diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 1c63a94ff6..665b8baaba 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -9,6 +9,7 @@
 use Drupal\Core\Asset\AttachedAssetsInterface;
 use Drupal\Core\Database\Database;
 use Drupal\Core\File\Exception\FileException;
+use Drupal\Core\File\FileSystemInterface;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Test\TestDatabase;
@@ -16,6 +17,7 @@
 use Drupal\Tests\Listeners\SimpletestUiPrinter;
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Process\PhpExecutableFinder;
+use Symfony\Component\Process\Process;
 use Drupal\Core\Test\JUnitHelper;
 use Drupal\Core\Test\JUnitHelperLegacy;
 use Drupal\Core\Test\TestStatus;
@@ -217,7 +219,7 @@ function simpletest_run_phpunit_tests($test_id, array $unescaped_test_classnames
       'test_id' => $test_id,
       'test_class' => implode(",", $unescaped_test_classnames),
       'status' => TestStatus::label($status),
-      'message' => 'PHPunit Test failed to complete; Error: ' . implode("\n", $output),
+      'message' => 'PHPunit Test failed to complete; Error: ' . $output,
       'message_group' => 'Other',
       'function' => implode(",", $unescaped_test_classnames),
       'line' => '0',
@@ -314,7 +316,10 @@ function simpletest_summarize_phpunit_result($results) {
  *   Path to the PHPUnit XML file to use for the current $test_id.
  */
 function simpletest_phpunit_xml_filepath($test_id) {
-  return \Drupal::service('file_system')->realpath('public://simpletest') . '/phpunit-' . $test_id . '.xml';
+  $file_system = \Drupal::service('file_system');
+  $simpletest_path = $file_system->realpath('public://simpletest');
+  $file_system->prepareDirectory($simpletest_path, FileSystemInterface::CREATE_DIRECTORY | FileSystemInterface::MODIFY_PERMISSIONS);
+  return $simpletest_path . '/phpunit-' . $test_id . '.xml';
 }
 
 /**
@@ -355,29 +360,29 @@ function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpun
   global $base_url;
   // Setup an environment variable containing the database connection so that
   // functional tests can connect to the database.
-  putenv('SIMPLETEST_DB=' . Database::getConnectionInfoAsUrl());
-  putenv('SIMPLETEST_JUNIT_FILE=' . $phpunit_file);
+  $process_environment_variables = [
+    'SIMPLETEST_DB' => Database::getConnectionInfoAsUrl(),
+    'SIMPLETEST_JUNIT_FILE' => $phpunit_file,
+  ];
 
   // Setup an environment variable containing the base URL, if it is available.
   // This allows functional tests to browse the site under test. When running
   // tests via CLI, core/phpunit.xml.dist or core/scripts/run-tests.sh can set
   // this variable.
   if ($base_url) {
-    putenv('SIMPLETEST_BASE_URL=' . $base_url);
-    putenv('BROWSERTEST_OUTPUT_DIRECTORY=' . \Drupal::service('file_system')->realpath('public://simpletest'));
+    $process_environment_variables['SIMPLETEST_BASE_URL'] = $base_url;
+    $process_environment_variables['BROWSERTEST_OUTPUT_DIRECTORY'] = \Drupal::service('file_system')->realpath('public://simpletest');
   }
   $phpunit_bin = simpletest_phpunit_command();
 
   $command = [
     $phpunit_bin,
-    '--printer',
-    escapeshellarg(SimpletestUiPrinter::class),
   ];
 
   // Optimized for running a single test.
   if (count($unescaped_test_classnames) == 1) {
     $class = new \ReflectionClass($unescaped_test_classnames[0]);
-    $command[] = escapeshellarg($class->getFileName());
+    $command[] = $class->getFileName();
   }
   else {
     // Double escape namespaces so they'll work in a regexp.
@@ -388,27 +393,18 @@ function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpun
     $filter_string = implode("|", $escaped_test_classnames);
     $command = array_merge($command, [
       '--filter',
-      escapeshellarg($filter_string),
+      $filter_string,
     ]);
   }
 
-  // Need to change directories before running the command so that we can use
-  // relative paths in the configuration file's exclusions.
-  $old_cwd = getcwd();
-  chdir(\Drupal::root() . "/core");
-
-  // exec in a subshell so that the environment is isolated when running tests
-  // via the simpletest UI.
-  $ret = exec(implode(" ", $command), $output, $status);
+  $process = new Process($command, \Drupal::root() . "/core", $process_environment_variables);
+  $process->inheritEnvironmentVariables();
+  $process->setTimeout(NULL);
+  $process->run();
+  $output = $process->getOutput();
+  $status = $process->getExitCode();
 
-  chdir($old_cwd);
-  putenv('SIMPLETEST_DB=');
-  putenv('SIMPLETEST_JUNIT_FILE=');
-  if ($base_url) {
-    putenv('SIMPLETEST_BASE_URL=');
-    putenv('BROWSERTEST_OUTPUT_DIRECTORY=');
-  }
-  return $ret;
+  return '';
 }
 
 /**
@@ -426,7 +422,7 @@ function simpletest_phpunit_command() {
 
   // The file in Composer's bin dir is a *nix link, which does not work when
   // extracted from a tarball and generally not on Windows.
-  $command = escapeshellarg($vendor_dir . '/phpunit/phpunit/phpunit');
+  $command = $vendor_dir . '/phpunit/phpunit/phpunit';
   if (substr(PHP_OS, 0, 3) == 'WIN') {
     // On Windows it is necessary to run the script using the PHP executable.
     $php_executable_finder = new PhpExecutableFinder();
diff --git a/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php b/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php
index 69593a269e..74c8bf957c 100644
--- a/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php
+++ b/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php
@@ -56,7 +56,7 @@ public function testTestingThroughUI() {
         "tests[$test]" => TRUE,
       ];
       $this->drupalPostForm($url, $edit, t('Run tests'));
-      $assertion->pageTextContains('0 fails, 0 skipped, 0 exceptions');
+      $assertion->pageTextContains('0 fails, 0 risky, 0 skipped, 0 incomplete, 0 exceptions');
     }
   }