diff --git a/src/Composer/Command/RequireCommand.php b/src/Composer/Command/RequireCommand.php
index 508514eb4..9b12941b8 100644
--- a/src/Composer/Command/RequireCommand.php
+++ b/src/Composer/Command/RequireCommand.php
@@ -26,6 +26,7 @@ use Composer\Plugin\PluginEvents;
 use Composer\Repository\CompositeRepository;
 use Composer\Repository\PlatformRepository;
 use Composer\IO\IOInterface;
+use Composer\Util\Silencer;
 
 /**
  * @author Jérémy Romey <jeremy@free-agent.fr>
@@ -103,11 +104,6 @@ EOT
 
             return 1;
         }
-        if (!is_writable($this->file)) {
-            $io->writeError('<error>'.$this->file.' is not writable.</error>');
-
-            return 1;
-        }
 
         if (filesize($this->file) === 0) {
             file_put_contents($this->file, "{\n}\n");
@@ -116,6 +112,14 @@ EOT
         $this->json = new JsonFile($this->file);
         $this->composerBackup = file_get_contents($this->json->getPath());
 
+        // check for writability by writing to the file as is_writable can not be trusted on network-mounts
+        // see https://github.com/composer/composer/issues/8231 and https://bugs.php.net/bug.php?id=68926
+        if (!is_writable($this->file) && !Silencer::call('file_put_contents', $this->file, $this->composerBackup)) {
+            $io->writeError('<error>'.$this->file.' is not writable.</error>');
+
+            return 1;
+        }
+
         $composer = $this->getComposer(true, $input->getOption('no-plugins'));
         $repos = $composer->getRepositoryManager()->getRepositories();