nganhkhoa/CTF-All-In-One
1
0
Fork 0
mirror of https://github.com/nganhkhoa/CTF-All-In-One.git synced 2024-12-25 11:41:16 +07:00
Code Issues Projects Releases Wiki Activity
716a9ca51c
CTF-All-In-One/doc/6.2.5_re_picoctf2014_baleful.md
firmianay 7b743fc91e update 6.2.5
2017-12-06 17:19:49 +08:00

317 lines
13 KiB
Markdown
Raw Blame History

This file contains ambiguous Unicode characters

This file contains Unicode characters that might be confused with other characters. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

# 6.2.5 re PicoCTF2014 Baleful
- [题目解析](#题目解析)
- [逆向 VM 求解](#逆向-vm-求解)
- [使用 Pin 求解](#使用-pin-求解)
- [参考资料](#参考资料)
[下载文件](../src/writeup/6.2.5_re_picoctf2014_baleful)
## 题目解析
```
$ file baleful
baleful: ELF 32-bit LSB executable, Intel 80386, version 1 (GNU/Linux), statically linked, stripped
$ strings baleful | grep -i upx
@UPX!
$Info: This file is packed with the UPX executable packer http://upx.sf.net $
$Id: UPX 3.91 Copyright (C) 1996-2013 the UPX Team. All Rights Reserved. $
UPX!u
UPX!
UPX!
$ upx -d baleful -o baleful_unpack
Ultimate Packer for eXecutables
Copyright (C) 1996 - 2017
UPX 3.94 Markus Oberhumer, Laszlo Molnar & John Reiser May 12th 2017
File size Ratio Format Name
-------------------- ------ ----------- -----------
144956 <- 6752 4.66% linux/i386 baleful_unpack
Unpacked 1 file.
$ file baleful_unpack
baleful_unpack: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=35d1a373cbe6a675ecbbc904722a86f853f20ce3, stripped
```
经过简单地检查，我们发现二进制文件被加了壳，使用 upx 脱掉就好了。
运行下看看，典型的密码验证题：
```
$ ./baleful_unpack
Please enter your password: ABCD
Sorry, wrong password!
```
#### 逆向 VM 求解
打开 r2 开干吧！
```
[0x08048540]> pdf @ main
/ (fcn) main 96
| main ();
| ; var int local_8h @ ebp-0x8
| ; var int local_10h @ esp+0x10
| ; var int local_8ch @ esp+0x8c
| ; DATA XREF from 0x08048557 (entry0)
| 0x08049c82 55 push ebp
| 0x08049c83 89e5 mov ebp, esp
| 0x08049c85 57 push edi
| 0x08049c86 53 push ebx
| 0x08049c87 83e4f0 and esp, 0xfffffff0
| 0x08049c8a 81ec90000000 sub esp, 0x90
| 0x08049c90 65a114000000 mov eax, dword gs:[0x14] ; [0x14:4]=-1 ; 20
| 0x08049c96 8984248c0000. mov dword [local_8ch], eax
| 0x08049c9d 31c0 xor eax, eax
| 0x08049c9f 8d442410 lea eax, [local_10h] ; 0x10 ; 16
| 0x08049ca3 89c3 mov ebx, eax
| 0x08049ca5 b800000000 mov eax, 0
| 0x08049caa ba1f000000 mov edx, 0x1f ; 31
| 0x08049caf 89df mov edi, ebx
| 0x08049cb1 89d1 mov ecx, edx
| 0x08049cb3 f3ab rep stosd dword es:[edi], eax
| 0x08049cb5 8d442410 lea eax, [local_10h] ; 0x10 ; 16
| 0x08049cb9 890424 mov dword [esp], eax
| 0x08049cbc e8caecffff call fcn.0804898b
| 0x08049cc1 b800000000 mov eax, 0
| 0x08049cc6 8b94248c0000. mov edx, dword [local_8ch] ; [0x8c:4]=-1 ; 140
| 0x08049ccd 653315140000. xor edx, dword gs:[0x14]
| ,=< 0x08049cd4 7405 je 0x8049cdb
| | 0x08049cd6 e8e5e7ffff call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void)
| | ; JMP XREF from 0x08049cd4 (main)
| `-> 0x08049cdb 8d65f8 lea esp, [local_8h]
| 0x08049cde 5b pop ebx
| 0x08049cdf 5f pop edi
| 0x08049ce0 5d pop ebp
\ 0x08049ce1 c3 ret
```
`fcn.0804898b` 是程序主要的逻辑所在，很容易看出来它其实是实现了一个虚拟机：
```
[0x08048540]> pdf @ fcn.0804898b
Linear size differs too much from the bbsum, please use pdr instead.
