First, NOBODY TRY THIS!

DO NOT FOLLOW THE LINK!

Forget the porn, its the viruses that'll get you.

Second, archaugust, bad news. Your site/server has been hacked.

The way I know this is I have seen this exact attack before. The way the attack works is as follows:

If someone comes directly to your site, the site works fine. If someone follows a link to your site, the site works fine. If someone follows a link from a search engine, they are redirected to the malicious site. This is an attempt to prevent the attack from being detected - how many webmasters actually search for their own site? This attack is based on the HTTP_REFERER - if the referer tag contains google.com or yahoo.com, the visitor is redirected. I tested this by blocking my referer tag and clicking your site in the SERPS - using that method I was not redirected.

Fixing this is your top priority, because Google and Yahoo are very likely to detect this issue quickly. When they do, your site will be penalized, and may get marked as a malware site.

Generally, the attack uses .htaccess. That should be where the compromise code is. However, there is probably a virus on the server that will simply reapply the attack code when you correct the issue. My first step would be to delete the entire site and restore from a known good backup if possible. Then, get someone very familiar with the server and network security to fix the problem and plug whatever exploit was used to accomplish the attack.

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