Debian – bad interpreter: Text file busy

Your configuration is telling Apache to serve all CGI scripts under FastCGI, which isn't possible. FastCGI isn't directly compatible with CGI scripts.

You should move the AddHandler directive into your client's VirtualHost, so it doesn't apply to the whole configuration, e.g.

<VirtualHost *:80>
    ServerName clienthost.com
    ...
    AddHandler fcgid-script .fcgi .cgi
</VirtualHost>

If your client's scripts are on the same VirtualHost as your other cgi scripts, you can restrict the FastCGI handling to a specific Location or Directory, e.g.

<Location /clientarea>
    AddHandler fcgid-script .fcgi .cgi
</Location>

(Strange post, so hopefully this won't be as strange a reply).

On the subject of security flaws: it is considered general bad practice to store cgi-bin scripts within the document root of the web server. Even the W3C eludes to it under "Are compiled languages such as C safer ..." in their World Wide Web Security FAQ:

Consider the following scenario. For convenience's sake, you've decided to identify CGI scripts to the server using the .cgi extension. Later on, you need to make a small change to an interpreted CGI script. You open it up with the Emacs text editor and modify the script. Unfortunately the edit leaves a backup copy of the script source code lying around in the document tree. Although the remote user can't obtain the source code by fetching the script itself, he can now obtain the backup copy by blindly requesting the URL:

    http://your-site/a/path/your_script.cgi~

(This is another good reason to limit CGI scripts to cgi-bin and to make sure that cgi-bin is separate from the document root.)

This is not as significant a threat as the ability to write a file within the document root. However, an attacker could obtain the source code of the cgi, devise a directed attack against it, and use it as a stepping stone into the server.

To mitigate this, you can add the following lines to the lighttpd.conf (or some variation therein) to direct cgi-bin to a directory separate from the /var/www/lighttpd document root.

$HTTP["url"] =~ "/cgi-bin/" { cgi.assign = ( "" => "" ) }
alias.url = ( "/cgi-bin/" => "/usr/lib/cgi-bin/" )

This requires both the cgi and alias modules for lighttpd.