diff --git a/core/includes/file.inc b/core/includes/file.inc
index effb71c..13df19f 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -422,7 +422,7 @@ function file_prepare_directory(&$directory, $options = FILE_MODIFY_PERMISSIONS)
     return FALSE;
   }
   // The directory exists, so check to see if it is writable.
-  $writable = is_writable($directory);
+  $writable = file_directory_is_writable($directory);
   if (!$writable && ($options & FILE_MODIFY_PERMISSIONS)) {
     return drupal_chmod($directory);
   }
@@ -431,6 +431,46 @@ function file_prepare_directory(&$directory, $options = FILE_MODIFY_PERMISSIONS)
 }
 
 /**
+ * Determines if a directory is writable by the web server.
+ *
+ * In order to be able to write files within the directory, the directory
+ * itself must be writable, and it must also have the executable bit set. This
+ * helper function checks both at the same time.
+ *
+ * @param $uri
+ *   A URI or pathname pointing to the directory that will be checked.
+ *
+ * @return
+ *   TRUE if the directory is writable and executable; FALSE otherwise.
+ */
+function file_directory_is_writable($uri) {
+  // By converting the URI to a normal path using drupal_realpath(), we can
+  // correctly handle both stream wrappers and normal paths.
+  return is_writable(drupal_realpath($uri)) && drupal_is_executable($uri);
+}
+
+/**
+ * Determines if a file or directory is executable.
+ *
+ * PHP's is_executable() does not fully support stream wrappers, so this
+ * function fills that gap.
+ *
+ * @param $uri
+ *   A URI or pathname pointing to the file or directory that will be checked.
+ *
+ * @return
+ *   TRUE if the file or directory is executable; FALSE otherwise.
+ *
+ * @see is_executable()
+ * @ingroup php_wrappers
+ */
+function drupal_is_executable($uri) {
+  // By converting the URI to a normal path using drupal_realpath(), we can
+  // correctly handle both stream wrappers and normal paths.
+  return is_executable(drupal_realpath($uri));
+}
+
+/**
  * Creates a .htaccess file in each Drupal files directory if it is missing.
  */
 function file_ensure_htaccess() {
@@ -472,7 +512,7 @@ function file_save_htaccess($directory, $private = TRUE, $force_overwrite = FALS
   $htaccess_lines = file_htaccess_lines($private);
 
   // Write the .htaccess file.
-  if (file_exists($directory) && is_writable($directory) && file_put_contents($htaccess_path, $htaccess_lines)) {
+  if (file_exists($directory) && file_directory_is_writable($directory) && file_put_contents($htaccess_path, $htaccess_lines)) {
     return drupal_chmod($htaccess_path, 0444);
   }
   else {
@@ -1568,7 +1608,7 @@ function file_directory_os_temp() {
   $directories[] = sys_get_temp_dir();
 
   foreach ($directories as $directory) {
-    if (is_dir($directory) && is_writable($directory)) {
+    if (is_dir($directory) && file_directory_is_writable($directory)) {
       return $directory;
     }
   }
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 1ec434d..e525dd0 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1880,7 +1880,7 @@ function install_check_translations($langcode, $server_pattern) {
   // Get values so the requirements errors can be specific.
   if (drupal_verify_install_file($translations_directory, FILE_EXIST, 'dir')) {
     $readable = is_readable($translations_directory);
-    $writable = is_writable($translations_directory);
+    $writable = file_directory_is_writable($translations_directory);
     $translations_directory_exists = TRUE;
   }
 
