diff --git a/api/critter-details.php b/api/critter-details.php
index a289459..1cf94a4 100644
--- a/api/critter-details.php
+++ b/api/critter-details.php
@@ -5,6 +5,9 @@ start_app_session();
 
 header('Content-Type: application/json');
 
+$pdo = getDbConnection();
+enforce_session_version($pdo);
+
 if (!isset($_SESSION['user_id'])) {
     http_response_code(401);
     echo json_encode(['error' => 'Not authenticated']);
@@ -28,7 +31,6 @@ function fetchOwnedCritter(PDO $pdo, int $userId, int $critterId): array|false
     return $stmt->fetch(PDO::FETCH_ASSOC);
 }
 
-$pdo = getDbConnection();
 $method = $_SERVER['REQUEST_METHOD'];
 $critterId = $method === 'POST'
     ? filter_input(INPUT_POST, 'id', FILTER_VALIDATE_INT)
diff --git a/api/purchase-boolean.php b/api/purchase-boolean.php
index ca8efa8..eda0766 100644
--- a/api/purchase-boolean.php
+++ b/api/purchase-boolean.php
@@ -5,11 +5,13 @@ start_app_session();
 
 header('Content-Type: application/json');
 
+$pdo = getDbConnection();
+enforce_session_version($pdo);
+
 $userId = $_SESSION['user_id'] ?? null;
 $result = false;
 
 if ($userId !== null) {
-    $pdo = getDbConnection();
     $tableExists = $pdo->query("SELECT name FROM sqlite_master WHERE type = 'table' AND name = 'transactions'")->fetch();
 
     if ($tableExists) {
diff --git a/create-critter.php b/create-critter.php
index b93dc73..59ae450 100644
--- a/create-critter.php
+++ b/create-critter.php
@@ -5,6 +5,9 @@ require __DIR__ . '/lib/ticket.php';
 require __DIR__ . '/lib/mailer.php';
 start_app_session();
 
+$pdo = getDbConnection();
+enforce_session_version($pdo);
+
 if (!isset($_SESSION['username']) || !isset($_SESSION['user_id'])) {
     header('Location: /');
     exit;
@@ -16,8 +19,6 @@ const DAILY_CRITTER_LIMIT = 30;
 const CRITTER_COOLDOWN_SECONDS = 15;
 const VERIFICATION_RESEND_COOLDOWN_SECONDS = 60;
 
-$pdo = getDbConnection();
-
 $userStmt = $pdo->prepare('SELECT email, username, credits, email_verified_at FROM users WHERE id = :id');
 $userStmt->execute(['id' => $_SESSION['user_id']]);
 $userRow = $userStmt->fetch(PDO::FETCH_ASSOC);
diff --git a/db.php b/db.php
index 6ef48b6..7bfc9c9 100644
--- a/db.php
+++ b/db.php
@@ -26,6 +26,9 @@ function getDbConnection(): PDO
         if (!in_array('email_verified_at', $userColumns, true)) {
             $pdo->exec('ALTER TABLE users ADD COLUMN email_verified_at TEXT');
         }
+        if (!in_array('session_version', $userColumns, true)) {
+            $pdo->exec('ALTER TABLE users ADD COLUMN session_version INTEGER NOT NULL DEFAULT 0');
+        }
     }
 
     return $pdo;
diff --git a/index.php b/index.php
index ecca183..9f7cc26 100644
--- a/index.php
+++ b/index.php
@@ -4,6 +4,8 @@ require __DIR__ . '/db.php';
 start_app_session();
 
 $pdo = getDbConnection();
+enforce_session_version($pdo);
+
 $tableExists = $pdo->query("SELECT name FROM sqlite_master WHERE type = 'table' AND name = 'adjectives'")->fetch();
 
 if (!$tableExists) {
diff --git a/lib/header.php b/lib/header.php
index 3bb5f36..2ab42fc 100644
--- a/lib/header.php
+++ b/lib/header.php
@@ -48,9 +48,12 @@
                             <a href="#" onClick="openCheckout(itemsList)" class="flex items-center gap-2 px-4 py-2 text-gray-700 hover:bg-gray-50 sm:hidden">
                                 <i data-lucide="coins" class="h-4 w-4"></i> Buy 50 Credits
                             </a>
-                            <a href="logout.php" class="flex items-center gap-2 px-4 py-2 text-gray-700 hover:bg-gray-50">
-                                <i data-lucide="log-out" class="h-4 w-4"></i> Log out
-                            </a>
+                            <form action="logout.php" method="post">
+                                <?php echo csrf_field(); ?>
+                                <button type="submit" class="w-full flex items-center gap-2 px-4 py-2 text-left text-gray-700 hover:bg-gray-50">
+                                    <i data-lucide="log-out" class="h-4 w-4"></i> Log out
+                                </button>
+                            </form>
                         </div>
                     </div>
                 </div>
diff --git a/login.php b/login.php
index a52d51f..53554a0 100644
--- a/login.php
+++ b/login.php
@@ -74,6 +74,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
                 session_regenerate_id(true);
                 $_SESSION['user_id'] = $user['id'];
                 $_SESSION['username'] = $user['username'];
+                $_SESSION['session_version'] = (int) $user['session_version'];
 
                 header('Location: /');
                 exit;
diff --git a/logout.php b/logout.php
index 0150ba0..33bb5d3 100644
--- a/logout.php
+++ b/logout.php
@@ -3,6 +3,11 @@ require __DIR__ . '/session.php';
 
 start_app_session();
 
