Protecting the Heap: Encryption & Hooks

Looking into some heap encryption shenanigans...

Introduction

As Endpoint Protection gets better, and more of the community build tooling to detect malware in memory, the more evasive implants must become. In this blog I want to look at encrypting the "Heap". More on that in a moment, but for now, the Heap will hold data for a lot longer than the "Stack". The stack will clear as a a function returns. A typical example of Heap usage is a Command and Control (C2) Frameworks configuration; after all, the data to communicate must live somewhere. A generic example of this would be connection strings, and that is what we will use as sample data for this test.

Cobalt Strike introduced Sleep Mask in Cobalt Strike 4.4:

The sleep_mask is Cobalt Strike’s ability to mask and unmask itself in memory. The goal of this feature is to push memory detections away from content-based signatures. Although sleep_mask can encode Beacon’s data and code (if the agent is in RWX memory), the static stub is still a target for in-memory hunting based on content.

And then updated in 4.5 and is a recommended read... This was then popularised by MDSec again back on July 30th 2021 and is core functionality of their proprietary C2, Nighthawk.

Off the back of this, waldo-irc put together Hook Heaps and Live Free and LockdExeDemo in which he replicates this functionality to further protect a Cobalt Strike Beacon. Similarly, SolomonSklash then produced SleepyCrypt: Encrypting a running PE image while it sleeps which was aimed towards encrypting the sections of a PE in memory.

I've been writing Vulpes since around 2019 and is becoming more stable and the modules I want are almost all there, so now I'm looking into some more evasive behaviour, specifically when the implant is not operational, hence this blog post!

WTF is a Heap

I don't want to turn this into a Computer Science class, so I won't discuss this TOO much. So, what is the heap?

Well, its considered dynamic storage. Meaning it can house large pools of memory which aren't allocated in a contiguous order. Furthermore, the Heap is not managed, and to use it, it must be allocated specifically with functions such as malloc, and then freeing with free. If this is not done, then a memory leak can occur. This is where it differs from the stack. If something is allocated on the stack, it is cleared when he calling routine returns.

With this in mind, imagine if a configuration for an implant was a big struct. The config would be required quite often, so its likely going to be stored on the heap. This is because if the configuration was completely done at runtime, then the configuration object would be constantly created and deleted. Meaning, if settings are applied at runtime, then they will constantly need re-updating. Thus, the heap is better for this.

Two great references for this:

Identifying the Heaps

Microsoft have documented this quite well:

Stringing these two posts together got me 99% of the way there, so lets look at it.

First off, CreateToolhelp32Snapshot is used with the TH32CS_SNAPHEAPLIST, 0x00000001, value:

HANDLE hHeapSnap = CreateToolhelp32Snapshot(TH32CS_SNAPHEAPLIST, GetCurrentProcessId());

All the snapshot values:

Value

Meaning

In typical fashion with the snapshotting functions, the setup:

hl.dwSize = sizeof(HEAPLIST32);

if (hHeapSnap == INVALID_HANDLE_VALUE)
{
    printf("CreateToolhelp32Snapshot %ld\n", GetLastError());
    return 1;
}

At this point, the snapshot is ready to parse. But before that, something needs to actually be put on the heap. This can be done with HeapAlloc , GetProcessHeap and memcpy:

// Allocate space on the heap
LPVOID pHeapBase = HeapAlloc(GetProcessHeap(), 0, 14);
// copy in the value
memcpy(pHeapBase, "10.10.11.205\0", 14);

// show it!
printf("heapAllocHeap: %p\n", pHeapBase);

Next thing is to grab the first heap entry with Heap32ListFirst:

BOOL bFirstHeap = Heap32ListFirst(hHeapSnap, &hl);

Now loop over with Heap32First and Heap32Next:

do
{
    HEAPENTRY32 he;
    ZeroMemory(&he, sizeof(HEAPENTRY32));
    he.dwSize = sizeof(HEAPENTRY32);

    if (Heap32First(&he, GetCurrentProcessId(), hl.th32HeapID))
    {
        do
        {
            printf("Heap Handle: 0x%p\n", he.hHandle);
            he.dwSize = sizeof(HEAPENTRY32);
        } while (Heap32Next(&he));
    }
    hl.dwSize = sizeof(HEAPLIST32);
} while (Heap32ListNext(hHeapSnap, &hl));

