Ubuntu Security Notice 612-11 – USN-612-3 addressed a weakness in OpenSSL certificate and key generation and introduced openssl-blacklist to aid in detecting vulnerable certificates and keys. This update adds RSA-4096 blacklists to the openssl-blacklist-extra package and adjusts openssl-vulnkey to properly handle RSA-4096 and higher moduli. by OpenSSL on Debian and Ubuntu systems. As a result of this weakness, certain encryption keys are much more common than they should be, such that an attacker could guess the key through a brute-force attack given minimal knowledge of the system. This particularly affects the use of encryption keys in OpenSSH, OpenVPN and SSL certificates.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/67470/USN-612-11.txt
Source: https://packetstormsecurity.com/files/67470/Ubuntu-Security-Notice-612-11.html