New encryption flaw, LogJam, puts Web surfers at risk

The flaw is closely related to FREAK and involves downgrading TLS connections to a weak key

Matthew D. Green, assistant research professor, Department of Computer Science, Johns Hopkins University

Matthew D. Green, assistant research professor, Department of Computer Science, Johns Hopkins University

Computer security experts said they've found a new encryption flaw closely related to one found earlier this year that puts Web surfers' data at risk.

The flaw, called LogJam, can allow an attacker to significantly weaken the encrypted connection between a user and a Web or email server, said Matthew D. Green, an assistant research professor in the department of computer science at Johns Hopkins University.

About 7 percent of websites on the Internet are vulnerable to LogJam along with many email servers. A website has been set up with more information.

Green was part of a team including experts from the University of Michigan and the French research institute Inria who found LogJam a few months ago.

Information on the flaw has been quietly circulating. Microsoft fixed its Internet Explorer browser last week, and patches for Firefox and Apple's Safari browser should be released soon, Green said.

LogJam's discovery came as a result of follow-up investigations into the FREAK (Factoring attack on RSA-EXPORT Keys) flaw, which was revealed in March.

Most websites and email servers use SSL (Secure Sockets Layer) or its successor, TLS (Transport Layer Security), which are protocols used to create an encrypted connection. SSL/TLS prevents clear-text data from being transmitted across the Web, which poses a high security risk.

In the 1990s for national security reasons, the U.S. government prevented companies from shipping software products that used strong encryption keys, which made it harder for an attacker to decode encrypted traffic. Software often shipped with 512-bit keys, which are considered very weak and can be cracked.

Although the export restrictions were lifted long ago, many products still support using 512-bit keys. An attacker using the LogJam flaw can trick a Web server into thinking it is using a stronger encryption key when it's not.

Green said that attackers can then collect the weakly encrypted data traffic and decode it within minutes.

The LogJam flaw lies specifically within a set of Diffie-Hellman algorithms, which facilitate the exchange of encryption keys before a connection is secured.

When a Web browser talks to a server, the two agree on which algorithm to use for the encrypted connection. Usually, the strongest algorithm is picked, but Green said sometimes the Web server can be tricked into using a weak one.

"It's actually a flaw in the SSL protocol that has been around for almost two decades now," Green said.

To trick the Web server into using a weaker key, an attacker needs to be on the same network as the victim, such as on the same Wi-Fi network at a coffee shop, he said.

The good news is that organizations and companies that patched their software to fix the FREAK flaw will not be vulnerable to LogJam, Green said. Those patches involved removing the ability for software to use the weaker, export-grade ciphers.

Browser makers, however, didn't remove the ability for their software to use those weaker keys even following FREAK, Green said.

About 1 percent of websites still can use 512-bit keys, and the browser makers didn't want to break them. This time around, however, it appears they're ready to make that move.

Email servers, however, could be problematic, as many weren't upgraded after FREAK.

"The big problem is that software people use to run email servers is not as well maintained," Green said. "They don't think about them. They just set them up and forget them. A lot of the default configurations that are shipped with them are bad ones."

Send news tips and comments to jeremy_kirk@idg.com. Follow me on Twitter: @jeremy_kirk

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