Refactoring C: Do We Need a Security Review?

Now that I’m actually doing real work with input from the network, I thought it would be a good time to stop and take a look at whatever I’m exposing stuff. C is known for buffer overruns and security issues, and compounding that with network software that accepts untrusted input, that is something that we should take a look at.
The first line of defense is to use Valgrind and see if it reports any errors. It reported a memory leak (I didn’t free the command’s buffer, it seemed), which was easy to fix. But it also reported a much more serious issue: