Leave it to advogato to have a very sophisticated attack launched for testing purposes.
There are two issues at play here. First of all, all arbitrary text should be escaped before being displayed in html. Nothing particularly deep or difficult about that, although advogato text processing is borked in several ways - repeatedly editing old diary entries results in lots of <p> <p> <p>.
The other issues is that for many web sites it's fairly simple to trick a user into clicking on a link which causes something nasty to happen to their account. iframes just exacerbate this problem, they don't create it. A straightforward solution is that for every page which has links which have side effects, a one-time use string is randomly generate and put into the links. When the action is performed, the system checks to see if the string is valid and (very important!) that it was generated for the current user and action. This also stops accidental double-posting, since a second usage of the same string can be treated as a modification instead of a new post.
Unfortunately, this isn't just an issue on advogato, it's an issue on almost all web sites. Advogato's second problem of not escaping makes the attack worse by making it possible for it to be viral, but the attack is there nonetheless.
Props to whoever came up with and implemented the attack. That was very clever.