External Libraries: Upgrade PHPMailer from 6.3.0 to 6.4.0.

6.4.0 reverts a change that made the `mail()` and sendmail transports set the envelope sender if one isn't explicitly provided, as it was causing problems in specific PHP/server configurations.

Release post: https://github.com/PHPMailer/PHPMailer/releases/tag/v6.4.0
Changelog: https://github.com/PHPMailer/PHPMailer/compare/v6.3.0...v6.4.0

Props Synchro, tigertech, ayeshrajans, galbaras, audrasjb, SergeyBiryukov, desrosj, ocean90.
Merges [50628] to the 5.7 branch.
Fixes #52822.
Built from https://develop.svn.wordpress.org/branches/5.7@50630


git-svn-id: http://core.svn.wordpress.org/branches/5.7@50242 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/PHPMailer/PHPMailer.php

@@ -748,7 +748,7 @@ class PHPMailer
      *
      * @var string
      */
-    const VERSION = '6.3.0';
+    const VERSION = '6.4.0';
 
     /**
      * Error severity: message only, continue processing.
@@ -1199,7 +1199,11 @@ class PHPMailer
                     )
                 ) {
                     //Decode the name part if it's present and encoded
-                    if (property_exists($address, 'personal') && preg_match('/^=\?.*\?=$/', $address->personal)) {
+                    if (
+                        property_exists($address, 'personal') &&
+                        extension_loaded('mbstring') &&
+                        preg_match('/^=\?.*\?=$/', $address->personal)
+                    ) {
                         $address->personal = mb_decode_mimeheader($address->personal);
                     }
 
@@ -1682,16 +1686,11 @@ class PHPMailer
         //Sendmail docs: http://www.sendmail.org/~ca/email/man/sendmail.html
         //Qmail docs: http://www.qmail.org/man/man8/qmail-inject.html
         //Example problem: https://www.drupal.org/node/1057954
-        //CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
-        if ('' === $this->Sender) {
-            $this->Sender = $this->From;
-        }
         if (empty($this->Sender) && !empty(ini_get('sendmail_from'))) {
             //PHP config has a sender address we can use
             $this->Sender = ini_get('sendmail_from');
         }
         //CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
-        //But sendmail requires this param, so fail without it
         if (!empty($this->Sender) && static::validateAddress($this->Sender) && self::isShellSafe($this->Sender)) {
             if ($this->Mailer === 'qmail') {
                 $sendmailFmt = '%s -f%s';
@@ -1699,8 +1698,12 @@ class PHPMailer
                 $sendmailFmt = '%s -oi -f%s -t';
             }
         } else {
-            $this->edebug('Sender address unusable or missing: ' . $this->Sender);
+            //allow sendmail to choose a default envelope sender. It may
-            return false;
+            //seem preferable to force it to use the From header as with
+            //SMTP, but that introduces new problems (see
+            //<https://github.com/PHPMailer/PHPMailer/issues/2298>), and
+            //it has historically worked this way.
+            $sendmailFmt = '%s -oi -t';
         }
 
         $sendmail = sprintf($sendmailFmt, escapeshellcmd($this->Sendmail), $this->Sender);
@@ -1860,9 +1863,6 @@ class PHPMailer
         //Qmail docs: http://www.qmail.org/man/man8/qmail-inject.html
         //Example problem: https://www.drupal.org/node/1057954
         //CVE-2016-10033, CVE-2016-10045: Don't pass -f if characters will be escaped.
-        if ('' === $this->Sender) {
-            $this->Sender = $this->From;
-        }
         if (empty($this->Sender) && !empty(ini_get('sendmail_from'))) {
             //PHP config has a sender address we can use
             $this->Sender = ini_get('sendmail_from');

wp-includes/PHPMailer/SMTP.php

@@ -35,7 +35,7 @@ class SMTP
      *
      * @var string
      */
-    const VERSION = '6.3.0';
+    const VERSION = '6.4.0';
 
     /**
      * SMTP line break constant.
@@ -553,6 +553,8 @@ class SMTP
                 }
                 //Send encoded username and password
                 if (
+                    //Format from https://tools.ietf.org/html/rfc4616#section-2
+                    //We skip the first field (it's forgery), so the string starts with a null byte
                     !$this->sendCommand(
                         'User & Password',
                         base64_encode("\0" . $username . "\0" . $password),

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7.1-alpha-50609';
+$wp_version = '5.7.1-alpha-50630';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.