-style ajax callback.
+ *
+ * Drupal 6 AHAH callbacks used to operate directly on forms retrieved using
+ * form_get_cache and stored using form_set_cache after manipulation. This
+ * callback helps testing whether form_set_cache prevents resaving of immutable
+ * forms.
+ */
+function form_test_storage_legacy_handler($form_build_id) {
+  $form_state = array();
+  $form = form_get_cache($form_build_id, $form_state);
+
+  drupal_json_output(array(
+    'form' => $form,
+    'form_state' => $form_state,
+  ));
+
+  $form['#poisoned'] = TRUE;
+  $form_state['poisoned'] = TRUE;
+
+  form_set_cache($form_build_id, $form, $form_state);
+}
+
 /**
  * Form element validation handler for 'value' element in form_test_storage_form().
  *
@@ -785,6 +827,56 @@ function form_test_storage_form_submit($form, &$form_state) {
   $form_state['redirect'] = 'node';
 }
 
+/**
+ * A simple form for testing form storage when page caching is enabled.
+ */
+function form_test_storage_page_cache_form($form, &$form_state) {
+  $form['title'] = array(
+    '#type' => 'textfield',
+    '#title' => 'Title',
+    '#required' => TRUE,
+  );
+
+  $form['test_build_id_old'] = array(
+    '#type' => 'item',
+    '#title' => 'Old build id',
+    '#markup' => 'No old build id',
+  );
+
+  $form['submit'] = array(
+    '#type' => 'submit',
+    '#value' => 'Save',
+  );
+
+  $form['rebuild'] = array(
+    '#type' => 'submit',
+    '#value' => 'Rebuild',
+    '#submit' => array('form_test_storage_page_cache_rebuild'),
+  );
+
+  $form['#after_build'] = array('form_test_storage_page_cache_old_build_id');
+  $form_state['cache'] = TRUE;
+
+  return $form;
+}
+
+/**
+ * Form element #after_build callback: output the old form build-id.
+ */
+function form_test_storage_page_cache_old_build_id($form) {
+  if (isset($form['#build_id_old'])) {
+    $form['test_build_id_old']['#markup'] = check_plain($form['#build_id_old']);
+  }
+  return $form;
+}
+
+/**
+ * Form submit callback: Rebuild the form and continue.
+ */
+function form_test_storage_page_cache_rebuild($form, &$form_state) {
+  $form_state['rebuild'] = TRUE;
+}
+
 /**
  * A form for testing form labels and required marks.
  */