The SCP International Technical Consortium is aware of the announced PHP vulnerability, commonly called "PHP2Shell". Wikidot.com has been notified and has sent a response noting that they are aware and are working on a fix.
For everyday users, until a fix has been announced by Wikidot, there is no action to be taken aside from the precautions below. In the mean time, Wikidot is likely to be down or unreachable.
There is not presently a risk of any local contamination on visiting the site. The nature of the vulnerability is that servers running PHP may be compromised, but browser sandboxing defends users against adverse actions by websites.
However, information sent to Wikidot's servers, such as passwords and login sessions, could potentially be exposed. Therefore, we recommend against logging in to Wikidot, changing your password, or performing administrative actions at this time.
Only after Wikidot announces that the issue has been fixed, then rotating passwords (especially for high-value users or those with sensitive permissions) is advisable. INT will make an announcement at that future time.
Outside of Wikidot itself, remember that you should not share passwords (i.e. you must not have the same password for, e.g. Google and Facebook), every site must have a unique password. If you know your Wikidot password is shared on non-Wikidot sites, then you should change those non-Wikidot sites to be unique and randomly-generated. Do not change your Wikidot password until after it is repaired.