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HTTPS-crippling attack threatens tens of thousands of Web and mail servers


The vulnerability affects an estimated 8.4 percent of the top one million websites and a slightly bigger percentage of mail servers populating the IPv4 address space, the researchers said. The threat stems from a flaw in the transport layer security protocol that websites and mail servers use to establish encrypted connections with end users. The new attack, which its creators have dubbed Logjam, can be exploited against a subset of servers that support the widely used Diffie-Hellman key exchange, which allows two parties that have never met before to negotiate a secret key even though they’re communicating over an unsecured, public channel.

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The weakness is the result of export restrictions the US government mandated in the 1990s on US developers who wanted their software to be used abroad. The regime was established by the Clinton administration so the FBI and other agencies could break the encryption used by foreign entities. Attackers with the ability to monitor the connection between an end user and a Diffie-Hellman-enabled server that supports the export cipher can inject a special payload into the traffic that downgrades encrypted connections to use extremely weak 512-bit key material. Using precomputed data prepared ahead of time, the attackers can then deduce the encryption key negotiated between the two parties.

“Logjam shows us once again why it’s a terrible idea to deliberately weaken cryptography, as the FBI and some in law enforcement are now calling for,” J. Alex Halderman, one of the scientists behind the research, wrote in an e-mail to Ars. “That’s exactly what the US did in the 1990s with crypto export restrictions, and today that backdoor is wide open, threatening the security of a large part of the Web.”

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In the short term, the researchers recommend all server administrators disable support for the DHE_EXPORT ciphersuites that allow Diffie-Hellman connections to be downgraded. The researchers have provided a guide with step-by-step instructions for securely deploying Diffie-Hellman in TLS. And of course, they also strongly encourage all end users to install browser and e-mail client patches that enforce minimum restrictions on the primes used to negotiate ephemeral keys. Over the longer term, they say, developers should transition to so-called elliptic curve Diffie-Hellman key exchange, since the scheme is less vulnerable to precomputed attacks.


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One Comment
  1. Reblogged this on TheFlippinTruth.


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