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TLS, SSH, VPN: weakening Diffie-Hellman via common primes
Synthesis of the vulnerability
An attacker, located as a Man-in-the-Middle, can obtain the DH keys used by the TLS/SSH/VPN client/server, in order to more easily capture or alter exchanged data.
Vulnerable software: Apache httpd, AnyConnect VPN Client, IVE OS, Juniper SA, lighttpd, nginx, OpenSSH, OpenSSL, Openswan, Postfix, SSL protocol, Sendmail.
Severity of this announce: 2/4.
Consequences of an intrusion: data reading, data creation/edition.
Attacker's origin: internet server.
Creation date: 20/05/2015.
Références of this computer vulnerability: VIGILANCE-VUL-16951.
Description of the vulnerability
The Diffie-Hellman algorithm is used to exchange cryptographic keys. It is used by TLS, SSH and VPNs (IPsec).
Most servers use the same prime numbers (standardized in RFC 3526). An attacker can thus pre-compute values (100000 core CPU hours, so during a week for 512 bits with 100 computers approximately) and use the "number field sieve discrete log algorithm" attack to quickly obtain the used DH keys, and decrypt a session.
The 512 bits sets are considered as broken, and the 1024 bits sets are considered as breakable by a state.
For TLS, this vulnerability can be exploited after Logjam (VIGILANCE-VUL-16950).
An attacker, located as a Man-in-the-Middle, can therefore obtain the DH keys used by the TLS/SSH/VPN client/server, in order to more easily capture or alter exchanged data.
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