Invoice Ninja before 5.13.28 contains a broken access control vulnerability that allows authenticated vendor-portal contacts to access documents belonging to other vendors and clients by exploiting authorization checks that validate only company_id scope instead of vendor ownership. Attackers can request GET /vendor/documents/{hashid}/download or POST /vendor/documents/download_multiple, enumerating predictable sequential Hashids document identifiers due to a default empty HASH_SALT, to read any invoice PDFs, contracts, payment receipts, and expense attachments within the company.