diff --git a/core/tests/Drupal/BuildTests/Framework/BuildTestBase.php b/core/tests/Drupal/BuildTests/Framework/BuildTestBase.php
index cebae20f0b..ea477fa4a9 100644
--- a/core/tests/Drupal/BuildTests/Framework/BuildTestBase.php
+++ b/core/tests/Drupal/BuildTests/Framework/BuildTestBase.php
@@ -322,11 +322,13 @@ protected function instantiateServer($hostname, $port, $docroot = NULL) {
       ->start();
     // Wait until the web server has started. It is started if the port is no
     // longer available.
-    while ($available_port = $this->findAvailablePort($port)) {
-      if ($available_port !== $port) {
+    for ($i = 0; $i < 1000; $i++) {
+      if (!$this->checkPortIsAvailable($port)) {
         return $ps;
       }
+      usleep(1000);
     }
+    throw new \RuntimeException('Unable to start the web server.');
   }
 
   /**
@@ -345,37 +347,49 @@ protected function stopServer() {
   /**
    * Discover an available port number.
    *
-   * @param int $min
-   *   The lowest port number to start with.
-   * @param int $max
-   *   The highest port number to allow.
-   *
    * @return int
    *   The available port number that we discovered.
    *
    * @throws \RuntimeException
    *   Thrown when there are no available ports within the range.
    */
-  protected function findAvailablePort($min = 8000, $max = 8100) {
-    // If we can open the port, then it's unavailable to us.
-    for ($port = $min; $port <= $max; $port++) {
-      if ($this->lockAcquired($port)) {
-        $fp = @fsockopen(static::$hostName, $port, $errno, $errstr, 1);
-        // If fsockopen() fails to connect, probably nothing is listening.
-        // It could be a firewall but that's impossible to detect, so as a
-        // best guess let's return it as available.
-        if ($fp === FALSE) {
-          return $port;
-        }
-        else {
-          $this->lockRelease($port);
-          fclose($fp);
-        }
+  protected function findAvailablePort() {
+    $counter = 100;
+    while ($counter--) {
+      $port = unpack('n', random_bytes(2))[1] % 64511 + 1024;
+      if ($this->checkPortIsAvailable($port)) {
+        return $port;
       }
     }
     throw new \RuntimeException('Unable to find a port available to run the web server.');
   }
 
+  /**
+   * Checks whether a port is available.
+   *
+   * @param $port
+   *   A number between 1024 and 65536.
+   *
+   * @return bool
+   */
+  protected function checkPortIsAvailable($port) {
+    if ($this->lockAcquired($port)) {
+      $fp = @fsockopen(static::$hostName, $port, $errno, $errstr, 1);
+      // If fsockopen() fails to connect, probably nothing is listening.
+      // It could be a firewall but that's impossible to detect, so as a
+      // best guess let's return it as available.
+      if ($fp === FALSE) {
+        return TRUE;
+      }
+      else {
+        $this->lockRelease($port);
+        fclose($fp);
+      }
+    }
+    return FALSE;
+  }
+
+
   /**
    * Get the port number for requests.
    *
diff --git a/core/tests/Drupal/BuildTests/Framework/Tests/BuildTestTest.php b/core/tests/Drupal/BuildTests/Framework/Tests/BuildTestTest.php
index 99ae014de6..4b21421250 100644
--- a/core/tests/Drupal/BuildTests/Framework/Tests/BuildTestTest.php
+++ b/core/tests/Drupal/BuildTests/Framework/Tests/BuildTestTest.php
@@ -126,26 +126,6 @@ public function testCopyCodebaseExclude() {
     $base->tearDown();
   }
 
-  /**
-   * @covers ::findAvailablePort
-   */
-  public function testPort() {
-    $iterator = (new Finder())->in($this->getDrupalRoot())
-      ->ignoreDotFiles(FALSE)
-      ->path('/^.ht.router.php$/')
-      ->exclude(['sites/simpletest'])
-      ->getIterator();
-    $this->copyCodebase($iterator);
-    // We should find the same port twice in a row, since we don't use it.
-    $this->assertEquals($this->findAvailablePort(), $this->findAvailablePort());
-
-    // Grab a port, make a server, make sure the next port we grab is not the
-    // same.
-    $port = $this->findAvailablePort();
-    $this->instantiateServer(static::$hostName, $port);
-    $this->assertNotEquals($port, $this->findAvailablePort());
-  }
-
   /**
    * @covers ::findAvailablePort
    */
@@ -165,6 +145,7 @@ public function testPortMany() {
       $processes[$port] = $this->instantiateServer(static::$hostName, $port);
       $this->assertNotEmpty($processes[$port]);
       $this->assertTrue($processes[$port] ->isRunning(), 'Process on port ' . $port . ' is not still running.');
+      $this->assertFalse($this->checkPortIsAvailable($port));
     }
   }