Update: I can't confirm if it was a hoax or not, because my client hasn't reported any problems. They were talking with their webhost about it, in any case. I did try to access their site during the indicated takedown period, and it was inaccessible, but it could very well be due to the webhost taking preventive measures by taking down his server. Will update when I find out. It is also interesting to note that Simon Willison has received the same email (see his comments in that blog entry).

One of my clients told me today that they received this message via the contact form at their website:

Dear Administrator

We are sorry to inform you that your site has been targeted for hacking
practice. The monthly hacking practice is organized since 1991. The targets
are randomly chosen.

We are sorry if your server is down, damged or formated. The first round
of attack starts on 15th Sept 23:56, there will be at most 5 rounds, each
last for 5-10 hours. Please backup your data.

The hackers assigned to your server are level 2 hackers. (ranking from
level 1 to 7 by increasing ability)

note: this message is sent from an randomly picked unknown singapore host.
please go ahead tracing. reply will not be receivable.

CHU - Chinese Hackers' Union 2003

This is the first time I've encountered something like this and I'm pleasantly surprised by the forewarning given. I've backed everything up but I also wonder if there's anything their web hosting provider can or should do. Does anyone have any experience with hacking attempts and can provide any advice?

Anyhow, this is just a theory, but I can pretty much guess that some of these hackers are members of some educational institutions (specifically universities) in Singapore, considering the nature of the client's website. Interesting. It's not like I'm going to do anything to them though, I think partly because they were ever so nice to give a warning and partly because there's really nothing we can do if the hackers are skilled enough.

Anyway, if you check out their (CHU's) website, the message there translates roughly to this:

The onion reorganization is now completed, Everyone please pass the message that this China Hacker Union will soon be reopened.