diff --git a/src/Process/HostProcessFactory.php b/src/Process/HostProcessFactory.php
index 7271f27e..af7196d8 100644
--- a/src/Process/HostProcessFactory.php
+++ b/src/Process/HostProcessFactory.php
@@ -9,6 +9,11 @@
 
 final class HostProcessFactory implements ProcessFactory
 {
+    /**
+     * Inherited env vars that are kept, everything else is unset so we don't leak secrets to the process
+     */
+    private const ALLOWED_ENV = ['PATH', 'HOME', 'COMPOSER_HOME'];
+
     private ExecutableFinder $executableFinder;
 
     public function __construct(?ExecutableFinder $executableFinder = null)
@@ -28,19 +33,27 @@ public function create(ProcessInput $processInput): Process
         return new Process(
             [$executablePath, ...$processInput->getArgs()],
             $processInput->getWorkingDirectory(),
-            $this->getDefaultEnv() + $processInput->getEnv(),
+            array_merge($this->getDefaultEnv(), $processInput->getEnv()),
             $processInput->getInput(),
             10,
         );
     }
 
     /**
-     * @return array<false>
+     * Symfony Process inherits the parent env, so we unset every inherited var by setting it to false.
+     * We read from getenv() rather than $_ENV, as $_ENV is empty when variables_order does not include E.
+     *
+     * @return array<string, false>
      */
     private function getDefaultEnv(): array
     {
-        $env = array_map(fn() => false, $_ENV);
-        $env + array_map(fn() => false, $_SERVER);
+        $env = [];
+
+        foreach (array_keys(getenv() + $_ENV) as $name) {
+            if (!in_array($name, self::ALLOWED_ENV, true)) {
+                $env[(string) $name] = false;
+            }
+        }
 
         return $env;
     }
diff --git a/test/Process/HostProcessFactoryTest.php b/test/Process/HostProcessFactoryTest.php
index e2172e59..8e4c6cce 100644
--- a/test/Process/HostProcessFactoryTest.php
+++ b/test/Process/HostProcessFactoryTest.php
@@ -68,7 +68,23 @@ public function testCreateWithEnv(): void
         $input = new ProcessInput('composer', ['one', 'two'], __DIR__, ['SOME_VAR' => 'value']);
 
         $process = $factory->create($input);
-        static::assertSame(['SOME_VAR' => 'value'], $process->getEnv());
+        static::assertSame('value', $process->getEnv()['SOME_VAR']);
+    }
+
+    public function testInheritedEnvIsRemovedExceptAllowed(): void
+    {
+        putenv('PHPSCHOOL_SECRET=secret');
+
+        try {
+            $process = (new HostProcessFactory())->create(
+                new ProcessInput('php', ['-r', 'echo json_encode([getenv("PHPSCHOOL_SECRET"), getenv("PATH") !== false]);'], __DIR__, []),
+            );
+            $process->mustRun();
+
+            static::assertSame('[false,true]', $process->getOutput());
+        } finally {
+            putenv('PHPSCHOOL_SECRET');
+        }
     }
 
     public function testWithInput(): void