diff --git a/tests/phpunit/tests/admin/includes/plugin/validateActivePlugins.php b/tests/phpunit/tests/admin/includes/plugin/validateActivePlugins.php
new file mode 100644
index 0000000000000..a0470bfb56688
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/plugin/validateActivePlugins.php
@@ -0,0 +1,199 @@
+<?php
+
+/**
+ * Tests for validate_active_plugins().
+ *
+ * @group admin
+ * @group plugins
+ *
+ * @covers ::validate_active_plugins
+ * @covers ::validate_plugin
+ */
+class Tests_Admin_Includes_Plugin_ValidateActivePlugins extends WP_UnitTestCase {
+
+	/**
+	 * The directory of the test plugin without a header.
+	 *
+	 * @var string
+	 */
+	const NO_HEADER_DIR = 'validate-test-no-header';
+
+	/**
+	 * Super admin user ID.
+	 *
+	 * @var int
+	 */
+	private static $super_admin_id;
+
+	/**
+	 * Sets up the user.
+	 *
+	 * @param WP_UnitTest_Factory $factory Test factory.
+	 */
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$super_admin_id = $factory->user->create( array( 'role' => 'administrator' ) );
+
+		if ( is_multisite() ) {
+			grant_super_admin( self::$super_admin_id );
+		}
+	}
+
+	/**
+	 * Revokes the super admin grant before the fixture users are deleted.
+	 */
+	public static function wpTearDownAfterClass() {
+		if ( is_multisite() ) {
+			revoke_super_admin( self::$super_admin_id );
+		}
+	}
+
+	/**
+	 * Creates a plugin file without a plugin header and resets the active plugins.
+	 */
+	public function set_up() {
+		parent::set_up();
+
+		$this->remove_plugin();
+
+		$dir = WP_PLUGIN_DIR . '/' . self::NO_HEADER_DIR;
+		$this->assertTrue( mkdir( $dir ), 'The test directory should be created.' );
+		$this->assertNotFalse( file_put_contents( $dir . '/functions.php', "<?php\n// No plugin header.\n" ), 'The file should be written.' );
+
+		wp_cache_delete( 'plugins', 'plugins' );
+		update_site_option( 'active_sitewide_plugins', array() );
+	}
+
+	/**
+	 * Removes the plugin file without a header.
+	 */
+	public function tear_down() {
+		$this->remove_plugin();
+
+		wp_cache_delete( 'plugins', 'plugins' );
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Tests that an active plugins option that is not an array is reset.
+	 *
+	 * @ticket 65819
+	 *
+	 * @dataProvider data_invalid_options
+	 *
+	 * @param mixed $value The option value.
+	 */
+	public function test_should_reset_option_that_is_not_an_array( $value ) {
+		update_option( 'active_plugins', $value );
+
+		$this->assertSame( array(), validate_active_plugins(), 'No invalid plugins should be returned.' );
+		$this->assertSame( array(), get_option( 'active_plugins' ), 'The option should be reset.' );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public static function data_invalid_options() {
+		return array(
+			'string'  => array( 'hello.php' ),
+			'integer' => array( 1 ),
+		);
+	}
+
+	/**
+	 * Tests that valid active plugins are kept.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_keep_valid_plugins() {
+		update_option( 'active_plugins', array( 'hello.php', 'internationalized-plugin.php' ) );
+
+		$this->assertSame( array(), validate_active_plugins(), 'No invalid plugins should be returned.' );
+		$this->assertSame( array( 'hello.php', 'internationalized-plugin.php' ), get_option( 'active_plugins' ), 'The plugins should stay active.' );
+	}
+
+	/**
+	 * Tests that invalid active plugins are reported and deactivated silently.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_should_report_and_deactivate_invalid_plugins() {
+		$no_header = self::NO_HEADER_DIR . '/functions.php';
+		update_option( 'active_plugins', array( 'hello.php', '../outside.php', 'missing/missing.php', $no_header ) );
+
+		$deactivate = new MockAction();
+		add_action( 'deactivate_plugin', array( $deactivate, 'action' ) );
+
+		$invalid = validate_active_plugins();
+
+		$this->assertSame( array( '../outside.php', 'missing/missing.php', $no_header ), array_keys( $invalid ), 'Each invalid plugin should be reported.' );
+		$this->assertSame(
+			array(
+				'../outside.php'      => array( 'plugin_invalid', 'Invalid plugin path.' ),
+				'missing/missing.php' => array( 'plugin_not_found', 'Plugin file does not exist.' ),
+				$no_header            => array( 'no_plugin_header', 'The plugin does not have a valid header.' ),
+			),
+			array_map(
+				static function ( $error ) {
+					return array( $error->get_error_code(), $error->get_error_message() );
+				},
+				$invalid
+			),
+			'Each invalid plugin should have its error.'
+		);
+		$this->assertSame( array( 'hello.php' ), array_values( get_option( 'active_plugins' ) ), 'Only the valid plugin should stay active.' );
+		$this->assertSame( 0, $deactivate->get_call_count(), 'Invalid plugins should be deactivated silently.' );
+	}
+
+	/**
+	 * Tests that network-active plugins are validated for a user who can manage network plugins.
+	 *
+	 * @ticket 65819
+	 *
+	 * @group ms-required
+	 */
+	public function test_should_validate_network_plugins_for_network_admin() {
+		wp_set_current_user( self::$super_admin_id );
+
+		update_option( 'active_plugins', array() );
+		update_site_option(
+			'active_sitewide_plugins',
+			array(
+				'hello.php'           => 1000,
+				'missing/missing.php' => 1000,
+			)
+		);
+
+		$this->assertSame( array( 'missing/missing.php' ), array_keys( validate_active_plugins() ), 'The invalid network plugin should be reported.' );
+		$this->assertSame( array( 'hello.php' => 1000 ), get_site_option( 'active_sitewide_plugins' ), 'The invalid network plugin should be deactivated.' );
+	}
+
+	/**
+	 * Tests that network-active plugins are not validated for other users.
+	 *
+	 * @ticket 65819
+	 *
+	 * @group ms-required
+	 */
+	public function test_should_not_validate_network_plugins_for_other_users() {
+		update_option( 'active_plugins', array() );
+		update_site_option( 'active_sitewide_plugins', array( 'missing/missing.php' => 1000 ) );
+
+		$this->assertSame( array(), validate_active_plugins(), 'No plugins should be validated.' );
+		$this->assertSame( array( 'missing/missing.php' => 1000 ), get_site_option( 'active_sitewide_plugins' ), 'The network plugins should not change.' );
+	}
+
+	/**
+	 * Removes the plugin file without a header, including one left over by an interrupted run.
+	 */
+	private function remove_plugin() {
+		$dir = WP_PLUGIN_DIR . '/' . self::NO_HEADER_DIR;
+
+		if ( is_dir( $dir ) ) {
+			$this->rmdir( $dir );
+			rmdir( $dir );
+		}
+	}
+}