diff --git a/core/lib/Drupal/Core/Session/Session.php b/core/lib/Drupal/Core/Session/Session.php
index 65b6ce1..2939cc5 100644
--- a/core/lib/Drupal/Core/Session/Session.php
+++ b/core/lib/Drupal/Core/Session/Session.php
@@ -44,9 +44,9 @@ class Session {
    * This value has to persist, since a potentially wrong or disallowed session
    * would be written otherwise.
    *
-   * @var bool
+   * @var int
    */
-  protected static $enabled = TRUE;
+  protected static $disableLevel = 0;
 
   /**
    * Whether the session has been started.
@@ -110,9 +110,10 @@ public function initialize() {
    * Starts a session forcefully, preserving already set session data.
    */
   public function start() {
-    if ($this->isCli() || $this->isStarted()) {
+    if ($this->isCli() || $this->isAnyActive()) {
       return;
     }
+
     // Save current session data before starting it, as PHP will destroy it.
     $session_data = isset($_SESSION) ? $_SESSION : NULL;
 
@@ -164,10 +165,17 @@ public function save() {
   }
 
   /**
-   * Returns whether a session has been started.
+   * Returns whether this session has been started.
    */
   public function isStarted() {
-    return $this->started && session_status() === \PHP_SESSION_ACTIVE;
+    return $this->started && $this->isAnyActive();
+  }
+
+  /**
+   * Returns whether any PHP session is active.
+   */
+  public function isAnyActive() {
+    return session_status() === \PHP_SESSION_ACTIVE;
   }
 
   /**
@@ -249,7 +257,7 @@ public function migrate() {
    *   FALSE if writing session data has been disabled. TRUE otherwise.
    */
   public function isEnabled() {
-    return static::$enabled;
+    return static::$disableLevel == 0;
   }
 
   /**
@@ -264,7 +272,7 @@ public function isEnabled() {
    * @return $this
    */
   public function disable() {
-    static::$enabled = FALSE;
+    static::$disableLevel++;
     return $this;
   }
 
@@ -274,7 +282,9 @@ public function disable() {
    * @return $this
    */
   public function enable() {
-    static::$enabled = TRUE;
+    if (static::$disableLevel > 0) {
+      static::$disableLevel--;
+    }
     return $this;
   }