Try something like this:
SECURITY_ATTRIBUTES lpSecurityAttributes = new SECURITY_ATTRIBUTES();
DirectorySecurity security = new DirectorySecurity();
lpSecurityAttributes.nLength = Marshal.SizeOf(lpSecurityAttributes);
byte[] src = security.GetSecurityDescriptorBinaryForm();
IntPtr dest = Marshal.AllocHGlobal(src.Length);
Marshal.Copy(src, 0, dest, src.Length);
lpSecurityAttributes.lpSecurityDescriptor = dest;
Found the code in the article
Manipulating NTFS Junction Points in .NET[
^] Its a good read and may just do the trick for you.
Hogan