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IDG News Service - A well-known cryptographic attack could be used by hackers to log into Web applications used by millions of users, according to two security experts who plan to discuss the issue at an upcoming security conference.

Researchers Nate Lawson and Taylor Nelson say they've discovered a basic security flaw that affects dozens of open-source software libraries -- including those used by software that implements the OAuth and OpenID standards -- that are used to check passwords and user names when people log into websites. OAuth and OpenID authentication are accepted by popular Web sites such as Twitter and Digg.

They found that some versions of these login systems are vulnerable to what's known as a timing attack. Cryptographers have known about timing attacks for 25 years, but they are generally thought to be very hard to pull off over a network. The researchers aim to show that's not the case.

The attack is thought to be so difficult because it requires very precise measurements. It cracks authentication tokens by measuring the time it takes for a computer to verify a digital signature. On some systems, the server will check a cryptographic signature on a token sent by the user to prove that he has logged into the system. It will kick back an error message as soon as it spots a bad character. This means a computer returns an error for a completely bad token a tiny bit faster than one where the first character is correct.

By submitting signatures again and again, cycling through characters and measuring the time it takes for the computer to respond, hackers can ultimately figure out the correct digital signature.

The attack lets someone masquerade as a legitimate Web site user without actually having to log in.

This all sounds very theoretical, but timing attacks can actually succeed in the real world. Three years ago, one was used to hack Microsoft's Xbox 360 gaming system, and people who build smart cards have added timing attack protection for years.

But Internet developers have long assumed that there are too many other factors -- called network jitter -- that slow down or speed up response times and make it almost impossible to get the kind of precise results, where nanoseconds make a difference, required for a successful timing attack.

Those assumptions are wrong, according to Lawson, founder of the security consultancy Root Labs. He and Nelson tested attacks over the Internet, local-area networks and in cloud computing environments and found they were able to crack passwords in all the environments by using algorithms to weed out the network jitter.