[0x08048540]> pdr @ fcn.0804898b
/ (fcn) fcn.0804898b 226
| fcn.0804898b (int arg_8h);
| ; var int local_b4h @ ebp-0xb4
| ; var int local_38h @ ebp-0x38
| ; var int local_34h @ ebp-0x34
| ; var int local_30h @ ebp-0x30
| ; var int local_2ch @ ebp-0x2c
| ; var int local_28h @ ebp-0x28
| ; var int local_24h @ ebp-0x24
| ; var int local_20h @ ebp-0x20
| ; var int local_1ch @ ebp-0x1c
| ; var int local_18h @ ebp-0x18
| ; var int local_14h @ ebp-0x14
| ; var int local_10h @ ebp-0x10
| ; var int local_ch @ ebp-0xc
| ; arg int arg_8h @ ebp+0x8
| ; CALL XREF from 0x08049cbc (main)
| 0x0804898b 55 push ebp
| 0x0804898c 89e5 mov ebp, esp
| 0x0804898e 81ecc8000000 sub esp, 0xc8
| 0x08048994 c745cc001000. mov dword [local_34h], 0x1000
| 0x0804899b 837d0800 cmp dword [arg_8h], 0
| 0x0804899f 742a je 0x80489cb
| ----------- true: 0x080489cb false: 0x080489a1
| 0x080489a1 c745d0000000. mov dword [local_30h], 0
| 0x080489a8 eb19 jmp 0x80489c3
| ----------- true: 0x080489c3
| ; JMP XREF from 0x080489c7 (fcn.0804898b)
| 0x080489aa 8b45d0 mov eax, dword [local_30h]
| 0x080489ad c1e002 shl eax, 2
| 0x080489b0 034508 add eax, dword [arg_8h]
| 0x080489b3 8b10 mov edx, dword [eax]
| 0x080489b5 8b45d0 mov eax, dword [local_30h]
| 0x080489b8 8994854cffff. mov dword [ebp + eax*4 - 0xb4], edx
| 0x080489bf 8345d001 add dword [local_30h], 1
| ----------- true: 0x080489c3
| ; JMP XREF from 0x080489a8 (fcn.0804898b)
| 0x080489c3 837dd01e cmp dword [local_30h], 0x1e ; [0x1e:4]=-1 ; 30
| 0x080489c7 7ee1 jle 0x80489aa
| ----------- true: 0x080489aa false: 0x080489c9
| 0x080489c9 eb21 jmp 0x80489ec
| ----------- true: 0x080489ec
| ; JMP XREF from 0x0804899f (fcn.0804898b)
| 0x080489cb c745d4000000. mov dword [local_2ch], 0
| 0x080489d2 eb12 jmp 0x80489e6
| ----------- true: 0x080489e6
| ; JMP XREF from 0x080489ea (fcn.0804898b)
| 0x080489d4 8b45d4 mov eax, dword [local_2ch]
| 0x080489d7 c784854cffff. mov dword [ebp + eax*4 - 0xb4], 0
| 0x080489e2 8345d401 add dword [local_2ch], 1
| ----------- true: 0x080489e6
| ; JMP XREF from 0x080489d2 (fcn.0804898b)
| 0x080489e6 837dd41e cmp dword [local_2ch], 0x1e ; [0x1e:4]=-1 ; 30
| 0x080489ea 7ee8 jle 0x80489d4
| ----------- true: 0x080489d4 false: 0x080489ec
| ; JMP XREF from 0x080489c9 (fcn.0804898b)
| 0x080489ec c745c800f000. mov dword [local_38h], 0xf000
