diff --git a/display.php b/display.php
index 37ad0c8..e76680a 100644
--- a/display.php
+++ b/display.php
@@ -5,6 +5,8 @@ require_once('./get_data.lib.php');
 $queue = get_queue();
 if ($queue) session_cache_limiter('nocache');
 else session_cache_limiter('none');
+// Refuse a session id that PHP did not make, so nobody can plant one
+ini_set('session.use_strict_mode', '1');
 session_set_cookie_params(array('lifetime' => 0, 'path' => '/', 'domain' => cookie_domain(), 'secure' => TRUE, 'httponly' => TRUE, 'samesite' => 'Lax'));
 session_start();
 if (isset($_SESSION['editor']) && editor_exists($_SESSION['editor'])) {
diff --git a/session.lib.php b/session.lib.php
index 0a0d8f6..0466146 100644
--- a/session.lib.php
+++ b/session.lib.php
@@ -13,6 +13,8 @@ if (isset($_SERVER['HTTPS'])) {
   // Browsers use HTTPS for this host for a year, without a plain request first
   header('Strict-Transport-Security: max-age=31536000');
   session_cache_limiter('private');
+  // Refuse a session id that PHP did not make, so nobody can plant one
+  ini_set('session.use_strict_mode', '1');
   session_set_cookie_params(array('lifetime' => 0, 'path' => '/', 'domain' => cookie_domain(), 'secure' => TRUE, 'httponly' => TRUE, 'samesite' => 'Lax'));
   session_start();
 
diff --git a/tests/php/Pages/FrontPagesTest.php b/tests/php/Pages/FrontPagesTest.php
index 4078b40..a753275 100644
--- a/tests/php/Pages/FrontPagesTest.php
+++ b/tests/php/Pages/FrontPagesTest.php
@@ -57,6 +57,17 @@ class FrontPagesTest extends PageTestCase {
     $this->assertNull($this->header($this->get('/', array('X-Test-Plain' => '1')), 'Strict-Transport-Security'));
   }
 
+  // PHP makes a new id for a session id that it did not make, so nobody can plant one
+  public function testAnUnknownSessionIdIsReplaced(): void {
+    $planted = str_repeat('ab', 16);
+    foreach (array('/', '/display?id=1') as $path) {
+      $this->cookies = array('PHPSESSID' => $planted);
+      $this->get($path);
+      $this->assertNotSame($planted, $this->cookies['PHPSESSID'] ?? NULL, $path);
+      $this->assertFileDoesNotExist(self::$state . "/sessions/sess_$planted", $path);
+    }
+  }
+
   public function testPlainHttpStartsNoSession(): void {
     $page = $this->get('/', array('X-Test-Plain' => '1'));
     $this->assertSame(200, $page['status']);