diff -Naurp squirrelmail/src/compose.php htdocs/src/compose.php
--- squirrelmail/src/compose.php	2019-02-23 08:09:00.000000000 +0100
+++ htdocs/src/compose.php	2020-06-20 10:24:44.531930554 +0200
@@ -132,7 +132,7 @@ if ( sqgetGlobalVar('mailtodata', $mailt
                  'bcc'          => 'send_to_bcc',
                  'body'         => 'body',
                  'subject'      => 'subject');
-    $mtdata = unserialize($mailtodata);
+    $mtdata = json_decode($mailtodata, TRUE);
 
     foreach ($trtable as $f => $t) {
         if ( !empty($mtdata[$f]) ) {
diff -Naurp squirrelmail/src/mailto.php htdocs/src/mailto.php
--- squirrelmail/src/mailto.php	2019-01-08 05:27:15.000000000 +0100
+++ htdocs/src/mailto.php	2020-06-20 10:24:29.712232742 +0200
@@ -109,12 +109,12 @@ sqsession_is_active();
 
 if (!$force_login && sqsession_is_registered('user_is_logged_in')) {
     if ($compose_only) {
-        $redirect = 'compose.php?mailtodata=' . urlencode(serialize($data));
+        $redirect = 'compose.php?mailtodata=' . urlencode(json_encode($data));
     } else {
-        $redirect = 'webmail.php?right_frame=compose.php&mailtodata=' . urlencode(serialize($data));
+        $redirect = 'webmail.php?right_frame=compose.php&mailtodata=' . urlencode(json_encode($data));
     }
 } else {
-    $redirect = 'login.php?mailtodata=' . urlencode(serialize($data));
+    $redirect = 'login.php?mailtodata=' . urlencode(json_encode($data));
 }
 
 session_write_close();