+if ($_SERVER['REQUEST_METHOD'] !== 'POST' || !csrf_verify($_POST['csrf_token'] ?? null)) {
+    header('Location: /');
+    exit;
+}
+
 $_SESSION = [];
 session_destroy();
 
diff --git a/receive-purchase.php b/receive-purchase.php
index 608c5e5..ce4e19f 100644
--- a/receive-purchase.php
+++ b/receive-purchase.php
@@ -115,8 +115,14 @@ if (($event['event_type'] ?? '') === 'transaction.completed') {
     $creditsToAdd = 0;
     foreach ($items as $item) {
         $priceId = $item['price']['id'] ?? null;
+        $quantity = $item['quantity'] ?? 1;
+
+        if (!is_int($quantity) || $quantity < 1 || $quantity > 100) {
+            continue;
+        }
+
         if (is_string($priceId) && isset($creditsPerPrice[$priceId])) {
-            $creditsToAdd += $creditsPerPrice[$priceId];
+            $creditsToAdd += $creditsPerPrice[$priceId] * $quantity;
         }
     }
 
diff --git a/register.php b/register.php
index c0d9340..74e757e 100644
--- a/register.php
+++ b/register.php
@@ -115,6 +115,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
                 session_regenerate_id(true);
                 $_SESSION['user_id'] = $newUserId;
                 $_SESSION['username'] = $username;
+                $_SESSION['session_version'] = 0;
 
                 $verifyToken = createTicket($newUserId, 'email_verify', EMAIL_VERIFICATION_LIFETIME_SECONDS);
                 $appUrl = rtrim(getenv('APP_URL') ?: 'https://ourapp.local', '/');
diff --git a/reset-password.php b/reset-password.php
index a86abf1..a5663a4 100644
--- a/reset-password.php
+++ b/reset-password.php
@@ -36,7 +36,9 @@ if ($tokenIsWellFormed && $_SERVER['REQUEST_METHOD'] === 'POST') {
             $errors['password'] = 'This reset link has already been used or has expired. Please request a new one.';
         } else {
             $hashedPassword = password_hash($password, PASSWORD_DEFAULT);
-            $stmt = $pdo->prepare('UPDATE users SET password = :password WHERE id = :id');
+            // Bumping session_version invalidates every existing session for this user
+            // (including a stolen one) the next time enforce_session_version() runs.
+            $stmt = $pdo->prepare('UPDATE users SET password = :password, session_version = session_version + 1 WHERE id = :id');
             $stmt->execute(['password' => $hashedPassword, 'id' => $userId]);
 
             header('Location: login.php?reset=success');
diff --git a/session.php b/session.php
index 015c154..3c53353 100644
--- a/session.php
+++ b/session.php
@@ -56,3 +56,27 @@ function csrf_verify(?string $token): bool
 {
     return is_string($token) && $token !== '' && isset($_SESSION['csrf_token']) && hash_equals($_SESSION['csrf_token'], $token);
 }
+
+// Logs the session out if the user's session_version has moved on since login (e.g. a
+// password reset bumped it), so a stolen/old session cookie stops working immediately
+// instead of staying valid until it naturally expires. Call this right after
+// start_app_session() + getDbConnection() on any page that trusts $_SESSION['user_id'].
+function enforce_session_version(PDO $pdo): void
+{
+    if (!isset($_SESSION['user_id'])) {
+        return;
+    }
+
+    $stmt = $pdo->prepare('SELECT session_version FROM users WHERE id = :id');
+    $stmt->execute(['id' => $_SESSION['user_id']]);
+    $currentVersion = $stmt->fetchColumn();
+
+    $sessionVersionMatches = $currentVersion !== false
+        && isset($_SESSION['session_version'])
+        && (int) $currentVersion === (int) $_SESSION['session_version'];
+
+    if (!$sessionVersionMatches) {
+        $_SESSION = [];
+        session_destroy();
+    }
+}
diff --git a/single-critter.php b/single-critter.php
index 229f630..5f3c469 100644
--- a/single-critter.php
+++ b/single-critter.php
@@ -3,13 +3,14 @@ require __DIR__ . '/session.php';
 require __DIR__ . '/db.php';
 start_app_session();
 
+$pdo = getDbConnection();
+enforce_session_version($pdo);
+
 if (!isset($_SESSION['username']) || !isset($_SESSION['user_id'])) {
     header('Location: /');
     exit;
 }
 
-$pdo = getDbConnection();
-
 $critterId = filter_input(INPUT_GET, 'id', FILTER_VALIDATE_INT);
 
 function fetchOwnedCritter(PDO $pdo, int $userId, int $critterId)