diff --git a/phpBB/includes/acp/acp_extensions.php b/phpBB/includes/acp/acp_extensions.php
index aaac3b04fd..69a1063e4d 100644
--- a/phpBB/includes/acp/acp_extensions.php
+++ b/phpBB/includes/acp/acp_extensions.php
@@ -605,7 +605,7 @@ class acp_extensions
 
 					$this->config->set('exts_composer_enable_on_install', $enable_on_install);
 					$this->config->set('exts_composer_purge_on_remove', $purge_on_remove);
-					$this->config->set('exts_composer_repositories', serialize($repositories));
+					$this->config->set('exts_composer_repositories', json_encode($repositories, JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES));
 
 					if (!$this->config['exts_composer_packagist'] && $enable_packagist)
 					{
@@ -651,7 +651,7 @@ class acp_extensions
 					'enable_packagist' => $this->config['exts_composer_packagist'],
 					'enable_on_install' => $this->config['exts_composer_enable_on_install'],
 					'purge_on_remove' => $this->config['exts_composer_purge_on_remove'],
-					'repositories' => unserialize($this->config['exts_composer_repositories']),
+					'repositories' => json_decode($this->config['exts_composer_repositories'], true),
 				]);
 				$this->template->assign_var('enabled', $manager->check_requirements());
 
diff --git a/phpBB/phpbb/composer/installer.php b/phpBB/phpbb/composer/installer.php
index 9a02060316..9bfff31337 100644
--- a/phpBB/phpbb/composer/installer.php
+++ b/phpBB/phpbb/composer/installer.php
@@ -77,7 +77,7 @@ class installer
 	{
 		if ($config)
 		{
-			$repositories        = unserialize($config['exts_composer_repositories']);
+			$repositories        = json_decode($config['exts_composer_repositories'], true);
 
 			if (!is_array($repositories) && !empty($repositories))
 			{
@@ -606,7 +606,7 @@ class installer
 	 *
 	 * @param array $repositories An array of composer repositories to use
 	 */
-	public function set_repositories($repositories)
+	public function set_repositories(array $repositories)
 	{
 		$this->repositories = $repositories;
 	}
diff --git a/phpBB/phpbb/db/migration/data/v320/extensions_composer.php b/phpBB/phpbb/db/migration/data/v320/extensions_composer.php
index 71456afd49..438c412158 100644
--- a/phpBB/phpbb/db/migration/data/v320/extensions_composer.php
+++ b/phpBB/phpbb/db/migration/data/v320/extensions_composer.php
@@ -18,7 +18,7 @@ class extensions_composer extends \phpbb\db\migration\migration
 	public function update_data()
 	{
 		return array(
-			array('config.add', array('exts_composer_repositories', serialize([]))),
+			array('config.add', array('exts_composer_repositories', json_encode([], JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES))),
 			array('config.add', array('exts_composer_packagist', true)),
 			array('config.add', array('exts_composer_json_file', 'composer-ext.json')),
 			array('config.add', array('exts_composer_vendor_dir', 'vendor-ext/')),