| 0x080489f3 c745d8000000. mov dword [local_28h], 0
| 0x080489fa c745ec000000. mov dword [local_14h], 0
| 0x08048a01 c745f0000000. mov dword [local_10h], 0
| 0x08048a08 c745f4000000. mov dword [local_ch], 0
| 0x08048a0f c745e8000000. mov dword [local_18h], 0
| 0x08048a16 8b45e8 mov eax, dword [local_18h]
| 0x08048a19 8945e4 mov dword [local_1ch], eax
| 0x08048a1c 8b45e4 mov eax, dword [local_1ch]
| 0x08048a1f 8945e0 mov dword [local_20h], eax
| 0x08048a22 8b45e0 mov eax, dword [local_20h]
| 0x08048a25 8945dc mov dword [local_24h], eax
| 0x08048a28 e93a120000 jmp 0x8049c67
| ----------- true: 0x08049c67
| ; JMP XREF from 0x08049c74 (fcn.0804898b)
| 0x08048a2d 8b45cc mov eax, dword [local_34h]
| 0x08048a30 05c0c00408 add eax, 0x804c0c0
| 0x08048a35 0fb600 movzx eax, byte [eax]
| 0x08048a38 0fbec0 movsx eax, al
| 0x08048a3b 83f820 cmp eax, 0x20 ; 32
| 0x08048a3e 0f871e120000 ja 0x8049c62
| ----------- true: 0x08049c62 false: 0x08048a44
| 0x08048a44 8b0485d49d04. mov eax, dword [eax*4 + 0x8049dd4] ; [0x8049dd4:4]=0x8048a4d
| 0x08048a4b ffe0 jmp eax
| ; JMP XREF from 0x08048a3e (fcn.0804898b)
| 0x08049c62 8345cc01 add dword [local_34h], 1
| 0x08049c66 90 nop
| ----------- true: 0x08049c67
| ; XREFS: JMP 0x08048a28 JMP 0x08048a51 JMP 0x08048a8a JMP 0x08048bbf JMP 0x08048cf4 JMP 0x08048e2a JMP 0x08048f8c JMP 0x080490c1
| ; XREFS: JMP 0x080491f6 JMP 0x0804932b JMP 0x08049462 JMP 0x08049599 JMP 0x080495f0 JMP 0x08049644 JMP 0x08049698 JMP 0x080496cc
| ; XREFS: JMP 0x080496e7 JMP 0x08049710 JMP 0x08049739 JMP 0x08049762 JMP 0x0804978b JMP 0x080497b4 JMP 0x080497dd JMP 0x080498eb
| ; XREFS: JMP 0x080499fd JMP 0x08049a81 JMP 0x08049ab4 JMP 0x08049ae7 JMP 0x08049b3e JMP 0x08049b8d JMP 0x08049bf6 JMP 0x08049c2c
| ; XREFS: JMP 0x08049c60
| 0x08049c67 8b45cc mov eax, dword [local_34h]
| 0x08049c6a 05c0c00408 add eax, 0x804c0c0
| 0x08049c6f 0fb600 movzx eax, byte [eax]
| 0x08049c72 3c1d cmp al, 0x1d ; 29
| 0x08049c74 0f85b3edffff jne 0x8048a2d
| ----------- true: 0x08048a2d false: 0x08049c7a
| 0x08049c7a 8b854cffffff mov eax, dword [local_b4h]
| ; JMP XREF from 0x08048a6f (fcn.0804898b + 228)
| 0x08049c80 c9 leave
\ 0x08049c81 c3 ret
```
```
[0x08048540]> px @ 0x0804c060
- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
0x0804c060 7c86 0408 ad86 0408 d486 0408 a987 0408 |...............
