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();
}

$util = new \OCA\Encryption\Util(new \OC_FilesystemView('/'), $username);
$recoveryAdminEnabled = OC_Appconfig::getValue( 'files_encryption', 'recoveryAdminEnabled' );
$recoveryEnabledForUser = $util->recoveryEnabledForUser();

if ($recoveryAdminEnabled && $recoveryEnabledForUser && $recoveryPassword == '') {
	OC_JSON::error(array("data" => array( "message" => "Please provide a admin recovery password, otherwise all user data will be lost" )));
}elseif ( $recoveryPassword && ! $util->checkRecoveryPassword($recoveryPassword) ) {
	OC_JSON::error(array("data" => array( "message" => "Wrong admin recovery password. Please check the password and try again." )));
}elseif(!is_null($password) && OC_User::setPassword( $username, $password, $recoveryPassword )) {
	OC_JSON::success(array("data" => array( "username" => $username )));
}
else{
	OC_JSON::error(array("data" => array( "message" => "Unable to change password" )));
}