[原创]KCTF2022春季赛第3题石像病毒-CTF对抗-看雪-安全社区|安全招聘|kanxue.com

[原创]KCTF2022春季赛第3题石像病毒

发表于: 2022-5-15 11:03 7685

[原创]KCTF2022春季赛第3题石像病毒

trackL

2022-5-15 11:03

7685

SEH 相关题目，我这次主要采取静态分析方法解题，核心是人脑修复 SEH 的控制流跳转，从而让 IDA 可以 F5。
main 函数中的异常处理并没有什么特别的地方，大多是内存的初始化。关键引起我注意的是这么几个地方：

使用 IDA findcrypt 插件可以发现，sub_401109 和 sub_401104 分别是 MD5 和 aesEncrypt。首先根据特征找到作者使用的 github 上的开源实现：

但还不能肯定没有魔改。首先人工修复一下 SEH 的异常处理：主要思路是把触发异常的语句 patch 为 jmp 到 except filter，并且把 retn 改成 nop 让执行流顺序执行到 except 块，然后跳转回 try 块中触发异常的下一条指令，最后再跳出来。以 MD5 函数中的这个为例：

同样，AES 中的密钥扩展、行移位和列混淆中也有几处。
修复完阅读可以发现，作者在下面几处地方做了魔改：

最后去解密就好，拿到 flag{db5c6a8dfec4d0ec5569899640}

......

strcpy((char *)&v8[9], "Enj0y_1t_4_fuuuN");
......

sub_401109(&v8[9], 0x10u, (int)&v8[2]);

sub_401104(&v8[2], 0x10u, (int)&v8[30], (int)&v8[18], 32);

......

strcpy((char *)&v8[9], "Enj0y_1t_4_fuuuN");

......

sub_401109(&v8[9], 0x10u, (int)&v8[2]);

sub_401104(&v8[2], 0x10u, (int)&v8[30], (int)&v8[18], 32);

.text:00402D13 ;   __try { // __except at loc_402D34

.text:00402D13                 mov     [ebp+ms_exc.registration.TryLevel], 0

.text:00402D1A                 xor     ebx, ebx

.text:00402D1C                 jmp     short loc_402D27 ; [Keypatch] modified this from:

.text:00402D1C ;   } // starts at 402D13               ;   div ebx

.text:00402D1E ; ---------------------------------------------------------------------------

.text:00402D1E

.text:00402D1E loc_402D1E:                             ; CODE XREF: sub_402CE0+57↓j

.text:00402D1E                 mov     [ebp+ms_exc.registration.TryLevel], 0FFFFFFFEh

.text:00402D25                 jmp     short loc_402D3E

.text:00402D27 ; ---------------------------------------------------------------------------

.text:00402D27

.text:00402D27 loc_402D27:                             ; CODE XREF: sub_402CE0+3C↑j

.text:00402D27                                         ; DATA XREF: .rdata:stru_40A080↓o

.text:00402D27 ;   __except filter // owned by 402D13

.text:00402D27                 mov     eax, [ebp+ms_exc.exc_ptr]

.text:00402D2A                 push    eax

.text:00402D2B                 call    sub_4010FA

.text:00402D30                 add     esp, 4

.text:00402D33                 nop                     ; [Keypatch] modified this from:

.text:00402D33                                         ;   retn

.text:00402D34

.text:00402D34 loc_402D34:                             ; DATA XREF: .rdata:stru_40A080↓o

.text:00402D34 ;   __except(loc_402D27) // owned by 402D13

.text:00402D34                 mov     esp, [ebp+ms_exc.old_esp]

.text:00402D37                 jmp     short loc_402D1E ; [Keypatch] modified this from:

.text:00402D37                                         ;   mov [ebp+ms_exc.registration.TryLevel], 0FFFFFFFEh

.text:00402D37                                         ; [Keypatch] padded NOP to next boundary: 5 bytes

.text:00402D39 ; ---------------------------------------------------------------------------

.text:00402D39                 nop

.text:00402D3A                 nop

.text:00402D3B                 nop

.text:00402D3C                 nop

.text:00402D3D                 nop

.text:00402D3E

.text:00402D3E loc_402D3E:                             ; CODE XREF: sub_402CE0+45↑j

