On September 12, in a security notice Canonical published details about a Light Display Manager vulnerability in its Ubuntu 13.04 (Saucy Salamander) operating system.
According to Canonical, SPICE could have been made to expose sensitive information.
It was discovered that Light Display Manager created .Xauthority files with incorrect permissions. A local attacker could have used this flaw to bypass access restrictions gain access to information.
For a more detailed description of the security problems, you can see Canonical's security notification.
The security flaws can be fixed if you upgrade your system(s) to the latest lightdm package specific to each distribution. To apply the update, run the Update Manager application.
In general, a standard system update will make all the necessary changes. A system restart will be necessary to implement the changes.