Microsoft has revealed a vulnerability in Microsoft Windows Kerberos KDC. It affects all modern versions of Windows, including Windows Server 2003/2008/2012, Vista, 7, 8, 8.1.
Please refer to Microsoft TechNet article for details:
Microsoft has revealed a vulnerability in Microsoft Windows Kerberos KDC that could allow an attacker to elevate unprivileged domain user account privileges to those of the domain administrator account. An attacker could use these elevated privileges to compromise any computer in the domain, including domain controllers. An attacker must have valid domain credentials to exploit this vulnerability. The affected component is available remotely to users who have standard user accounts with domain credentials; this is not the case for users with local account credentials only. When the security bulletin was issued, Microsoft was aware of limited, targeted attacks that attempt to exploit this vulnerability.
This security update is rated Critical for all supported editions of Windows Server 2003, Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, and Windows Server 2012 R2.
To close the vulnerability on Windows-based hosts install the security patch from Windows Update.
For specific Parallels products, here is the list of articles which you may refer to: