I found a number of things interesting when reading the following post:

http://www.offensive-security.com/vulndev/freepbx-exploit-phone-home/

Too bad that nmap’s interactive mode was taken out, but there are a great number of other such methods, most notably VI’s shell mode. 

But when I started looking into appending or inserting lines into /etc/sudoers for CCDC, I happened upon an interesting function of that file. Near the end of the file there are two lines:

# See sudoers(5) for more information on "#include" directives:  
#includedir /etc/sudoers.d

Both look commented out, but in actuality, exactly as-is the #includedir line is interpreted and acted upon. So any file that you put in the /etc/sudoers.d directory counts as an extension of the /etc/sudoers file. Make a small edit to the default README file with a bunch of added # commented out lines copied directly from the sudo man page, with a

nobody ALL = NOPASSWD: ALL

or www-data plus a webshell makes for easy re-exploitation. Just an evil way to stay hidden on a ‘nix box… 

Update: 

nmap --script <(echo "os.execute('/bin/sh')")

‘nuf said…  (thanks @bonsaiviking )