The "BlueHat Prize" contest, which debuted in August 2011, as a first prize, $50,000 for second, and a subscription to Microsoft's developer network for third place. The three winners will be flown to Las Vegas this July, when Microsoft will announce the results at the Black Hat security conference.

Microsoft collected 20 entries before the April 1 deadline, said Katie Moussouris, a senior security strategist lead at Microsoft, on a yesterday.

Between now and Black Hat -- which runs July 21-24 -- Microsoft will evaluate the submissions and pick winners, Moussouris said.

BlueHat Prize was not a bug bounty system, where vulnerability experts are rewarded for uncovering specific flaws in software -- but instead was designed to prod researchers to invent novel technologies that would protect Windows from entire classes of memory bugs.

When Microsoft rolled out BlueHat Prize last year, some experts assumed that the company was after a technology or technique to defeat or at least deflect exploits of "return-oriented programming," or ROP vulnerabilities.