USN-4203-1 fixed a vulnerability in NSS. This update provides the corresponding update for Ubuntu 12.04 ESM and Ubuntu 14.04 ESM.
Original advisory details:
It was discovered that NSS incorrectly handled certain memory operations. A remote attacker could use this issue to cause NSS to crash, resulting in a denial of service, or possibly execute arbitrary code.
{
    "availability": "Available with Ubuntu Pro (Infra-only): https://ubuntu.com/pro",
    "binaries": [
        {
            "binary_name": "libnss3",
            "binary_version": "2:3.28.4-0ubuntu0.14.04.5+esm2"
        },
        {
            "binary_name": "libnss3-1d",
            "binary_version": "2:3.28.4-0ubuntu0.14.04.5+esm2"
        },
        {
            "binary_name": "libnss3-dev",
            "binary_version": "2:3.28.4-0ubuntu0.14.04.5+esm2"
        },
        {
            "binary_name": "libnss3-nssdb",
            "binary_version": "2:3.28.4-0ubuntu0.14.04.5+esm2"
        },
        {
            "binary_name": "libnss3-tools",
            "binary_version": "2:3.28.4-0ubuntu0.14.04.5+esm2"
        }
    ]
}