0x0804c070 fb86 0408 1b87 0408 4e87 0408 d887 0408 ........N.......
0x0804c080 1986 0408 1388 0408 3488 0408 7b88 0408 ........4...{...
0x0804c090 b688 0408 f188 0408 2c89 0408 6086 0408 ........,...`...
0x0804c0a0 6a86 0408 6789 0408 0000 0000 0000 0000 j...g...........
```
#### 使用 Pin 求解
就像上面那样逆向实在是太难了，不如 Pin 的黑科技。
编译 32 位 pintool：
```
[ManualExamples]$ make obj-ia32/inscount0.so TARGET=
```
随便输入几个长度不同的密码试试：
```
[ManualExamples]$ echo "A" | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack ; cat inscount.out
Please enter your password: Sorry, wrong password!
Count 437603
[ManualExamples]$ echo "AA" | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack ; cat inscount.out
Please enter your password: Sorry, wrong password!
Count 438397
[ManualExamples]$ echo "AAA" | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack ; cat inscount.out
Please enter your password: Sorry, wrong password!
Count 439191
```
```
$ python -c 'print(439191 - 438397)'
794
$ python -c 'print(438397 - 437603)'
794
```
指令执行的次数呈递增趋势，完美，这样只要递增到这个次数有不同时，就可以得到正确的密码长度：
```python
import os
def get_count(flag):
cmd = "echo " + "\"" + flag + "\"" + " | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack"
os.system(cmd)
with open("inscount.out") as f:
count = int(f.read().split(" ")[1])
return count
flag = "A"
p_count = get_count(flag)
for i in range(50):
flag += "A"
count = get_count(flag)
print("count: ", count)
diff = count - p_count
print("diff: ", diff)
if diff != 794:
break
p_count = count
print("length of password: ", len(flag))
```
```
Please enter your password: Sorry, wrong password!
count: 459041
diff: 794
Please enter your password: Sorry, wrong password!
count: 459835
diff: 794
Please enter your password: Sorry, wrong password!
count: 508273
diff: 48438
length of password: 30
```
好，密码长度为 30，接下来是逐字符爆破，首先要确定字符不同对 count 没有影响：
```
[ManualExamples]$ echo "A" | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack ; cat inscount.out
Please enter your password: Sorry, wrong password!
Count 437603
[ManualExamples]$ echo "b" | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack ; cat inscount.out
Please enter your password: Sorry, wrong password!
Count 437603
[ManualExamples]$ echo "_" | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack ; cat inscount.out
Please enter your password: Sorry, wrong password!
Count 437603
```
确实没有，写下脚本：
```python
import os
def get_count(flag):
cmd = "echo " + "\"" + flag + "\"" + " | ../../../pin -t obj-ia32/inscount0.so -o inscount.out -- ~/baleful_unpack"
os.system(cmd)
with open("inscount.out") as f:
count = int(f.read().split(" ")[1])
return count
charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_-+*'"
flag = list("A" * 30)
p_count = get_count("".join(flag))
for i in range(30):
for c in charset:
flag[i] = c
print("".join(flag))
count = get_count("".join(flag))
print("count: ", count)
if count != p_count:
break
p_count = count
print("password: ", "".join(flag))
```
```
packers_and_vms_and_xors_oh_mx
Please enter your password: Sorry, wrong password!
count: 507925
packers_and_vms_and_xors_oh_my
Please enter your password: Congratulations!
count: 505068
password: packers_and_vms_and_xors_oh_my
```
简单到想哭。
## 参考资料
- [Pico CTF 2014 : Baleful](https://github.com/ctfs/write-ups-2014/tree/master/pico-ctf-2014/master-challenge/baleful-200)
Reference in New Issue View Git Blame Copy Permalink