Summary

If a password stored with password_hash starts with a null byte (\x00), testing a blank string as the password via password_verify will incorrectly return true.

If a user were able to create a password with a leading null byte (unlikely, but syntactically valid), an attacker could trivially compromise the victim's account by attempting to sign in with a blank string.

Details

$ php -v
PHP 8.3.2 (cli) (built: Jan 16 2024 13:46:41) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.3.2, Copyright (c) Zend Technologies
    with Xdebug v3.3.0, Copyright (c) 2002-2023, by Derick Rethans
    with Zend OPcache v8.3.2, Copyright (c), by Zend Technologies

This appears to exist at least back to 8.1.0: https://3v4l.org/Z0pcl

PoC

$ cat pw_bug.php