Virtual DOS machine
In January 2010, Google security researcher Tavis Ormandy discovered a large security problem in NTVDM system letting non-Administrators to increase their permissions to the SYSTEM level. Ormandy claimed it was a problem for all x86 versions of Windows NT after 1993. 32-bit versions of NT 3.X/4.0, 2000, XP, 2003, Vista, 2008, and 7 were affected. Ormandy published how the problem works. Before Microsoft could fix it, you could turn off 16-bit application support to solve the problem, but it made so you could not run 16-bit programs. x64 Windows never had this problem because it doesn't use NTVDM. After applying the security fixes Microsoft made available, NTVDM could be turned on safely.