nextcloud/settings/ajax/changepassword.php

<?php

// Check if we are a user
OCP\JSON::callCheck();
OC_JSON::checkLoggedIn();

// Manually load apps to ensure hooks work correctly (workaround for issue 1503)
OC_APP::loadApps();

$username = isset($_POST['username']) ? $_POST['username'] : OC_User::getUser();
$password = isset($_POST['password']) ? $_POST['password'] : null;
$oldPassword = isset($_POST['oldpassword']) ? $_POST['oldpassword'] : '';
$recoveryPassword = isset($_POST['recoveryPassword']) ? $_POST['recoveryPassword'] : null;

$userstatus = null;
if (OC_User::isAdminUser(OC_User::getUser())) {
	$userstatus = 'admin';
}
if (OC_SubAdmin::isUserAccessible(OC_User::getUser(), $username)) {
	$userstatus = 'subadmin';
}
if (OC_User::getUser() === $username && OC_User::checkPassword($username, $oldPassword)) {
	$userstatus = 'user';
}

if (is_null($userstatus)) {
	OC_JSON::error(array('data' => array('message' => 'Authentication error')));
	exit();
}

if (\OCP\App::isEnabled('files_encryption') && $userstatus !== 'user') {
	//handle the recovery case
	$util = new \OCA\Encryption\Util(new \OC_FilesystemView('/'), $username);
	$recoveryAdminEnabled = OC_Appconfig::getValue('files_encryption', 'recoveryAdminEnabled');

	$validRecoveryPassword = false;
	$recoveryPasswordSupported = false;
	if ($recoveryAdminEnabled) {
		$validRecoveryPassword = $util->checkRecoveryPassword($recoveryPassword);
		$recoveryEnabledForUser = $util->recoveryEnabledForUser();
	}

	if ($recoveryEnabledForUser && $recoveryPassword === '') {
		OC_JSON::error(array('data' => array('message' => 'Please provide a admin recovery password, otherwise all user data will be lost')));
	} elseif ($recoveryEnabledForUser && ! $validRecoveryPassword) {
		OC_JSON::error(array('data' => array('message' => 'Wrong admin recovery password. Please check the password and try again.')));
	} else { // now we know that everything is fine regarding the recovery password, let's try to change the password
	$result = OC_User::setPassword($username, $password, $recoveryPassword);
	if (!$result && $recoveryPasswordSupported) {
		OC_JSON::error(array("data" => array( "message" => "Back-end doesn't support password change, but the users encryption key was successfully updated." )));
	} elseif (!$result && !$recoveryPasswordSupported) {
		OC_JSON::error(array("data" => array( "message" => "Unable to change password" )));
	} else {
		OC_JSON::success(array("data" => array( "username" => $username )));
	}

	}
} else { // if user changes his own password or if encryption is disabled, proceed
	if (!is_null($password) && OC_User::setPassword($username, $password)) {
		OC_JSON::success(array('data' => array('username' => $username)));
	} else {
		OC_JSON::error(array('data' => array('message' => 'Unable to change password')));
	}
}