.text:00402D3E                 mov     ecx, 1

.text:00402D13 ; __try { // __except at loc_402D34

.text:00402D13 mov [ebp+ms_exc.registration.TryLevel], 0

.text:00402D1A xor ebx, ebx

.text:00402D1C jmp short loc_402D27 ; [Keypatch] modified this from:

.text:00402D1C ; } // starts at 402D13 ; div ebx

.text:00402D1E ; ---------------------------------------------------------------------------

.text:00402D1E

.text:00402D1E loc_402D1E: ; CODE XREF: sub_402CE0+57↓j

.text:00402D1E mov [ebp+ms_exc.registration.TryLevel], 0FFFFFFFEh

.text:00402D25 jmp short loc_402D3E

.text:00402D27 ; ---------------------------------------------------------------------------

.text:00402D27

.text:00402D27 loc_402D27: ; CODE XREF: sub_402CE0+3C↑j

.text:00402D27 ; DATA XREF: .rdata:stru_40A080↓o

.text:00402D27 ; __except filter // owned by 402D13

.text:00402D27 mov eax, [ebp+ms_exc.exc_ptr]

.text:00402D2A push eax

.text:00402D2B call sub_4010FA

.text:00402D30 add esp, 4

.text:00402D33 nop ; [Keypatch] modified this from:

.text:00402D33 ; retn

.text:00402D34

.text:00402D34 loc_402D34: ; DATA XREF: .rdata:stru_40A080↓o

.text:00402D34 ; __except(loc_402D27) // owned by 402D13

.text:00402D34 mov esp, [ebp+ms_exc.old_esp]

.text:00402D37 jmp short loc_402D1E ; [Keypatch] modified this from:

.text:00402D37 ; mov [ebp+ms_exc.registration.TryLevel], 0FFFFFFFEh

.text:00402D37 ; [Keypatch] padded NOP to next boundary: 5 bytes

.text:00402D39 ; ---------------------------------------------------------------------------

.text:00402D39 nop

.text:00402D3A nop

.text:00402D3B nop

.text:00402D3C nop

.text:00402D3D nop

.text:00402D3E

.text:00402D3E loc_402D3E: ; CODE XREF: sub_402CE0+45↑j

.text:00402D3E mov ecx, 1

int sub_401AE0()
{

  int *v0; // ebp

  int v1; // ecx

  int v3; // [esp+10h] [ebp-38h]

  int v4; // [esp+14h] [ebp-34h]

  int v5; // [esp+18h] [ebp-30h]

  int v6; // [esp+1Ch] [ebp-2Ch]

  int v7; // [esp+20h] [ebp-28h]

  int v8; // [esp+24h] [ebp-24h]

  int v9; // [esp+28h] [ebp-20h]

  int v10; // [esp+2Ch] [ebp-1Ch]

  int savedregs; // [esp+48h] [ebp+0h] BYREF
 
  v0 = &savedregs;

  v4 = -858993460;

  v9 = -858993460;

  v10 = -858993460;

  v5 = 0;

  v6 = 0;

  v7 = 0;

  v8 = 0;

  v3 = 0;

  while ( *(v0 - 14) < 4 )

  {

    v0[*(v0 - 14) - 12] = _byteswap_ulong(*(_DWORD *)(v0[2] + 4 * *(v0 - 14)));

    *(v0 - 1) = 0;

    v0 = (int *)((char *)v0 - 1);

    v1 = *(v0 - 14);

    v0[v1 - 12] = __ROR4__(v0[v1 - 12], 8 * v1);

    v0[*(v0 - 14) - 12] = ((unsigned int)v0[*(v0 - 14) - 12] >> (32 - 8 * *((_BYTE *)v0 - 56))) | (v0[*(v0 - 14) - 12] << v1);

    *(v0 - 1) = -2;

    *(_BYTE *)(v0[2] + 4 * *(v0 - 14)) = HIBYTE(v0[*(v0 - 14) - 12]);

    *(_BYTE *)(v0[2] + 4 * *(v0 - 14) + 1) = BYTE2(v0[*(v0 - 14) - 12]);

    *(_BYTE *)(v0[2] + 4 * *(v0 - 14) + 2) = BYTE1(v0[*(v0 - 14) - 12]);

    *(_BYTE *)(v0[2] + 4 * *(v0 - 14) + 3) = v0[*(v0 - 14) - 12];

    ++*(v0 - 14);

  }

  return 0;
}