diff --git a/phpBB/phpbb/update/controller.php b/phpBB/phpbb/update/controller.php
index 45d11d2f0c..cdbc53495b 100644
--- a/phpBB/phpbb/update/controller.php
+++ b/phpBB/phpbb/update/controller.php
@@ -53,7 +53,7 @@ class controller
 	/**
 	 * Handle requests.
 	 *
-	 * @param string $download	The download URL.
+	 * @param string $download The download URL.
 	 *
 	 * @return string[] Unencoded json response.
 	 */
@@ -61,65 +61,51 @@ class controller
 	{
 		$update_path = $this->phpbb_root_path . 'store/update.zip';
 		$status = ['status' => 'continue'];
-		if (!file_exists($update_path))
+		if (!$this->filesystem->exists($update_path))
 		{
 			$result = $this->updater->download($download, $update_path);
 			if (!$result)
 			{
-				return [
-					'status' => 'error',
-					'error' => $this->language->lang('UPDATE_PACKAGE_DOWNLOAD_FAILURE')
-				];
+				return $this->error_response('UPDATE_PACKAGE_DOWNLOAD_FAILURE');
 			}
 
 			return $status;
 		}
 
-		if (!file_exists($update_path . '.sig'))
+		if (!$this->filesystem->exists($update_path . '.sig'))
 		{
 			$result = $this->updater->download($download . '.sig', $update_path . '.sig');
 			if (!$result)
 			{
-				return [
-					'status' => 'error',
-					'error' => $this->language->lang('UPDATE_SIGNATURE_DOWNLOAD_FAILURE')
-				];
+				return $this->error_response('UPDATE_SIGNATURE_DOWNLOAD_FAILURE');
 			}
+
 			return $status;
 		}
 
-		if (!is_dir($this->phpbb_root_path . 'store/update'))
+		if (!$this->filesystem->exists($this->phpbb_root_path . 'store/update') || !is_dir($this->phpbb_root_path . 'store/update'))
 		{
 			$result = $this->updater->validate($update_path, $update_path . '.sig');
 			if (!$result)
 			{
-				return [
-					'status' => 'error',
-					'error' => $this->language->lang('UPDATE_SIGNATURE_INVALID')
-				];
+				return $this->error_response('UPDATE_SIGNATURE_INVALID');
 			}
 
 			$result = $this->updater->extract($update_path, $this->phpbb_root_path . 'store/update');
 			if (!$result)
 			{
-				return [
-					'status' => 'error',
-					'error' => $this->language->lang('UPDATE_PACKAGE_EXTRACT_FAILURE')
-				];
+				return $this->error_response('UPDATE_PACKAGE_EXTRACT_FAILURE');
 			}
 
 			return $status;
 		}
 
-		if (!is_dir($this->phpbb_root_path . 'install'))
+		if (!$this->filesystem->exists($this->phpbb_root_path . 'install') || !is_dir($this->phpbb_root_path . 'install'))
 		{
 			$result = $this->updater->copy($this->phpbb_root_path . 'store/update');
 			if (!$result)
 			{
-				return [
-					'status' => 'error',
-					'error' => $this->language->lang('UPDATE_FILES_COPY_FAILURE')
-				];
+				return $this->error_response('UPDATE_FILES_COPY_FAILURE');
 			}
 
