excessive "access denied" logs?
Yes, this is a common problem. A spam bot has crawled your site, realizes it's a Drupal site, and it going through its functions to spam content (e.g. user register &amp; node posting). This isn't exclusive to Drupal; there many kinds spam bot software that target specifics CMSes and any other comment/content submission forms.

You should look into various Drupal spam prevention modules like CAPTCHA, Botcha, etc.