HomeBox is a home inventory and organization system. Prior to 0.27.0, POST /api/v1/users/register and the users.email uniqueness constraint treat email addresses as case-sensitive while HomeBox uses a case-insensitive login lookup. An unauthenticated attacker who knows a victim's email can register a differently cased variant, creating duplicate rows that cause login to return 401 for both variants and deny the victim access, while also bypassing the one-account-per-email policy. This issue is fixed in version 0.27.0.