Vectored Overloading Pe Injection

Vectored Overloading PE Injection

A Windows PE injection primitive that fuses classic Module Overloading with Vectored Exception Handlers (VEHs) and hardware breakpoints to disguise malicious code as legitimate DLLs while bypassing kernel-level telemetry.

Technique Overview

Instead of patching LoadLibrary or writing a custom loader, this technique:

1. Creates a SEC_IMAGE section backed by a legitimate DLL (e.g., wmp.dll)
2. Overwrites the mapped view with a fully relocated malicious PE while keeping the section object pointing to the benign image on disk
3. Registers a VEH and programs debug registers so every call to NtOpenSection, NtMapViewOfSection, and optionally NtClose raises a user-mode breakpoint
4. Calls LoadLibrary("amsi.dll") (or any other benign target). When the Windows loader invokes those syscalls, the VEH skips the kernel transition and returns the handles and base addresses of the prepared malicious image

Key advantage: Tooling that only looks at section backing files sees the legitimate DLL (e.g., wmp.dll) even though memory contains the attacker's payload. Imports/TLS callbacks are still resolved by the genuine loader, reducing custom PE-parsing logic.

Stage 1 – Build the Disguised Section

Step 1: Create and map a section for the decoy DLL

HANDLE DecoySection;
PVOID DecoyView;
SIZE_T DecoySize;

NtCreateSection(&DecoySection, SECTION_ALL_ACCESS, NULL,
                NULL, 0, PAGE_READWRITE, SEC_IMAGE, L"\\??\\C:\\Windows\\System32\\wmp.dll");
NtMapViewOfSection(DecoySection, GetCurrentProcess(), &DecoyView, 0, 0,
                   NULL, &DecoySize, ViewShare, 0, PAGE_READWRITE);

Step 2: Copy the malicious PE into that view

Copy section by section, honoring SizeOfRawData/VirtualSize and updating protections afterwards (PAGE_EXECUTE_READ, PAGE_READWRITE, etc.).

Step 3: Apply relocations and resolve imports

Apply relocations and resolve imports exactly as a reflective loader would. Because the view is already mapped as SEC_IMAGE, section alignments and guard pages match what the Windows loader expects later.

Step 4: Normalize the PE header

• If the payload is an EXE, set IMAGE_FILE_HEADER.Characteristics |= IMAGE_FILE_DLL and zero the entry point to keep LdrpCallTlsInitializers from jumping into EXE-specific stubs
• DLL payloads can keep their headers unchanged

Result: The process owns a RWX-capable view whose backing object is still wmp.dll, yet the bytes in memory are attacker-controlled.

Stage 2 – Hijack the Loader with VEHs

Step 1: Register a VEH and arm hardware breakpoints

Program Dr0 (or another debug register) with the address of ntdll!NtOpenSection and set DR7 so every execution raises STATUS_SINGLE_STEP. Repeat later for NtMapViewOfSection and optionally NtClose.

Minimal HWBP setup (x64):

CONTEXT ctx = {0};
ctx.ContextFlags = CONTEXT_DEBUG_REGISTERS;
GetThreadContext(GetCurrentThread(), &ctx);
ctx.Dr0 = (DWORD64)NtOpenSection;
ctx.Dr7 = 1;  // Enable breakpoint on Dr0
SetThreadContext(GetCurrentThread(), &ctx);
AddVectoredExceptionHandler(1, VehHandler);

Step 2: Trigger DLL loading

Call LoadLibrary("amsi.dll"). LdrLoadDll will eventually call NtOpenSection to obtain the real section handle.

Step 3: VEH hook for NtOpenSection

LONG WINAPI VehHandler(EXCEPTION_POINTERS* ExceptionInfo) {
    if (ExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_SINGLE_STEP) {
        // Locate the stack slot for the [out] PHANDLE SectionHandle argument
        // Write the previously created DecoySection handle into that slot
        // Advance RIP/EIP to the ret instruction so the kernel is never called
        // Re-arm the hardware breakpoint to watch NtMapViewOfSection next
        return EXCEPTION_CONTINUE_EXECUTION;
    }
    return EXCEPTION_CONTINUE_SEARCH;
}

Step 4: VEH hook for NtMapViewOfSection

• Overwrite the [out] PVOID *BaseAddress (and size/protection outputs) with the address of the already mapped malicious view
• Skip the syscall body just like before

Step 5: (Optional) VEH hook for NtClose

Verify that the fake section handle is cleaned up, preventing resource leaks and providing a final sanity check.

Why this works: Because the syscalls are never executed, kernel callbacks (ETWti, minifilter, etc.) do not observe the suspicious NtOpenSection/NtMapViewOfSection events, drastically lowering telemetry. From the loader's point of view everything succeeded and amsi.dll is in memory, so it proceeds with import/TLS resolution against the attacker's bytes.

Implementation Notes

Hardware Breakpoints are Per-Thread

The VEH must run on the same thread that triggers the loader. Set CONTEXT_DEBUG_REGISTERS on the current thread before calling LoadLibrary.

Syscall Emulation

The VEH sets RAX = 0 and advances RIP to the ret inside the ntdll stub (scan for 0xC3) so the kernel transition never happens, then resumes with NtContinue.

Output Parameters

For NtMapViewOfSection, the VEH overwrites the returned BaseAddress, ViewSize, and Win32Protect outputs so the loader believes the mapping succeeded and continues with imports/TLS using the attacker's view.

Stealth Variation

Recent VEH research highlights that handlers can be registered by manually manipulating the VEH list instead of calling AddVectoredExceptionHandler, which reduces reliance on user-mode APIs that may be monitored or hooked. This is not required for Vectored Overloading but can be combined with it to reduce observable API activity.

Stage 3 – Execute the Payload

EXE Payload

The injector simply jumps to the original entry point once relocations are done. When the loader thinks it would call DllMain, the custom code instead executes the EXE-style entry.

DLL Payload / Node.js Addon

Resolve and call the intended export. Because the module is already registered with LdrpModuleBaseAddressIndex, subsequent lookups see it as the benign DLL.

When combined with a Node.js native addon (.node file), all of the Windows-internals heavy lifting stays outside the JavaScript layer.

Detection Considerations

Indicators to Monitor

1. Hardware breakpoint usage: Monitor SetThreadContext calls with CONTEXT_DEBUG_REGISTERS
2. VEH registration: Track AddVectoredExceptionHandler calls, especially before LoadLibrary
3. Section mapping anomalies: NtMapViewOfSection returning addresses that don't match expected DLL bases
4. Syscall skipping: Unusual patterns where syscalls appear to complete without kernel transitions
5. PE header modifications: EXE files with IMAGE_FILE_DLL characteristics set

Evasion Challenges

• Manual VEH list manipulation can hide handler registration
• Hardware breakpoints are per-thread, making them harder to detect globally
• The technique uses legitimate Windows APIs, making behavioral detection difficult

References

• Check Point Research – GachiLoader: Defeating Node.js Malware with API Tracing
• VectoredOverloading – PoC implementation
• IBM X-Force – You just got vectored: Using VEH for defense evasion and process injection
• Module Overloading

Safety Notice

This technique is for security research and defensive purposes only. Use only in controlled environments with proper authorization. Unauthorized use of injection techniques may violate laws and terms of service.