diff --git a/wp-includes/class-phpmailer.php b/wp-includes/class-phpmailer.php
index 7f5e353578..4e05a4340b 100644
--- a/wp-includes/class-phpmailer.php
+++ b/wp-includes/class-phpmailer.php
@@ -1294,9 +1294,12 @@ class PHPMailer
 
             // Sign with DKIM if enabled
             if (!empty($this->DKIM_domain)
-                && !empty($this->DKIM_selector)
-                && (!empty($this->DKIM_private_string)
-                   || (!empty($this->DKIM_private) && file_exists($this->DKIM_private))
+                and !empty($this->DKIM_selector)
+                and (!empty($this->DKIM_private_string)
+                    or (!empty($this->DKIM_private)
+                        and self::isPermittedPath($this->DKIM_private)
+                        and file_exists($this->DKIM_private)
+                    )
                 )
             ) {
                 $header_dkim = $this->DKIM_Add(
@@ -1461,6 +1464,39 @@ class PHPMailer
         return true;
     }
 
+    /**
+     * Check whether a file path is of a permitted type.
+     * Used to reject URLs and phar files from functions that access local file paths,
+     * such as addAttachment.
+     * @param string $path A relative or absolute path to a file.
+     * @return bool
+     */
+    protected static function isPermittedPath($path)
+    {
+        //Matches scheme definition from https://tools.ietf.org/html/rfc3986#section-3.1
+        return !preg_match('#^[a-z][a-z\d+.-]*://#i', $path);
+    }
+
+    /**
+     * Check whether a file path is safe, accessible, and readable.
+     *
+     * @param string $path A relative or absolute path to a file
+     *
+     * @return bool
+     */
+    protected static function fileIsAccessible($path)
+    {
+        if (!self::isPermittedPath($path)) {
+                return false;
+        }
+        $readable = file_exists($path);
+        //If not a UNC path (expected to start with \\), check read permission, see #2069
+        if (strpos($path, '\\\\') !== 0) {
+            $readable = $readable && is_readable($path);
+        }
+        return $readable;
+    }
+
     /**
      * Send mail using the PHP mail() function.
      * @param string $header The message headers
@@ -1784,7 +1820,7 @@ class PHPMailer
         // There is no English translation file
         if ($langcode != 'en') {
             // Make sure language file path is readable
-            if (!is_readable($lang_file)) {
+            if (!self::fileIsAccessible($lang_file)) {
                 $foundlang = false;
             } else {
                 // Overwrite language-specific strings.
@@ -2495,6 +2531,8 @@ class PHPMailer
      * Add an attachment from a path on the filesystem.
      * Never use a user-supplied path to a file!
      * Returns false if the file could not be found or read.
+     * Explicitly *does not* support passing URLs; PHPMailer is not an HTTP client.
+     * If you need to do that, fetch the resource yourself and pass it in via a local file or string.
      * @param string $path Path to the attachment.
      * @param string $name Overrides the attachment name.
      * @param string $encoding File encoding (see $Encoding).
@@ -2506,7 +2544,7 @@ class PHPMailer
     public function addAttachment($path, $name = '', $encoding = 'base64', $type = '', $disposition = 'attachment')
     {
         try {
-            if (!@is_file($path)) {
+            if (!self::fileIsAccessible($path)) {
                 throw new phpmailerException($this->lang('file_access') . $path, self::STOP_CONTINUE);
             }
 
@@ -2687,7 +2725,7 @@ class PHPMailer
     protected function encodeFile($path, $encoding = 'base64')
     {
         try {
-            if (!is_readable($path)) {
+            if (!self::fileIsAccessible($path)) {
                 throw new phpmailerException($this->lang('file_open') . $path, self::STOP_CONTINUE);
             }
             $magic_quotes = get_magic_quotes_runtime();
@@ -3031,7 +3069,7 @@ class PHPMailer
      */
     public function addEmbeddedImage($path, $cid, $name = '', $encoding = 'base64', $type = '', $disposition = 'inline')
     {
-        if (!@is_file($path)) {
+        if (!self::fileIsAccessible($path)) {
             $this->setError($this->lang('file_access') . $path);
             return false;
         }