Shared hosts are a reality for many small businesses or businesses that aren't oriented around moving massive amounts of data. This is a given - we can't all afford racks full of dedicated servers. With that in mind, I would urge people to be more careful about what they do on shared hosting accounts. You should assume that anything you do is being watched.
Take, for example, the /tmp directory. I was doing some work for a friend this weekend whose account is housed on the servers of a certain very large hosting company. While tweaking some of his scripts, I noticed via
phpinfo() that sessions were file-based and were being stored in /tmp. This made me curious as to whether any of that session data could possibly be available for public viewing.
My first move was to simply try FTP'ing up and CD'ing to /tmp directory. No go - they have the FTP accounts chrooted into a jail, so the obvious door is closed. However, the accounts have PHP installed, so I can do something like this in a PHP script:
<?php system("ls -al /tmp"); ?>
With this little bit of code, I can look into the tmp directory even if my FTP login is chrooted. Fortunately, sessions on this host are 600, so they're not publically readable - this was my primary concern and the reason I took some time to check this out. But people are putting lots of things into the tmp directory with the misguided idea that it is their private temporary file dump, including one idiot who put a month's worth of PayPal transaction data into tmp and left it 644 so that it was publically viewable.
Now, I'm a nice guy and the only thing I'm going to do with this information is laugh at it. But keeping in mind how dirt cheap hosting accounts are, there's not a high entry barrier for someone with fewer scruples.
The key thing to remember is that, if you need temporary file storage on a shared host, do it someplace less obvious, set the permissions so that only you can read/write to it (600), and clean up by deleting files as soon as you possibly can.