diff --git a/Http/Firewall/SwitchUserListener.php b/Http/Firewall/SwitchUserListener.php
index 4aff3b458..dac5e02e2 100644
--- a/Http/Firewall/SwitchUserListener.php
+++ b/Http/Firewall/SwitchUserListener.php
@@ -93,7 +93,8 @@ public function handle(GetResponseEvent $event)
             try {
                 $this->tokenStorage->setToken($this->attemptSwitchUser($request, $username));
             } catch (AuthenticationException $e) {
-                throw new \LogicException(sprintf('Switch User failed: "%s"', $e->getMessage()));
+                // Generate 403 in any conditions to prevent user enumeration vulnerabilities
+                throw new AccessDeniedException('Switch User failed: '.$e->getMessage(), $e);
             }
         }
 
@@ -130,7 +131,24 @@ private function attemptSwitchUser(Request $request, $username)
             throw new \LogicException(sprintf('You are already switched to "%s" user.', $token->getUsername()));
         }
 
-        $user = $this->provider->loadUserByUsername($username);
+        $currentUsername = $token->getUsername();
+        $nonExistentUsername = '_'.md5(random_bytes(8).$username);
+
+        // To protect against user enumeration via timing measurements
+        // we always load both successfully and unsuccessfully
+        try {
+            $user = $this->provider->loadUserByUsername($username);
+
+            try {
+                $this->provider->loadUserByUsername($nonExistentUsername);
+                throw new \LogicException('AuthenticationException expected');
+            } catch (AuthenticationException $e) {
+            }
+        } catch (AuthenticationException $e) {
+            $this->provider->loadUserByUsername($currentUsername);
+
+            throw $e;
+        }
 
         if (false === $this->accessDecisionManager->decide($token, [$this->role], $user)) {
             $exception = new AccessDeniedException();
diff --git a/Http/Tests/Firewall/SwitchUserListenerTest.php b/Http/Tests/Firewall/SwitchUserListenerTest.php
index 6e3b0e30f..31cd45809 100644
--- a/Http/Tests/Firewall/SwitchUserListenerTest.php
+++ b/Http/Tests/Firewall/SwitchUserListenerTest.php
@@ -17,6 +17,7 @@
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 use Symfony\Component\Security\Core\Authentication\Token\Storage\TokenStorage;
 use Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken;
+use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
 use Symfony\Component\Security\Core\Role\SwitchUserRole;
 use Symfony\Component\Security\Core\User\User;
 use Symfony\Component\Security\Http\Event\SwitchUserEvent;
@@ -161,6 +162,7 @@ public function testExitUserDoesNotDispatchEventWithStringUser()
     public function testSwitchUserIsDisallowed()
     {
         $token = new UsernamePasswordToken('username', '', 'key', ['ROLE_FOO']);
+        $user = new User('username', 'password', []);
 
         $this->tokenStorage->setToken($token);
         $this->request->query->set('_switch_user', 'kuba');
@@ -169,6 +171,33 @@ public function testSwitchUserIsDisallowed()
             ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'])
             ->willReturn(false);
 
+        $this->userProvider->expects($this->exactly(2))
+            ->method('loadUserByUsername')
+            ->withConsecutive(['kuba'])
+            ->will($this->onConsecutiveCalls($user, $this->throwException(new UsernameNotFoundException())));
+
+        $listener = new SwitchUserListener($this->tokenStorage, $this->userProvider, $this->userChecker, 'provider123', $this->accessDecisionManager);
+        $listener->handle($this->event);
+    }
+
+    /**
+     * @expectedException \Symfony\Component\Security\Core\Exception\AccessDeniedException
+     */
+    public function testSwitchUserTurnsAuthenticationExceptionTo403()
+    {
+        $token = new UsernamePasswordToken('username', '', 'key', ['ROLE_ALLOWED_TO_SWITCH']);
+
+        $this->tokenStorage->setToken($token);
+        $this->request->query->set('_switch_user', 'kuba');
+
+        $this->accessDecisionManager->expects($this->never())
+            ->method('decide');
+
+        $this->userProvider->expects($this->exactly(2))
+            ->method('loadUserByUsername')
+            ->withConsecutive(['kuba'], ['username'])
+            ->will($this->onConsecutiveCalls($this->throwException(new UsernameNotFoundException())));
+
         $listener = new SwitchUserListener($this->tokenStorage, $this->userProvider, $this->userChecker, 'provider123', $this->accessDecisionManager);
         $listener->handle($this->event);
     }
@@ -185,9 +214,10 @@ public function testSwitchUser()
             ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $user)
             ->willReturn(true);
 
-        $this->userProvider->expects($this->once())
-            ->method('loadUserByUsername')->with('kuba')
-            ->willReturn($user);
+        $this->userProvider->expects($this->exactly(2))
+            ->method('loadUserByUsername')
+            ->withConsecutive(['kuba'])
+            ->will($this->onConsecutiveCalls($user, $this->throwException(new UsernameNotFoundException())));
         $this->userChecker->expects($this->once())
             ->method('checkPostAuth')->with($user);
 
@@ -215,9 +245,10 @@ public function testSwitchUserKeepsOtherQueryStringParameters()
             ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $user)
             ->willReturn(true);
 
-        $this->userProvider->expects($this->once())
-            ->method('loadUserByUsername')->with('kuba')
-            ->willReturn($user);
+        $this->userProvider->expects($this->exactly(2))
+            ->method('loadUserByUsername')
+            ->withConsecutive(['kuba'])
+            ->will($this->onConsecutiveCalls($user, $this->throwException(new UsernameNotFoundException())));
         $this->userChecker->expects($this->once())
             ->method('checkPostAuth')->with($user);
 
@@ -243,9 +274,10 @@ public function testSwitchUserWithReplacedToken()
             ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $user)
             ->willReturn(true);
 
-        $this->userProvider->expects($this->any())
-            ->method('loadUserByUsername')->with('kuba')
-            ->willReturn($user);
+        $this->userProvider->expects($this->exactly(2))
+            ->method('loadUserByUsername')
+            ->withConsecutive(['kuba'])
+            ->will($this->onConsecutiveCalls($user, $this->throwException(new UsernameNotFoundException())));
 
         $dispatcher = $this->getMockBuilder('Symfony\Component\EventDispatcher\EventDispatcherInterface')->getMock();
         $dispatcher
@@ -290,9 +322,10 @@ public function testSwitchUserStateless()
             ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $user)
             ->willReturn(true);
 
-        $this->userProvider->expects($this->once())
-            ->method('loadUserByUsername')->with('kuba')
-            ->willReturn($user);
+        $this->userProvider->expects($this->exactly(2))
+            ->method('loadUserByUsername')
+            ->withConsecutive(['kuba'])
+            ->will($this->onConsecutiveCalls($user, $this->throwException(new UsernameNotFoundException())));
         $this->userChecker->expects($this->once())
             ->method('checkPostAuth')->with($user);