Logjam attack exposes data passed over TLS connections

All servers that support DHE_EXPORT ciphers, as well as all modern browsers, are affected by the Logjam attack.
All servers that support DHE_EXPORT ciphers, as well as all modern browsers, are affected by the Logjam attack.

Computer scientists have identified weaknesses in the way popular cryptographic algorithm Diffie-Hellman key exchange is deployed – notably, they discovered an attack that could enable the reading and modifying of data passed over TLS connections.

All servers that support DHE_EXPORT ciphers, as well as all modern browsers, are affected, according to a website advisory. A Skyhigh blog post indicated that 575 cloud services are vulnerable, and that the average enterprise is using 71 vulnerable cloud services.

The attack – referred to as Logjam – can be used by a man-in-the-middle (MitM) attacker to downgrade TLS connections to 512-bit export-grade cryptography that is weaker and easier to crack, thus enabling the reading and modifying of data, the website advisory explained.

Ken Westin, senior security analyst at Tripwire, noted in a Wednesday blog post that the issue has been around longer than two decades, adding it affects HTTPS, SSH, IPsec, SMTPS and protocols that rely on TLS, making it very widespread. 

The Logjam attack is similar to the FREAK attack that was identified in March, the website advisory said, except it attacks Diffie-Hellman key exchange as opposed to RSA key exchange, and is the result of a flaw in TLS protocol and is not an implementation vulnerability.

“The population of affected sites is slightly lower than for FREAK,” Bill Weinberg, senior director of Open Source Strategy with Black Duck Software, told SCMagazine.com in a Wednesday email correspondence. “8.4 percent of the top million web domains are reported vulnerable and 3.4 percent of HTTPS sites, as well as email servers using SMTP with several standard types of authentication.”

Weinberg also noted, “Like FREAK, Logjam is the result of encryption export restrictions implemented by the U.S. Government twenty years ago to protect then-strategic encryption tech.” 

Rapid7 Security Engineering Manager Tod Beardsley, in a statement emailed to SCMagazine.com on Wednesday, said, “The only two groups really in a position to take advantage of this vulnerability are 1) criminals on coffee shop Wi-Fi networks; and 2) state actors who already control a huge chunk of the local Internet – [the] usual rogues gallery of internet criminals are not really a risk here.”

Currently the issue has been addressed in Internet Explorer. Richard Barnes, cryptographic engineering manager with Mozilla, told SCMagazine.com in a Wednesday email that a fix in Firefox is expected to be released in a few days. A Google spokesperson told SCMagazine.com in a Wednesday email that a fix in the stable version of Chrome will be available in the coming weeks, and that a fix in Chrome Canary should be live in a day or two.

However, coming up with a fix was not easy and making it so browsers reject keys with fewer than 1,024 bits could result in as many as 20,000 websites becoming inaccessible, The Wall Street Journal reported.

Those that run a web or mail server should disable support for export cipher suites and generate a unique 2048-bit Diffie Hellman group, and those using SSH should upgrade server and client installations to the most recent version of OpenSSH, according to the website advisory.

Sysadmins and developers should use updated TLS libraries and reject Diffie-Hellman groups smaller than 1024-bit, and vulnerable browser users should remain on the lookout for updates as fixes are deployed, the advisory added.

The computer scientists published a research paper with further details, in which they examine evidence from leaked Edward Snowden documents that suggested the NSA has been exploiting the capability to decrypt VPN traffic.

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