 			return $status;
@@ -134,4 +120,18 @@ class controller
 		$status['status'] = 'done';
 		return $status;
 	}
+
+	/**
+	 * Create error response
+	 *
+	 * @param string $error_key
+	 * @return array Error response
+	 */
+	protected function error_response(string $error_key): array
+	{
+		return [
+			'status' => 'error',
+			'error' => $this->language->lang($error_key),
+		];
+	}
 }
diff --git a/tests/update/controller_test.php b/tests/update/controller_test.php
new file mode 100644
index 0000000000..bba0186b7f
--- /dev/null
+++ b/tests/update/controller_test.php
@@ -0,0 +1,351 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+use phpbb\update\controller;
+use phpbb\update\get_updates;
+use phpbb\filesystem\filesystem;
+use phpbb\language\language;
+
+class phpbb_update_controller_test extends \phpbb_test_case
+{
+	private $filesystem;
+	private $filesystem_mock;
+	private $updater_mock;
+	private $language_mock;
+	private $phpbb_root_path;
+
+	protected function setUp(): void
+	{
+		global $phpbb_root_path;
+
+		$this->filesystem = new filesystem();
+		$this->filesystem_mock = $this->createMock(filesystem::class);
+		$this->updater_mock = $this->createMock(get_updates::class);
+		$this->language_mock = $this->createMock(language::class);
+		$this->phpbb_root_path = $phpbb_root_path;
+	}
+
+	protected function tearDown(): void
+	{
+		$this->filesystem->remove([
+			$this->phpbb_root_path . 'store/update.zip',
+			$this->phpbb_root_path . 'store/update.zip.sig',
+			$this->phpbb_root_path . 'store/update',
+		]);
+	}
+
+	public function test_download_fails(): void
+	{
+		$this->updater_mock->expects($this->once())
+			->method('download')
+			->willReturn(false);
+
+		$this->language_mock->expects($this->once())
+			->method('lang')
+			->with('UPDATE_PACKAGE_DOWNLOAD_FAILURE')
+			->willReturnArgument(0);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'error', 'error' => 'UPDATE_PACKAGE_DOWNLOAD_FAILURE'], $response);
+	}
+
+	public function test_download_success(): void
+	{
+		$this->updater_mock->expects($this->once())
+			->method('download')
+			->willReturn(true);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'continue'], $response);
+	}
+
+	public function test_download_signature_fails(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', false],
+				[$this->phpbb_root_path . 'store/update', false],
+				[$this->phpbb_root_path . 'install', false],
+			]);
+
+		$this->updater_mock->expects($this->once())
+			->method('download')
+			->with('https://example.com/update.zip.sig', $update_path . '.sig')
+			->willReturn(false);
+
+		$this->language_mock->expects($this->once())
+			->method('lang')
+			->with('UPDATE_SIGNATURE_DOWNLOAD_FAILURE')
+			->willReturnArgument(0);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'error', 'error' => 'UPDATE_SIGNATURE_DOWNLOAD_FAILURE'], $response);
+	}
+
+	public function test_download_signature_success(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', false],
+				[$this->phpbb_root_path . 'store/update', false],
+				[$this->phpbb_root_path . 'install', false],
+			]);
+
+		$this->updater_mock->expects($this->once())
+			->method('download')
+			->with('https://example.com/update.zip.sig', $update_path . '.sig')
+			->willReturn(true);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'continue'], $response);
+	}
+
+	public function test_signature_validation_fails(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', true],
+				[$this->phpbb_root_path . 'store/update', false],
+				[$this->phpbb_root_path . 'install', false],
+			]);
+
+		$this->updater_mock->expects($this->once())
+			->method('validate')
+			->willReturn(false);
+
+		$this->language_mock->expects($this->once())
+			->method('lang')
+			->with('UPDATE_SIGNATURE_INVALID')
+			->willReturnArgument(0);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'error', 'error' => 'UPDATE_SIGNATURE_INVALID'], $response);
+	}
+
+	public function test_extract_fails(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', true],
+				[$this->phpbb_root_path . 'store/update', false],
+				[$this->phpbb_root_path . 'install', false],
+			]);
+
+		$this->updater_mock->expects($this->once())
+			->method('validate')
+			->willReturn(true);
+
+		$this->updater_mock->expects($this->once())
+			->method('extract')
+			->willReturn(false);
+
+		$this->language_mock->expects($this->once())
+			->method('lang')
+			->with('UPDATE_PACKAGE_EXTRACT_FAILURE')
+			->willReturnArgument(0);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'error', 'error' => 'UPDATE_PACKAGE_EXTRACT_FAILURE'], $response);
+	}
+
+	public function test_extract_success(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', true],
+				[$this->phpbb_root_path . 'store/update', false],
+				[$this->phpbb_root_path . 'install', false],
+			]);
+
+		$this->updater_mock->expects($this->once())
+			->method('validate')
+			->willReturn(true);
+
+		$this->updater_mock->expects($this->once())
+			->method('extract')
+			->willReturn(true);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'continue'], $response);
+	}
+
+	public function test_copy_fails(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+		$this->filesystem->touch($update_path); // Simulate existing update file
+		$this->filesystem->touch($update_path . '.sig'); // Simulate existing signature file
+		$this->filesystem->mkdir($this->phpbb_root_path . 'store/update');
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', true],
+				[$this->phpbb_root_path . 'store/update', true],
+				[$this->phpbb_root_path . 'install', false],
+			]);
+
+		$this->updater_mock->expects($this->once())
+			->method('copy')
+			->willReturn(false);
+
+		$this->language_mock->expects($this->once())
+			->method('lang')
+			->with('UPDATE_FILES_COPY_FAILURE')
+			->willReturnArgument(0);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'error', 'error' => 'UPDATE_FILES_COPY_FAILURE'], $response);
+	}
+
+	public function test_copy_success(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+		$this->filesystem->touch($update_path); // Simulate existing update file
+		$this->filesystem->touch($update_path . '.sig'); // Simulate existing signature file
+		$this->filesystem->mkdir($this->phpbb_root_path . 'store/update');
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', true],
+				[$this->phpbb_root_path . 'store/update', true],
+				[$this->phpbb_root_path . 'install', false],
+			]);
+
+		$this->updater_mock->expects($this->once())
+			->method('copy')
+			->willReturn(true);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'continue'], $response);
+	}
+
+	public function test_successful_update_process(): void
+	{
+		$update_path = $this->phpbb_root_path . 'store/update.zip';
+		$signature_path = $update_path . '.sig';
+		$update_dir = $this->phpbb_root_path . 'store/update';
+
+		$this->filesystem->touch($update_path);
+		$this->filesystem->touch($signature_path);
+		$this->filesystem->mkdir($update_dir);
+
+		$this->filesystem_mock->expects($this->any())
+			->method('exists')
+			->willReturnMap([
+				[$update_path, true],
+				[$update_path . '.sig', true],
+				[$this->phpbb_root_path . 'store/update', true],
+				[$this->phpbb_root_path . 'install', true],
+			]);
+
+		$this->filesystem_mock->expects($this->once())
+			->method('remove')
+			->with([$update_dir, $update_path, $signature_path]);
+
+		$controller = new controller(
+			$this->filesystem_mock,
+			$this->updater_mock,
+			$this->language_mock,
+			$this->phpbb_root_path
+		);
+
+		$response = $controller->handle('https://example.com/update.zip');
+		$this->assertEquals(['status' => 'done'], $response);
+	}
+}
\ No newline at end of file