Quick introduction to reverse engineering for beginners

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7.2 SAP client network traffic compression case (Tracing connection between TDW_NOCOMPRESS SAPGUI 3 environment variable to nagging pop-up window and actual data compression routine.) It’s known that network traffic between SAPGUI and SAP is not crypted by default, it’s rather compressed (read here and here). It’s also known that by setting environment variable TDW_NOCOMPRESS to 1, it’s possible to turn network packets compression off. But you will see a nagging pop-up windows that cannot be closed: Figure 7.1: Screenshot Let’s see, if we can remove that window somehow. But before this, let’s see what we already know. First: we know that environment variable TDW_NOCOMPRESS is checked somewhere inside of SAPGUI client. Second: string like “data compression switched off” must be present somewhere too. With the help of FAR file manager I found that both of these strings are stored in the SAPguilib.dll file. So let’s open SAPguilib.dll in IDA 5 and search for “TDW_NOCOMPRESS” string. Yes, it is present and there is only one reference to it. We see the following piece of code (all file offsets are valid for SAPGUI 720 win32, SAPguilib.dll file version 7200,1,0,9009): .text:6440D51B lea eax, [ebp+2108h+var_211C] .text:6440D51E push eax ; int .text:6440D51F push offset aTdw_nocompress ; "TDW_NOCOMPRESS" .text:6440D524 mov byte ptr [edi+15h], 0 .text:6440D528 call chk_env .text:6440D52D pop ecx .text:6440D52E pop ecx .text:6440D52F push offset byte_64443AF8 .text:6440D534 lea ecx, [ebp+2108h+var_211C] ; demangled name: int ATL::CStringT::Compare(char const *)const .text:6440D537 call ds:mfc90_1603 3 SAP GUI client 183
.text:6440D53D test eax, eax .text:6440D53F jz short loc_6440D55A .text:6440D541 lea ecx, [ebp+2108h+var_211C] ; demangled name: const char* ATL::CSimpleStringT::operator PCXSTR .text:6440D544 call ds:mfc90_910 .text:6440D54A push eax ; Str .text:6440D54B call ds:atoi .text:6440D551 test eax, eax .text:6440D553 setnz al .text:6440D556 pop ecx .text:6440D557 mov [edi+15h], al String returned by chk_env() via second argument is then handled by MFC string functions and then atoi() 4 is called. After that, numerical value is stored to edi+15h. Also, take a look onto chk_env() function (I gave a name to it): .text:64413F20 ; int __cdecl chk_env(char *VarName, int) .text:64413F20 chk_env proc near .text:64413F20 .text:64413F20 DstSize = dword ptr -0Ch .text:64413F20 var_8 = dword ptr -8 .text:64413F20 DstBuf = dword ptr -4 .text:64413F20 VarName = dword ptr 8 .text:64413F20 arg_4 = dword ptr 0Ch .text:64413F20 .text:64413F20 push ebp .text:64413F21 mov ebp, esp .text:64413F23 sub esp, 0Ch .text:64413F26 mov [ebp+DstSize], 0 .text:64413F2D mov [ebp+DstBuf], 0 .text:64413F34 push offset unk_6444C88C .text:64413F39 mov ecx, [ebp+arg_4] ; (demangled name) ATL::CStringT::operator=(char const *) .text:64413F3C call ds:mfc90_820 .text:64413F42 mov eax, [ebp+VarName] .text:64413F45 push eax ; VarName .text:64413F46 mov ecx, [ebp+DstSize] .text:64413F49 push ecx ; DstSize .text:64413F4A mov edx, [ebp+DstBuf] .text:64413F4D push edx ; DstBuf .text:64413F4E lea eax, [ebp+DstSize] .text:64413F51 push eax ; ReturnSize .text:64413F52 call ds:getenv_s .text:64413F58 add esp, 10h .text:64413F5B mov [ebp+var_8], eax .text:64413F5E cmp [ebp+var_8], 0 .text:64413F62 jz short loc_64413F68 .text:64413F64 xor eax, eax .text:64413F66 jmp short loc_64413FBC .text:64413F68 ; --------------------------------------------------------------------------- 4 standard C library function, coverting number in string into number 184
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Quick introduction to reverse engin
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1.16 C++ classes . . . . . . . . .
