_www user is designed to never have a shell and many of the user attributes needed for ssh to run are missing. Once you log in as any other admin user, you can easily sudo over to user
_www to get file permissions correct.
mac:~ me$ dscl . -read /Users/_www
World Wide Web Server
RecordName: _www www
mac:~ me$ sudo -u _www -s
uid=70(_www) gid=70(_www) groups=70(_www),61(localaccounts),12(everyone)
You can compare with a real user to see the missing attributes. If you want to hack, you could try making the NFSHomeDirectory writable and editing the shell - but that would leave all sorts of user files in /Library/Webserver - and your custom mods might get wiped out each update or cause unintended effects down the road.
Any reasons why you don't just create a apache admin user with a UID less than 500 so that it won't show in the log in screen as a normal user?
(or simply use a normal user to ssh in before using sudo to become
It's a lot less work and more secure.
_www is intentionally placed in a sandboxbox to reduce the chance that web browsing does bad things to a running mac.