@@ -2075,7 +2075,7 @@ function install_check_requirements($install_state) {
     // Otherwise, if $file does not exist yet, we can try to copy
     // $default_file to create it.
     elseif (!$exists) {
-      $copied = drupal_verify_install_file($conf_path, FILE_EXIST | FILE_WRITABLE, 'dir') && @copy($default_file, $file);
+      $copied = drupal_verify_install_file($conf_path, FILE_EXIST|FILE_WRITABLE|FILE_EXECUTABLE, 'dir') && @copy($default_file, $file);
       if ($copied) {
         // If the new $file file has the same owner as $default_file this means
         // $default_file is owned by the webserver user. This is an inherent
diff --git a/core/includes/install.inc b/core/includes/install.inc
index ad911db..3b45c12 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -673,12 +673,12 @@ function drupal_verify_install_file($file, $mask = NULL, $type = 'file') {
             }
             break;
           case FILE_WRITABLE:
-            if (!is_writable($file) && !drupal_install_fix_file($file, $mask)) {
+            if (!file_directory_is_writable($file) && !drupal_install_fix_file($file, $mask)) {
               $return = FALSE;
             }
             break;
           case FILE_EXECUTABLE:
-            if (!is_executable($file) && !drupal_install_fix_file($file, $mask)) {
+            if (!drupal_is_executable($file) && !drupal_install_fix_file($file, $mask)) {
               $return = FALSE;
             }
             break;
@@ -688,12 +688,12 @@ function drupal_verify_install_file($file, $mask = NULL, $type = 'file') {
             }
             break;
           case FILE_NOT_WRITABLE:
-            if (is_writable($file) && !drupal_install_fix_file($file, $mask)) {
+            if (file_directory_is_writable($file) && !drupal_install_fix_file($file, $mask)) {
               $return = FALSE;
             }
             break;
           case FILE_NOT_EXECUTABLE:
-            if (is_executable($file) && !drupal_install_fix_file($file, $mask)) {
+            if (drupal_is_executable($file) && !drupal_install_fix_file($file, $mask)) {
               $return = FALSE;
             }
             break;
@@ -788,12 +788,12 @@ function drupal_install_fix_file($file, $mask, $message = TRUE) {
           }
           break;
         case FILE_WRITABLE:
-          if (!is_writable($file)) {
+          if (!file_directory_is_writable($file)) {
             $mod |= 0222;
           }
           break;
         case FILE_EXECUTABLE:
-          if (!is_executable($file)) {
+          if (!drupal_is_executable($file)) {
             $mod |= 0111;
           }
           break;
@@ -803,12 +803,12 @@ function drupal_install_fix_file($file, $mask, $message = TRUE) {
           }
           break;
         case FILE_NOT_WRITABLE:
-          if (is_writable($file)) {
+          if (file_directory_is_writable($file)) {
             $mod &= ~0222;
           }
           break;
         case FILE_NOT_EXECUTABLE:
-          if (is_executable($file)) {
+          if (drupal_is_executable($file)) {
             $mod &= ~0111;
           }
           break;
diff --git a/core/lib/Drupal/Core/Updater/Updater.php b/core/lib/Drupal/Core/Updater/Updater.php
index bbaecfb..d6b167e 100644
--- a/core/lib/Drupal/Core/Updater/Updater.php
+++ b/core/lib/Drupal/Core/Updater/Updater.php
@@ -275,7 +275,7 @@ public function prepareInstallDirectory(&$filetransfer, $directory) {
     // Make the parent dir writable if need be and create the dir.
     if (!is_dir($directory)) {
       $parent_dir = dirname($directory);
-      if (!is_writable($parent_dir)) {
+      if (!file_directory_is_writable($parent_dir)) {
         @chmod($parent_dir, 0755);
         // It is expected that this will fail if the directory is owned by the
         // FTP user. If the FTP user == web server, it will succeed.
@@ -317,7 +317,7 @@ public function prepareInstallDirectory(&$filetransfer, $directory) {
    *   If the chmod should be applied recursively.
    */
   public function makeWorldReadable(&$filetransfer, $path, $recursive = TRUE) {
-    if (!is_executable($path)) {
+    if (!drupal_is_executable($path)) {
       // Set it to read + execute.
       $new_perms = substr(sprintf('%o', fileperms($path)), -4, -1) . "5";
       $filetransfer->chmod($path, intval($new_perms, 8), $recursive);
diff --git a/core/modules/system/src/Form/PerformanceForm.php b/core/modules/system/src/Form/PerformanceForm.php
index 372b319..ba846e1 100644
--- a/core/modules/system/src/Form/PerformanceForm.php
+++ b/core/modules/system/src/Form/PerformanceForm.php
@@ -136,7 +136,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     );
 
     $directory = 'public://';
-    $is_writable = is_dir($directory) && is_writable($directory);
+    $is_writable = is_dir($directory) && file_directory_is_writable($directory);
     $disabled = !$is_writable;
     $disabled_message = '';
     if (!$is_writable) {
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index cc2079f..209b130 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -421,7 +421,7 @@ function system_requirements($phase) {
     if ($phase == 'install') {
       file_prepare_directory($directory, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS);
     }
-    $is_writable = is_writable($directory);
+    $is_writable = file_directory_is_writable($directory);
     $is_directory = is_dir($directory);
     if (!$is_writable || !$is_directory) {
       $description = '';
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 255449e..03bb066 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -776,7 +776,7 @@ function system_check_directory($form_element, FormStateInterface $form_state) {
     $logger->error('The directory %directory does not exist and could not be created.', array('%directory' => $directory));
   }
 
-  if (is_dir($directory) && !is_writable($directory) && !drupal_chmod($directory)) {
+  if (is_dir($directory) && !file_directory_is_writable($directory) && !drupal_chmod($directory)) {
     // If the directory is not writable and cannot be made so.
     $form_state->setErrorByName($form_element['#parents'][0], t('The directory %directory exists but is not writable and could not be made writable.', array('%directory' => $directory)));
     $logger->error('The directory %directory exists but is not writable and could not be made writable.', array('%directory' => $directory));