The Logjam Attack basically allows an attacker to downgrade a secure connection to a VPN or secure website so that the attacker is able to read or modify your communication. The issue was found in the way how Diffie-Hellman key exchange has been deployed. It has been extensively described at https://weakdh.org/.
You can test if your server is vulnerable via the Qualys SSLServer test or via a form on the weakdh.org website.
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