On October 26, in a security notice Canonical published details about an Exim vulnerability for its Ubuntu 12.10, Ubuntu 12.04 LTS, Ubuntu 11.10, Ubuntu 11.04, and Ubuntu 10.04 LTS operating systems.
According to Canonical, Exim could have been made to run programs if it received specially crafted network traffic.
It was discovered that Exim incorrectly handled DKIM DNS decoding. This flaw could have allowed a remote attacker to execute arbitrary code.
For a more detailed description of the security problems, you can visit Canonical's security notification.
Users can simply fix the security flaws by upgrading the operating systems to the latest exim4-daemon-light, exim4-daemon-custom, and exim4-daemon-heavy packages, specific to each distribution.
A normal system update, executed with the Update Manager, will implement all the necessary changes. A complete system restart is not necessary.