Como não posso comentar a resposta do PilotBobs:
Este é por design e não representa um risco de segurança, como eu entendo o recurso.
Há um artigo de technet que descreve como essa coisa funciona :
When any folder or file within a user’s profile is shared, the Windows sharing engine creates a single share to facilitate sharing out that content. This helps facilitates several sharing scenarios. It facilitates scenarios which allow the sharing of individual files while also reducing the management overhead for creating and maintaining multiple shares on the computer. It also allows standard users on that same computer to share their own content without the need to create additional shares, which requires UAC elevation.
Additionally, both Windows Vista and Windows 7 use a Server Message Block (SMB) feature called Access-based Enumeration to limit what content is shown to others when they access this share. With the Access-based Enumeration feature enabled, users accessing the share are only shown the content they have access to over SMB and everything else is hidden from them.
Eu acho esse recurso realmente muito legal porque você pode compartilhar arquivos únicos e, além disso, você mantém a estrutura de pastas.