Working with the heap

Before operating on the heap, it must be locked with HeapLock:

If the function succeeds, the calling thread owns the heap lock. Only the calling thread will be able to allocate or release memory from the heap. The execution of any other thread of the calling process will be blocked if that thread attempts to allocate or release memory from the heap. Such threads will remain blocked until the thread that owns the heap lock calls the HeapUnlock function.

By doing this, the heap now belongs to the calling thread, meaning no other threads will be accessing the heap whilst we encrypt it:

if (HeapLock(he.hHandle)) {
    // locked!
}

Walk and Encrypt

Conveniently, HeapWalk allows just that:

while (HeapWalk(he.hHandle, &heapEntry) != FALSE) {
    // do something
}

This is shown in Enumerating a Heap where a lot of prints are done, we don't care about that here.

When this returns, a PROCESS_HEAP_ENTRY struct will give us allows to all the following information:

typedef struct _PROCESS_HEAP_ENTRY {
  PVOID lpData;
  DWORD cbData;
  BYTE  cbOverhead;
  BYTE  iRegionIndex;
  WORD  wFlags;
  union {
    struct {
      HANDLE hMem;
      DWORD  dwReserved[3];
    } Block;
    struct {
      DWORD  dwCommittedSize;
      DWORD  dwUnCommittedSize;
      LPVOID lpFirstBlock;
      LPVOID lpLastBlock;
    } Region;
  } DUMMYUNIONNAME;
} PROCESS_HEAP_ENTRY, *LPPROCESS_HEAP_ENTRY, *PPROCESS_HEAP_ENTRY;

cbData is the size, and lpData is the actual data on the heap. There is all kinds of information here, and the only check we are going to make is that wFlags is PROCESS_HEAP_ENTRY_BUSY, 0x0004:

The heap element is an allocated block.
If PROCESS_HEAP_ENTRY_MOVEABLE is also specified, the Block structure becomes valid. The hMem member of the Block structure contains a handle to the allocated, moveable memory block.

This just means that memory is allocated here.

In terms of encrypting, a simple XOR will be used:

VOID Xor(unsigned char* lpData, DWORD cbData)
{
    for (int i = 0; i < cbData; i++)
    {
        lpData[i] = lpData[i] ^ 0xff;
    }
}

Before actually doing the encryption, lets set a breakpoint on the Xor call:

Note, there are two strings. Because the string is just stored in the PE, one of those strings will be from one of the data sections, this is not a focus of the blog for now.

Calling the encryption function:

while (HeapWalk(he.hHandle, &heapEntry) != FALSE) {
    if ((heapEntry.wFlags & PROCESS_HEAP_ENTRY_BUSY) && heapEntry.cbData > 0)
    {
        if (pHeapBase == heapEntry.lpData) {
            Xor((unsigned char*)heapEntry.lpData, heapEntry.cbData);
        }
    }
}

Rerunning the code:

Now there is only one, remember the other is in the PE data sections.

Before returning, the heap is unlocked with HeapUnlock, releasing ownership:

if (HeapUnlock(he.hHandle) == FALSE) {
    printf("HeapUnlock %ld\n", GetLastError());
}

Alternatively, everything in the heap can be encrypted, but this could end up giving unexpected results:

while (HeapWalk(he.hHandle, &heapEntry) != FALSE) {
    if ((heapEntry.wFlags & PROCESS_HEAP_ENTRY_BUSY) && heapEntry.cbData > 0)
    {
        Xor((unsigned char*)heapEntry.lpData, heapEntry.cbData);
    }
}

Hooking Heap Allocations

In order to hook heap allocations effectively, three fnctions are required:

As their names imply, they are responsible for allocating, reallocating, and freeing space on the heap. For hooking, and ease, minhook will be used.

