diff --git a/phpBB/docs/CHANGELOG.html b/phpBB/docs/CHANGELOG.html
index 3cd2116ddb..93c47b0477 100644
--- a/phpBB/docs/CHANGELOG.html
+++ b/phpBB/docs/CHANGELOG.html
@@ -88,6 +88,7 @@
 	<a name="v305"></a><h3>1.i. Changes since 3.0.5</h3>
 
 	<ul>
+		<li>[Fix] Correctly determine writable status of files on Windows operating system. (Bug #39035)</li>
 		<li>[Feature] Backported 3.2 cpatcha plugins.</li>
 		<li>[Feature] Introduced new ACM plugins: null</li>
 	</ul>
diff --git a/phpBB/includes/acp/acp_main.php b/phpBB/includes/acp/acp_main.php
index 8a92c06e04..365290e909 100644
--- a/phpBB/includes/acp/acp_main.php
+++ b/phpBB/includes/acp/acp_main.php
@@ -504,7 +504,7 @@ class acp_main
 			$template->assign_var('S_REMOVE_INSTALL', true);
 		}
 
-		if (!defined('PHPBB_DISABLE_CONFIG_CHECK') && file_exists($phpbb_root_path . 'config.' . $phpEx) && is_writable($phpbb_root_path . 'config.' . $phpEx))
+		if (!defined('PHPBB_DISABLE_CONFIG_CHECK') && file_exists($phpbb_root_path . 'config.' . $phpEx) && phpbb_is_writable($phpbb_root_path . 'config.' . $phpEx))
 		{
 			// World-Writable? (000x)
 			$template->assign_var('S_WRITABLE_CONFIG', (bool) (@fileperms($phpbb_root_path . 'config.' . $phpEx) & 0x0002));
diff --git a/phpBB/includes/functions.php b/phpBB/includes/functions.php
index 58601be65b..78905beff6 100644
--- a/phpBB/includes/functions.php
+++ b/phpBB/includes/functions.php
@@ -669,6 +669,67 @@ function phpbb_chmod($filename, $perms = CHMOD_READ)
 	return $result;
 }
 
+/**
+ * Test if a file/directory is writable
+ *
+ * This function calls the native is_writable() when not running under
+ * Windows and it is not disabled.
+ *
+ * @param string $file Path to perform write test on
+ * @return bool True when the path is writable, otherwise false.
+ */
+function phpbb_is_writable($file)
+{
+	if (substr(PHP_OS, 0, 3) === 'WIN' || !function_exists('is_writable'))
+	{
+		if (file_exists($file))
+		{
+			// Canonicalise path to absolute path
+			$file = phpbb_realpath($file);
+
+			if (is_dir($file))
+			{
+				// Test directory by creating a file inside the directory
+				$result = @tempnam($file, 'i_w');
+
+				if (is_string($result) && file_exists($result))
+				{
+					unlink($result);
+
+					// Ensure the file is actually in the directory (returned realpathed)
+					return (strpos($result, $file) === 0) ? true : false;
+				}
+			}
+			else
+			{
+				$handle = @fopen($file, 'r+');
+
+				if (is_resource($handle))
+				{
+					fclose($handle);
+					return true;
+				}
+			}
+		}
+		else
+		{
+			// file does not exist test if we can write to the directory
+
+			$dir = dirname($file);
+
+			if (file_exists($dir) && is_dir($dir) && phpbb_is_writable($dir))
+			{
+				return true;
+			}
+		}
+		return false;
+	}
+	else
+	{
+		return is_writable($file);
+	}
+}
+
 // Compatibility functions
 
 if (!function_exists('array_combine'))