diff --git a/composer.json b/composer.json
index 7d6ff50..f4afdf4 100644
--- a/composer.json
+++ b/composer.json
@@ -35,10 +35,12 @@
         "phpstan/phpstan": "^1.10",
         "phpstan/phpstan-symfony": "^1.3",
         "phpunit/phpunit": "^10.4",
-        "symfony/messenger": "^5.4 || ^6.0 || ^7.0"
+        "symfony/messenger": "^5.4 || ^6.0 || ^7.0",
+        "symfony/security-bundle": "^5.4 || ^6.0 || ^7.0"
     },
     "suggest": {
-        "ext-newrelic": "Install the New Relic PHP agent on your system"
+        "ext-newrelic": "Install the New Relic PHP agent on your system",
+        "symfony/security-bundle": "Track users impacted by transaction error"
     },
     "autoload": {
         "psr-4": {
diff --git a/src/EventListener/ExceptionListener.php b/src/EventListener/ExceptionListener.php
index b69258a..3b7998c 100644
--- a/src/EventListener/ExceptionListener.php
+++ b/src/EventListener/ExceptionListener.php
@@ -14,6 +14,7 @@
 namespace Check24\NewRelicBundle\EventListener;
 
 use Check24\NewRelicBundle\NewRelic\NewRelicInteractorInterface;
+use Symfony\Bundle\SecurityBundle\Security;
 use Symfony\Component\EventDispatcher\Attribute\AsEventListener;
 use Symfony\Component\HttpKernel\Event\ExceptionEvent;
 
@@ -26,12 +27,18 @@
     public function __construct(
         private array $excludedExceptions,
         private NewRelicInteractorInterface $interactor,
+        private ?Security $security = null,
     ) {
     }
 
     public function __invoke(ExceptionEvent $event): void
     {
         $exception = $event->getThrowable();
+        // link users to transactions (anonymously), so that we can monitor # of users impacted by errors in error inbox
+        // see https://docs.newrelic.com/docs/errors-inbox/error-users-impacted/#attributes
+        if ($user = $this->security?->getUser()) {
+            $this->interactor->addCustomParameter('enduser.id', crc32($user->getUserIdentifier()));
+        }
 
         foreach ($this->excludedExceptions as $excludedException) {
             if ($exception instanceof $excludedException) {
diff --git a/tests/App/TestKernel.php b/tests/App/TestKernel.php
index 91888bd..f2ed060 100644
--- a/tests/App/TestKernel.php
+++ b/tests/App/TestKernel.php
@@ -16,6 +16,7 @@
 use Check24\NewRelicBundle\Check24NewRelicBundle;
 use Symfony\Bundle\FrameworkBundle\FrameworkBundle;
 use Symfony\Bundle\MonologBundle\MonologBundle;
+use Symfony\Bundle\SecurityBundle\SecurityBundle;
 use Symfony\Component\Config\Loader\LoaderInterface;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\HttpKernel\Kernel;
@@ -29,6 +30,7 @@ public function registerBundles(): iterable
             new FrameworkBundle(),
             new MonologBundle(),
             new Check24NewRelicBundle(),
+            new SecurityBundle(),
         ];
     }
 
@@ -46,6 +48,7 @@ public function registerContainerConfiguration(LoaderInterface $loader): void
                         ],
                     ],
                 ],
+                'router' => ['resource' => 'kernel::loadRoutes', 'type' => 'service', 'utf8' => true],
             ]);
 
             $container->loadFromExtension('check24_new_relic', [
@@ -76,6 +79,10 @@ public function registerContainerConfiguration(LoaderInterface $loader): void
                     ],
                 ],
             ]);
+            $container->loadFromExtension('security', [
+                'providers' => ['users_in_memory' => ['memory' => null]],
+                'firewalls' => ['main' => ['provider' => 'users_in_memory']],
+            ]);
         });
     }
 
diff --git a/tests/EventListener/ExceptionListenerTest.php b/tests/EventListener/ExceptionListenerTest.php
index 9a9f6a2..657fcc8 100644
--- a/tests/EventListener/ExceptionListenerTest.php
+++ b/tests/EventListener/ExceptionListenerTest.php
@@ -17,21 +17,25 @@
 use Check24\NewRelicBundle\NewRelic\NewRelicInteractorInterface;
 use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
+use Symfony\Bundle\SecurityBundle\Security;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\Event\ExceptionEvent;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
+use Symfony\Component\Security\Core\User\InMemoryUser;
 use Tests\Check24\NewRelicBundle\DummyException;
 
 class ExceptionListenerTest extends TestCase
 {
     private ExceptionListener $listener;
     private NewRelicInteractorInterface&MockObject $interactor;
+    private Security&MockObject $security;
 
     protected function setUp(): void
     {
         $this->listener = new ExceptionListener(
             [DummyException::class],
             $this->interactor = $this->createMock(NewRelicInteractorInterface::class),
+            $this->security = $this->createMock(Security::class),
         );
     }
 
@@ -51,6 +55,19 @@ public function testItSendErrorToNewRelic(): void
         $this->listener->__invoke($event);
     }
 
+    public function testItAttachesUserId(): void
+    {
+        $this->security->method('getUser')->willReturn(new InMemoryUser('foo@bar.baz', null));
+        $this->interactor->expects(self::once())->method('addCustomParameter')->with('enduser.id', '3260418537');
+
+        $this->listener->__invoke(new ExceptionEvent(
+            $this->createMock(HttpKernelInterface::class),
+            $this->createMock(Request::class),
+            HttpKernelInterface::MAIN_REQUEST,
+            new \RuntimeException(),
+        ));
+    }
+
     public function testItIgnoreExcludedErrors(): void
     {
         $event = new ExceptionEvent(
diff --git a/tests/FunctionalTest.php b/tests/FunctionalTest.php
index c544574..4e20abe 100644
--- a/tests/FunctionalTest.php
+++ b/tests/FunctionalTest.php
@@ -13,6 +13,7 @@
 
 namespace Tests\Check24\NewRelicBundle;
 
+use Check24\NewRelicBundle\EventListener\ExceptionListener;
 use Check24\NewRelicBundle\NewRelic\Config;
 use Check24\NewRelicBundle\NewRelic\NewRelicInteractor;
 use Check24\NewRelicBundle\NewRelic\NewRelicInteractorInterface;
@@ -38,6 +39,7 @@ public function testKernelIsBootableAndServicesAreUsable(): void
         self::assertInstanceOf(TraceUuidFactory::class, $container->get(TraceIdFactoryInterface::class));
         self::assertInstanceOf(RouteNameStrategy::class, $container->get(RequestTransactionNameStrategyInterface::class));
         self::assertInstanceOf(MessageNameStrategy::class, $container->get(MessengerTransactionNameStrategyInterface::class));
+        self::assertInstanceOf(ExceptionListener::class, $container->get(ExceptionListener::class));
 
         $config = $container->get(Config::class);