If I call malloc and then overflow the buffer I created, I am then writing to unused memory. Is there any security impact from me being able to do so? I would think you'd want to call malloc twice, then overflow the first buffer you allocated. That way you can overwrite heap management metadata in the second buffer, or other data in the second buffer.
EDIT: Please assume I can only overflow a kilobyte and this is a on Debian Wheezy. The heap and stack are very far apart.