On Unix, CGIHTTPRequestHandler.run_cgi() uses the following code to run a CGI script:
"""
            pid = os.fork()
            [...]            
            # Child
            try:
                try:
                    os.setuid(nobody)
                except OSError:
                    pass
                os.dup2(self.rfile.fileno(), 0)
                os.dup2(self.wfile.fileno(), 1)
                os.execve(scriptfile, args, env)
"""

It's basically reimplementing subprocess.Popen, with a potential securiy issue: open file descriptors are not closed before exec, which means that the CGI script - which is run as 'nobody' on Unix to reduce its priviledges - can inherit open sockets or files (unless they're close-on-exec)...

The attached patch rewrites run_cgi() to use subprocess on all platorms.
I'm not at all familiar with CGI, so I don't guarantee it's correct, but  the regression test test_httpservers passes on Linux.

It leads to cleaner and safer code, so if someone with some httpsever/CGI background could review it, it would be great.