diff --git a/web/includes/auth.php b/web/includes/auth.php
index a1d45c74b..768ec578e 100644
--- a/web/includes/auth.php
+++ b/web/includes/auth.php
@@ -635,8 +635,8 @@ if (ZM_OPT_USE_AUTH) {
         } // end if success==false
       } // end if using reCaptcha
 
-      zm_session_clear(); # Closes session
-      zm_session_regenerate_id(); # starts session
+      # Drop the pre-auth session and issue a fresh id in a single Set-Cookie
+      zm_session_regenerate_id_login();
 
       $username = $_REQUEST['username'];
       $password = $_REQUEST['password'];
diff --git a/web/includes/session.php b/web/includes/session.php
index 3139336af..37e0dd761 100644
--- a/web/includes/session.php
+++ b/web/includes/session.php
@@ -92,6 +92,24 @@ function zm_session_regenerate_id() {
     : $_SERVER['REMOTE_ADDR'];
 } // function zm_session_regenerate_id()
 
+// Regenerate the session id at a privilege boundary (login).
+// When called with an already-started session (the normal login flow), this
+// should emit a single Set-Cookie via session_regenerate_id(true) while
+// discarding any pre-auth session data and deleting the old session server-side.
+// Assumes zm_session_start() has been called previously.
+function zm_session_regenerate_id_login() {
+  if (!is_session_started()) zm_session_start();
+  // Discard any pre-auth session contents so nothing carries across the
+  // authentication boundary.
+  $_SESSION = array();
+  // New id + delete the old session file server-side. Emits a single Set-Cookie.
+  session_regenerate_id(true);
+  $_SESSION['generated_at'] = time();
+  $_SESSION['remoteAddr'] = !empty($_SERVER['HTTP_X_FORWARDED_FOR'])
+    ? trim(explode(',', $_SERVER['HTTP_X_FORWARDED_FOR'])[0])
+    : $_SERVER['REMOTE_ADDR'];
+} // function zm_session_regenerate_id_login()
+
 function is_session_started() {
   if ( php_sapi_name() !== 'cli' ) {
     if ( version_compare(phpversion(), '5.4.0', '>=') ) {