To answer a few questions, this is on a professional hosting service, which is PCI compliant, and therefore has reasonably good isolation between clients, but one can never be too careful, thus this post.
Post-processing information is stored securely in a database, so outside the scope of this post. HTTPS, as was stated, is obviously in place and strictly enforced.
The OS in question is Linux, I've checked file permission security, and it's in place. Root obviously could get to it, but unless the hosting service is compromised, root can be trusted.
I'm expecting file sizes to be < 1 MB.
As for my level of experience, I have a BS in CS, almost have my CCNA, and have never had a security audit come back with a problem. I know enough when to ask about things I'm not sure about.
Encryption on disk is an option, but...
Thus far, it seems the consensus seems to be for keeping it in memory if the file is small enough. This is feasible, and the direction I'll head at this point, but any additional feedback would be greatly appreciated.

In reply to Re: Security on shared server by derekstucki
in thread Security on shared server by derekstucki

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