So I was attempting a CTF recently, and I needed to input some non-printing chars into stdin in order to overwrite a stack variable. I decided to pipe the output from a printf command into the program, and this worked to overwrite the variable, but had an unexpected side effect of killing the program whenever it hit a blocking call and was out of input.
The shortest C program I could write to demonstrate this issue is
#include <stdlib.h>
main()
{
system("/bin/sh");
}
running it normally produces the expected result
~$ ./a.out
> ls
. .. Documents etc etc
> _
but if I use a pipe on this program, I get
~$ echo "ls" | ./a.out
. .. Documents etc etc
~$
See the difference? If I use a pipe, system("/bin/sh") returns after it runs out of input and the program would normally block / wait for input. Maybe the pipe is sending an EOF causing system() to return? How can I make it not do this (act as though I typed it in normally) by changing the way I am using bash? (I cannot change the program in a CTF). Is there a better way to input non-printing chars?