OpenSSL may generate unsafe primes for use in the Diffie-Hellman protocol, which may lead to disclosure of enough information for an attacker to recover the private encryption key.
CWE-325: Missing Required Cryptographic Step - CVE-2016-0701
OpenSSL 1.0.2 introduced the ability to generate X9.42 style parameter files as required by RFC 5114. The primes generated in this mode may be 'unsafe', enabling generation of groups containing small subgroups, which may allow for cryptographic attacks that may recover the key. OpenSSL prior to 1.0.2f did not properly check for this possibility.
A malicious remote user may be able to learn sensitive information by obtaining the decryption key. A remote attacker may also be able to negotiate for weak SSLv2 ciphers even if the SSLv2 ciphers have been disabled, allowing for further known attacks.
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OpenSSL credits Antonio Sanso of Adobe for reporting CVE-2016-0701, and Nimrod Aviram and Sebastian Schinzel for reporting CVE-2015-3197.