A serious security vulnerability has been discovered in OpenSSL. All stable NixOS releases prior to version 13.10.35708.15a465c are vulnerable. (You can see your current version by running nixos-version.) To upgrade to the latest NixOS version, run nixos-rebuild switch --upgrade. You can verify whether you are safe by running
$ nix-store -qR /run/current-system | grep opensslIf this shows any OpenSSL version prior to 1.0.1g, you may be vulnerable.