In my case with a Kubuntu 12.04 Live USB, I wonder is it safe ? There have been a lot of updates (150+ or something) since it came out. Specifically with the USB, although I chose the "Discard on shutdown, unless you save them elsewhere" option creating the LiveUSB, is there a chance that something malicious can sneak trough ? Mount my partitions and place something there ? I mean it seems possible to me ( a slightly better noob, that is). And if that were true, shouldn't there be a warning ?
There aren't many risks to browsing from a Live session, at least none that you wouldn't encounter on a normal installation.
If you really care about the security bugs that have been patched since you made your Live USB, you can always run
„safe“ is always relative. You are safe from stuff that messed with your operating system. But you are not safe from everything else:
In theory, one could mount your partitions and store data on them. But this data would have to be read again after the next reboot, which would require a different hack.
There are some hacks that write themselves into the master boot record (MBR) of your hard drives, meaning that the attack starts before the operating starts. With a CD, that should be impossible. With the USB stick, I am not too sure.
So you are safer than before, but you are not „safe“ in an absolute sense.