Setting up the hooks

First thing required is to create the three functions as types:

typedef PVOID (NTAPI* _RtlAllocateHeap)
(
    PVOID  HeapHandle,
    ULONG  Flags,
    SIZE_T Size
);

typedef PVOID (NTAPI* _RtlReAllocateHeap)
(
    PVOID HeapHandle,
    ULONG Flags,
    PVOID MemoryPointer,
    ULONG Size
);

typedef BOOLEAN (NTAPI* _RtlFreeHeap)
(
    PVOID HeapHandle,
    ULONG Flags,
    PVOID BaseAddress
);

Then, three functions that will be used to replace the functionality once hooked:

PVOID NTAPI RtlAllocateHeapHook(PVOID HeapHandle, ULONG Flags, SIZE_T Size)
{
    return pRtlAllocateHeap(HeapHandle, Flags, Size);
}
 
PVOID NTAPI RtlReAllocateHeapHook(PVOID HeapHandle, ULONG Flags, PVOID MemoryPointer, ULONG Size)
{
    return pRtlReAllocateHeap(HeapHandle, Flags, MemoryPointer, Size);
}

BOOLEAN NTAPI RtlFreeHeapHook(PVOID HeapHandle, ULONG Flags, PVOID BaseAddress)
{
    return pRtlFreeHeap(HeapHandle, Flags, BaseAddress);
}

Once that is done, minhook needs to be initialised:

MH_STATUS status;

if (MH_Initialize() != MH_OK) {
    return -1;
}

Then place the hooks with MH_CreateHookApi:

status = MH_CreateHookApi(
    L"ntdll",
    "RtlAllocateHeap",
    RtlAllocateHeapHook,
    reinterpret_cast<LPVOID*>(&pRtlAllocateHeap)
);

status = MH_CreateHookApi(
    L"ntdll",
    "RtlReAllocateHeap",
    RtlReAllocateHeapHook,
    reinterpret_cast<LPVOID*>(&pRtlReAllocateHeap)
);

status = MH_CreateHookApi(
    L"ntdll",
    "RtlFreeHeap",
    RtlFreeHeapHook,
    reinterpret_cast<LPVOID*>(&pRtlFreeHeap)
);

One final thing is to create three variables to store the original address:

_RtlAllocateHeap pRtlAllocateHeap = nullptr;
_RtlReAllocateHeap pRtlReAllocateHeap = nullptr;
_RtlFreeHeap pRtlFreeHeap = nullptr;

So now, any call to RtlAllocateHeap will be replaced with RtlAllocateHeapHook and the original RtlAllocateHeap will be stored in pRtlAllocateHeap. Then, enable the hooks:

if (status == MH_OK)
{
    printf("Hooks Enabled!\n");
}
else
{
    printf("Hooks failed to start!\n");
    return -1;
}

Capturing the heap data

In order to do this, a new struct is created:

typedef struct _HEAP
{
    HANDLE hHeap;
    PVOID pAllocated;
    ULONG ulFlags;
} HEAP, *PHEAP;

This will hold:

The handle to the heap
The space allocated by the heap
The flags used to allocate the heap

As this is a POC, a global vector will be used to hold them:

std::vector<HEAP> heaps = {};

Updating the RtlAllocateHeapHook function:

PVOID NTAPI RtlAllocateHeapHook(PVOID HeapHandle, ULONG Flags, SIZE_T Size)
{
    HEAP heap = { 0 };
    heap.hHeap = HeapHandle;
    heap.ulFlags = Flags;
    heap.pAllocated = pRtlAllocateHeap(HeapHandle, Flags, Size);
    heaps.push_back(heap);
    return heap.pAllocated;
}

The original RtlAllocateHeap is used to allocate space and then stored in the struct, this same value is returned from the hook so the function operates as expected. Once that is done, it is added to the vector. However, std::vector is not thread safe, and that push_back will cause a crash.

That part is left as a task for the reader, look at:

These two should sort it out, good luck!

Conclusion

In this short blog, I wanted to take a look at working with the heap from both an allocation and hooking perspective.

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Last updated 1 year ago