diff --git a/src/Game/Controller/GameAdminController.php b/src/Game/Controller/GameAdminController.php
index 33877fa..20efaa3 100644
--- a/src/Game/Controller/GameAdminController.php
+++ b/src/Game/Controller/GameAdminController.php
@@ -54,11 +54,10 @@ final class GameAdminController extends AbstractController
     {
         $playersLogs = [];
         foreach ($session->getPlayers() as $player) {
-            $username = $player->getUser()->getUsername();
-            $logFile = $this->projectDir . '/var/log/sessions/' . $session->getId() . '/' . $username . '.txt';
+            $logFile = $this->projectDir . '/var/log/sessions/' . $session->getId() . '/' . $player->getLogFileBasename() . '.txt';
 
             $playersLogs[] = [
-                'username' => $username,
+                'username' => $player->getUser()->getUsername(),
                 'logs' => file_exists($logFile) ? file_get_contents($logFile) : '',
             ];
         }
diff --git a/src/Game/Entity/Player.php b/src/Game/Entity/Player.php
index a99bd7c..888f500 100644
--- a/src/Game/Entity/Player.php
+++ b/src/Game/Entity/Player.php
@@ -66,4 +66,19 @@ class Player
 
         return $this;
     }
+
+    /**
+     * A filesystem-safe basename derived from the player's username, for use when
+     * building per-player log file paths. Usernames are validated to only contain
+     * safe characters at registration time, but this sanitizes defensively too, so
+     * a path segment can never traverse outside its intended directory regardless
+     * of what ends up stored on the user.
+     */
+    public function getLogFileBasename(): string
+    {
+        $username = $this->user?->getUsername() ?? '';
+        $safe = preg_replace('/[^A-Za-z0-9_-]/', '_', $username);
+
+        return $safe !== null && $safe !== '' ? $safe : ('player-' . $this->id);
+    }
 }
diff --git a/src/Game/Service/GameResponseService.php b/src/Game/Service/GameResponseService.php
index c01cdfa..280601d 100644
--- a/src/Game/Service/GameResponseService.php
+++ b/src/Game/Service/GameResponseService.php
@@ -107,14 +107,13 @@ class GameResponseService
     private function logSessionActivity(Player $player, string $content): void
     {
         $sessionId = $player->getSession()->getId();
-        $username = $player->getUser()->getUsername();
         $logDir = $this->projectDir . '/var/log/sessions/' . $sessionId;
 
         if (!is_dir($logDir)) {
             mkdir($logDir, 0777, true);
         }
 
-        $logFile = $logDir . '/' . $username . '.txt';
+        $logFile = $logDir . '/' . $player->getLogFileBasename() . '.txt';
         $timestamp = date('Y-m-d H:i:s');
         $logMessage = sprintf("[%s] %s\n", $timestamp, $content);
 
diff --git a/src/Tech/Form/AdminUserType.php b/src/Tech/Form/AdminUserType.php
index d701d87..7d2be2e 100644
--- a/src/Tech/Form/AdminUserType.php
+++ b/src/Tech/Form/AdminUserType.php
@@ -16,6 +16,7 @@ use Symfony\Component\OptionsResolver\OptionsResolver;
 use Symfony\Component\Validator\Constraints\Email;
 use Symfony\Component\Validator\Constraints\Length;
 use Symfony\Component\Validator\Constraints\NotBlank;
+use Symfony\Component\Validator\Constraints\Regex;
 
 class AdminUserType extends AbstractType
 {
@@ -26,7 +27,14 @@ class AdminUserType extends AbstractType
                 'constraints' => [new NotBlank(), new Email()],
             ])
             ->add('username', TextType::class, [
-                'constraints' => [new NotBlank(), new Length(min: 2, max: 180)],
+                'constraints' => [
+                    new NotBlank(),
+                    new Length(min: 2, max: 32),
+                    new Regex(
+                        pattern: '/^[A-Za-z0-9_-]+$/',
+                        message: 'Username may only contain letters, numbers, underscores, and hyphens.',
+                    ),
+                ],
             ])
             ->add('plainPassword', PasswordType::class, [
                 'mapped' => false,
diff --git a/src/Tech/Form/RegistrationFormType.php b/src/Tech/Form/RegistrationFormType.php
index daabb19..e78a4f8 100644
--- a/src/Tech/Form/RegistrationFormType.php
+++ b/src/Tech/Form/RegistrationFormType.php
@@ -16,6 +16,7 @@ use Symfony\Component\OptionsResolver\OptionsResolver;
 use Symfony\Component\Validator\Constraints\IsTrue;
 use Symfony\Component\Validator\Constraints\Length;
 use Symfony\Component\Validator\Constraints\NotBlank;
+use Symfony\Component\Validator\Constraints\Regex;
 
 class RegistrationFormType extends AbstractType
 {
@@ -26,6 +27,11 @@ class RegistrationFormType extends AbstractType
             ->add('username', TextType::class, [
                 'constraints' => [
                     new NotBlank(message: 'Please enter a username'),
+                    new Length(min: 3, max: 32, minMessage: 'Your username should be at least {{ limit }} characters', maxMessage: 'Your username cannot be longer than {{ limit }} characters'),
+                    new Regex(
+                        pattern: '/^[A-Za-z0-9_-]+$/',
+                        message: 'Your username may only contain letters, numbers, underscores, and hyphens.',
+                    ),
                 ],
             ])
             ->add('plainPassword', RepeatedType::class, [
diff --git a/tests/Game/SessionLoggingTest.php b/tests/Game/SessionLoggingTest.php
index 5b52ee9..4e7e5fa 100644
--- a/tests/Game/SessionLoggingTest.php
+++ b/tests/Game/SessionLoggingTest.php
@@ -74,6 +74,7 @@ class SessionLoggingTest extends TestCase
         $player->method('getUser')->willReturn($user);
         $player->method('getSession')->willReturn($session);
         $player->method('getScreen')->willReturn(1);
+        $player->method('getLogFileBasename')->willReturn('player1');
 
         $this->security->method('getUser')->willReturn($user);
         $this->playerService->method('GetCurrentlyActiveAsPlayer')->willReturn($player);