The bug resides in the TLS (Transport Layer Security) protocol used to encrypt traffic between browsers and website servers. By interposing themselves between users and servers -- the classic is a "man-in-the-middle" (MITM) attack at a public Wi-Fi hotspot -- hackers can intercept that supposedly-secure traffic, then leverage the decades-old weakness to easily decipher it.
Like FREAK, Logjam -- uncovered by an international team of experts, including ones from Microsoft, the University of Michigan and INRIA, a French research institute -- is connected to long-discarded encryption standards, once the only ones eligible for export from the U.S.
Those encryption keys can be quickly broken with off-the-shelf software and computing power purchased from cloud services.
Logjam is different from FREAK in that it lets attackers dupe a Web server into thinking it is using a stronger encryption key when it's actually not.
Logjammin' the browser. On the client side, you can verify whether your browser is vulnerable by heading to weakdh.org, an informational site set up by the Logjam team. A message will appear, either "Good News! Your browser is safe against the Logjam attack!" or "Warning! Your web browser is vulnerable to Logjam and can be tricked into using weak encryption. You should update your browser."
Computerworld ran the Logjam test on the top browsers. As of early Wednesday, this was the result.
At the moment, only Microsoft's Internet Explorer (IE) -- specifically IE11, which was the version tested by Computerworld -- has been patched, although the researchers noted in their technical paper (download PDF) that other browser makers had been informed and are working on fixes.
Microsoft patched IE last week with MS15-055, one of 13 bulletins issued May 12.
Logjammin' the server. Testing servers is more tedious. The Logjam team has published a page on how to deploy the at-root Diffie-Hellman key exchange, a popular cryptographic algorithm, and included a quick server test there.
Enter a domain name of any website into the field to see the results. The preferred reply will be, "Good News! This site is safe from the Logjam attack. It supports ECDHE, and does not use DHE."