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Preface Here (will be) some of my n
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1.1 Hello, world! Let’s start wit
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1.2 Stack Stack — is one of the m
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(_snprintf() function works just li
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1.3 printf() with several arguments
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1.4 scanf() Now let’s use scanf()
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GCC replaced first printf() call to
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}; return 0; printf ("What you ente
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1.5 Passing arguments via stack Now
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1.6 One more word about results ret
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push OFFSET $SG741 ; ’a
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1.8 switch()/case/default 1.8.1 Few
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1.8.2 A lot of cases If switch() st
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loc_804840C: ; DATA XREF: .rodata:0
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et 0 _main ENDP Nothing very specia
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add esp, 1Ch xor eax, eax ; return
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C/C++ standard defines char type as
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eax=eax+ecx return eax ... and this
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mov eax, ecx imul edx sar edx, 1 mo
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; current stack state: ST(0) = resu
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; in local stack here 8 bytes still
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fld QWORD PTR _a$[esp-4] ; current
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In other words, fnstsw ax / sahf in
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1.13 Arrays Array is just a set of
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mov [esp+70h+var_70], eax call _pri
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jge SHORT $LN1@main mov ecx, DWORD
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ebp 0x15 0x15 esi 0x0 0 edi 0x0 0 e
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1.14 Bit fields A lot of functions
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So, let’s download Linux Kernel 2
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There are some redundant code prese
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y 10 just by dropping last digit (3
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_key$ = 8 ; size = 4 _len$ = 12 ; s
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1.15 Structures It can be defined t
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call DWORD PTR __imp__GetSystemTime
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1.15.4 Fields packing in structure
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}; int b; struct outer_struct { cha
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}; printf ("stepping=%d\n", tmp->st
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call ___printf_chk mov eax, esi shr
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mov edx, DWORD PTR _t$[ebp] or edx,
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1.16 C++ classes I placed a C++ cla
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push ecx mov DWORD PTR _this$[ebp],
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call _ZN1c4dumpEv lea eax, [esp+20h
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1.17 Unions 1.17.1 Pseudo-random nu
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xor eax, eax pop esi mov esp, ebp p
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Let’s compile it in MSVC 2010 (I
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comp() function: public comp comp p
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Some compilers can do vectorization
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mov ecx, [esp+10h+var_10] mov edx,
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GCC In all other cases, non-SSE2 co
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add ebx, edx lea esi, [esi+0] loc_8
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movdqa xmm1, XMMWORD PTR [ecx+16] a
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1.20 x86-64 It’s a 64-bit extensi
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} x40 = a3 | x31; x41 = x24 & ~x37;
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or ebx, DWORD PTR _x6$[esp+36] mov
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and r11, r8 and rsi, r8 and r10, r1
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pop rdi pop rsi ret 0 s1 ENDP Nothi
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2.1 LEA instruction LEA (Load Effec
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2.3 npad It’s an assembler macro
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2.4 Signed number representations T
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call function function: .. do somet
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3.2 String Debugging messages are o
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.text:3011E91B DD 1E fstp qword ptr
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loc_80483F2: pop ebp retn _f endp 4
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loc_8048444: loc_8048448: loc_80484
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public f2 f2 proc near push ebp mov
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loc_804840A: loc_804842E: loc_80484
Page 138 and 139: mov esi, DWORD PTR _k$[esp+16] push
Page 140 and 141: or eax, edx retn f endp 4.1.8 Task
Page 142 and 143: 4.2 Middle level 4.2.1 Task 2.1 Wel
Page 144 and 145: mov [edx+eax], bl mov ebx, edi mov
Page 146 and 147: loc_80485AB: ; CODE XREF: f+1E0 ; f
Page 148 and 149: mov [ecx+eax], dl inc eax cmp ebx,
Page 150 and 151: Chapter 5 Tools ∙ IDA as disassem
Page 152 and 153: Chapter 7 More examples 147
Page 154 and 155: .text:00541050 arg_0 = dword ptr 4
Page 156 and 157: .text:005410F3 .text:005410F3 loc_5
Page 158 and 159: .text:005411A9 pop ebx .text:005411
Page 160 and 161: .text:00541249 .text:00541249 next_
Page 162 and 163: .text:005412FC mov esi, offset cube
Page 164 and 165: .text:005413DC call _fwrite ; write
Page 166 and 167: .text:005414A7 push ecx ; Filename
Page 168 and 169: .text:005413A3 mov ecx, [esp+44h+pa
Page 170 and 171: .text:00541408 push offset aRb ; "r
Page 172 and 173: }; }; return; fseek (f, 0, SEEK_END
Page 174 and 175: .text:005411B5 mov ebp, eax Check f
Page 176 and 177: .text:0054124C inc ebp .text:005412
Page 178 and 179: This instruction clears bit, in oth
Page 180 and 181: .text:005410CD add esp, 40h .text:0
Page 182 and 183: }; for (i=0; i
Page 184 and 185: }; tmp[y][z]=get_bit (row, y, z); f
Page 186 and 187: }; fread (buf, flen, 1, f); fclose
Page 190 and 191: .text:64413F68 .text:64413F68 loc_6
Page 192 and 193: .text:64405171 call dbg .text:64405
Page 194 and 195: .text:64404FF7 push offset aLogging
Page 196 and 197: .text:64405113 loc_64405113: .text:
Page 198 and 199: Now let’s dig deeper and find con
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Page 202 and 203: Chapter 8 Other things 8.1 Compiler
Page 204 and 205: Chapter 9 Tasks solutions 9.1 Easy
Page 206 and 207: 9.1.5 Task 1.5 Hint #1: Keep in min
Page 208 and 209: } 9.1.8 Task 1.8 Solution: two 100*
Page 210 and 211: #define PUSH(low, high) ((void) ((t
Page 212 and 213: } } ignore one or both. Otherwise,
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