APT Overwriting Ntdll Text Sections To Remove EDR Hooks

Advanced Persistent Threat (APT) actors are employing a 15-year-old technique to bypass Endpoint Detection and Response (EDR) systems by overwriting the entire `.text` section of ntdll.dll. This method, though dated, remains effective against modern security solutions.

You Should Know:

How the Attack Works

1. Locating ntdll in Memory:

APTs first identify the base address of `ntdll.dll` in the target process.

Command to list loaded modules in Windows:

Get-Process | Select-Object -ExpandProperty Modules | Where-Object {$_.ModuleName -eq "ntdll.dll"} | Format-Table -AutoSize

2. Overwriting the .text Section:

The `.text` section contains executable code, including EDR hooks.
APTs overwrite this section with clean bytes from disk or a patched version.

Example in C:

include <windows.h>
void OverwriteNtdllTextSection() {
HMODULE hNtdll = GetModuleHandleA("ntdll.dll");
DWORD oldProtect;
VirtualProtect(hNtdll, 0x1000, PAGE_EXECUTE_READWRITE, &oldProtect);
// Overwrite with legitimate bytes (simplified)
memcpy(hNtdll, clean_ntdll_bytes, sizeof(clean_ntdll_bytes));
}

3. Bypassing EDR:

Since EDR relies on hooks in `ntdll` (e.g., NtReadVirtualMemory), overwriting them neutralizes monitoring.

Detection & Mitigation

Detecting Unusual ntdll Modifications:

Check for modified ntdll sections
(Get-Process).Modules | Where-Object { $<em>.ModuleName -eq "ntdll.dll" } | ForEach-Object {
$onDiskHash = (Get-FileHash $</em>.FileName).Hash
$memHash = (Get-ProcessMemoryHash -PID $<em>.Process.Id -ModuleBase $</em>.BaseAddress).Hash
if ($onDiskHash -ne $memHash) { Write-Warning "ntdll modified in PID $($_.Process.Id)" }
}

Blocking Unauthorized Memory Writes:

Enable Microsoft Attack Surface Reduction (ASR) rules:

Set-MpPreference -AttackSurfaceReductionRules_Ids "56a863a9-875e-4185-98a7-b882c64b5ce5" -AttackSurfaceReductionRules_Actions Enabled

Linux Equivalent (ELF Hooking Detection)

Check for modified `.text` sections in critical binaries:

Compare running libc with disk version
diff <(objdump -d /proc/$(pidof bash)/maps | grep libc) <(objdump -d /lib/x86_64-linux-gnu/libc.so.6)

What Undercode Say

This technique highlights the persistence of legacy attack methods in modern cyber warfare. Defenders must:
– Monitor critical DLL memory integrity.
– Deploy kernel-mode EDR (e.g., Microsoft Defender ATP).
– Use Code Integrity Guards (CI.dll policies).
– Hunt for direct syscalls (bypassing `ntdll` entirely).