Google researchers have found a severe flaw in an obsolete but still used encryption software, which could be exploited to steal sensitive data—and the fix could break the Web for users of older web browsers.

The flaw in SSL 3.0 is more than 15 years old but is still used by modern web browsers and servers. SSL stands for “Secure Sockets Layer,” which encrypts data between a client and server and secures most data sent over the Internet.

Bodo Möller, Thai Duong and Krzysztof Kotowicz of Google developed an attack called “POODLE,” which stands for Padding Oracle On Downgraded Legacy Encryption, according to their research paper.

Web browsers are designed to use newer versions of SSL or TLS (Transport Layer Security), but most browsers will accommodate SSL 3.0 if that’s all that a server can do on the other end.

The POODLE attack can force a connection to “fallback” to SSL 3.0, where it is then possible to steal cookies, which are small data files that enable persistent access to an online service. If stolen, a cookie could allow an attacker access to someone’s Web-based email account, for example.

An attacker would have to control the network a victim is connected to in order to conduct this kind of man-in-the-middle attack. That might be possible in a public area, such as over a Wi-Fi network in an airport.

Legacy concerns

Security experts have long known SSL 3.0 was problematic. Matthew Green, a cryptographer and research professor at Johns Hopkins University, wrote on his blog that many servers still support SSL 3.0 since they didn’t want to lockout users of Internet Explorer 6, a very dated but still used browser.

“The problem with the obvious solution is that our aging Internet infrastructure is still loaded with crappy browsers and servers that can’t function without SSLv3 support,” Green wrote.

“Browser vendors don’t want their customers to hit a blank wall anytime they access a server or load balancer that only supports SSLv3, so they enable fallback,” he wrote.

Google has already taken steps to stop encrypted connections from being made using less secure versions of SSL and TLS.

Adam Langley, who works on Google’s Chrome browser, wrote on his blog that connections made using Chrome to Google’s infrastructure are using a mechanism called “TLS_FALLBACK_SCSV”, which prevents downgrading.

“We are urging server operators and other browsers to implement it too,” Langley wrote. “It doesn’t just protect against this specific attack, it solves the fallback problem in general.”

Google is preparing a patch for Chrome that would forbid falling back to SSL 3.0 for all servers, but “this change will break things and so we don’t feel that we can jump it straight to Chrome’s stable channel. But we do hope to get it there within weeks and so buggy servers that currently function only because of SSL 3.0 fallback will need to be updated.”

Major internet companies are already making adjustments to prevent a POODLE attack. CloudFlare, which has a widely used caching service, has disabled SSL 3.0 across its network by default for all of its customers, wrote CEO Matthew Prince.

“This will have an impact on some older browsers, resulting in an SSL connection error,” Prince wrote. “The biggest impact is Internet Explorer 6 running on Windows XP or older.”

Prince wrote that just 0.65 percent of the HTTPS encrypted traffic on CloudFlare’s network uses SSL 3.0. “The good news is most of that traffic is actually attack traffic and some minor crawlers,” he wrote.