The problems can be fixed with a simple system update
On September 26th, in a security notice Canonical published details about a Transmission vulnerability for its Ubuntu 12.04 LTS (Precise Pangolin) operating system.According to Canonical, Transmission could be made to expose sensitive information over the network. The web client incorrectly escaped certain strings and if a user were tricked into opening a specially crafted torrent file, an attacker could possibly exploit this to conduct cross-site scripting (XSS) attacks.
The security flaws can be fixed if you upgrade your system(s) to the latest transmission-common, version 2.51-0ubuntu1.1. To apply the update, run the Update Manager application.
In general, a standard system update will make all the necessary changes. A system restart won't be necessary to implement the changes, but Transmission will have to be restarted in order for the changes to be implemented.