naviserver does nothing with permissions. I've checked to sites, and everything looks fine
file mkdir /tmp/testdir
exec ls -ld /tmp/testdir
returns
drwxr-xr-x 2 openacs www-data 4096 Aug 11 12:52 /tmp/testdir
Maybe the umask under which your server is run is incorrect. Check with:
exec bash -c umask
(should be per default 0022).