USN-6499-2: GnuTLS vulnerability

USN-6499-1 fixed vulnerabilities in GnuTLS. This update provides the
corresponding update for Ubuntu 18.04 LTS.

Original advisory details:

It was discovered that GnuTLS had a timing side-channel when handling
certain RSA-PSK key exchanges. A remote attacker could possibly use this
issue to recover sensitive information.

advisory attacker channel gnutls handling information issue key lts recover rsa sensitive sensitive information side-channel ubuntu update usn vulnerabilities vulnerability