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Computerworld - Just days after a pair of researchers outwitted major Windows 7 defenses to exploit Internet Explorer and Firefox, Microsoft said the measures aren't meant to "prevent every attack forever."

At the same time, it defended the security measures, saying they remain an effective way to hinder exploits.

Pete LePage, a product manager in IE's developer division, stood up for DEP (data execution prevention) and ASLR (address space layout randomization), the security features that two hackers sidestepped to win $10,000 each at the high-profile Pwn2Own hacking contest last Wednesday.

"Defense-in-depth techniques aren't designed to prevent every attack forever, but to instead make it significantly harder to exploit a vulnerability," LePage said, referring to DEP, ASLR and another feature specific to IE, called Protected Mode.

DEP, which Microsoft introduced in 2004 with Windows XP SP2, is designed to prevent attack code from executing in memory not intended for code execution. ASLR, a feature that debuted with Windows Vista three years ago, randomly shuffles the positions of key memory areas, such as the stack, to make it more difficult for hackers to predict whether their attack code will run. Protected Mode, a sandbox-like technology in which IE runs with restricted rights, is designed to reduce the ability of attack code to "escape" from the browser to write, alter or delete data elsewhere on the PC.

All three antiexploit features are also found in Windows 7, Microsoft's newest operating system.

Peter Vreugdenhil, a freelance vulnerability researcher from the Netherlands, and a German researcher who only goes by his first name of Nils, each bypassed Windows 7's DEP and ASLR when they successfully attacked IE8 and Firefox 3.6, respectively, at the Pwn2Own hacking challenge.

LePage's comments Friday were the first from Microsoft on the DEP and ASLR circumventions since Pwn2Own concluded.

In a post to the Windows Security blog, LePage used the analogy of a fireproof safe, comparing Windows' security to the safe's ability to withstand fire and heat. "Without defense-in-depth techniques, a fireproof safe may only protect its contents for an hour or two," LePage said. "A stronger fireproof safe with several defense-in-depth features still won't guarantee the valuables forever, but adds significant time and protection to how long the contents will last."

Microsoft has made the point before that DEP, ASLR and other defensive measures don't guarantee that Windows can withstand attack. Last summer, for example, Robert Hensing, an engineer at the Microsoft Security Response Center, said that "DEP by itself is generally not a robust mitigation."

But in the same post to Microsoft's Security Research & Defense blog, Hensing also said that "DEP and ASLR used together are very difficult to bypass."

Microsoft's LePage said much the same last Friday. "Defense-in-depth features, including DEP and ASLR, continue to be highly effective protection mechanisms," he claimed.

Vreugdenhil, who exploited IE8 on Windows 7, didn't seem to think they were that difficult to overcome. "It took me six or seven days to get everything to work," he said last week in an interview after his Pwn2Own victory. Although he didn't reveal the two exploits he used to bypass DEP and ASLR and then hack Microsoft's browser -- by the rules of the contest, he's not allowed to talk in detail until the flaws are fixed -- Vreugdenhil posted a paper (download PDF) where he explained how he evaded Windows 7's protections.