For a few hours recently, anybody trying to visit the forum would have seen, instead of the forum, something like this:
The writing seems to be in Turkish, but what it means, I can't tell you.
Evidently some mischievous types had found a way to inject some of their own files into the forum Web site, replacing some of the normal files. As far as I can tell, the intrusion did nothing harmful apart from preventing us from using the forum until it was repaired. I have no reason to believe these people were able to access confidential data from the website, such as forumites' email addresses.
I've been negligent in the past in applying upgrades to the forum software, and we had fallen about 6 versions behind the latest release. Since some of those upgrades dealt with security issues, this might be the reason we were vulnerable to such an attack. So I've taken the opportunity of this downtime to upgrade the software to the latest version from SMF.
As far as I can see, things are back to normal now, but let me know if there's anything not working properly in the forum.
Thanks to Fireworks, Linda and others for letting me know about these problems.