according to the Apache documentation, filter programs should be passed a full set of environment variables, including all the standard CGI ones. This seems to be true for internal filters (like Includes), but external filters get only a small subset of all variables. Important things, like REMOTE_ADDR or HTTP_REFERER, seem to be missing.
This has already been fixed in the stable branch and will be in the next 2.0.x release. Visit http://cvs.apache.org/viewcvs.cgi/httpd-2.0/modules/filters/mod_ext_filter.c and download the latest code if you want. Thanks for using Apache! *** This bug has been marked as a duplicate of 20944 ***