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Introduction:
The Tropic Trooper threat group (aka Earth Centaur, Pirate Panda) has launched a new campaign combining a trojanized SumatraPDF reader, a custom AdaptixC2 Beacon listener, and Visual Studio Code tunnels to gain persistent, stealthy remote access. This multi‑stage attack targets Chinese‑speaking individuals in Taiwan, plus victims in South Korea and Japan, using legitimate developer tools to bypass traditional network defenses.
Learning Objectives:
- Analyze the complete attack chain from trojanized PDF to in‑memory AdaptixC2 Beacon execution.
- Detect and block malicious VS Code tunnel usage for C2 communication.
- Implement memory forensics and endpoint hardening against shellcode injection.
You Should Know:
1. Understanding the Tropic Trooper Attack Chain
This campaign starts with a trojanized version of SumatraPDF – a legitimate open‑source PDF reader. When launched, it displays a decoy PDF matching the phishing lure’s theme, while silently downloading and executing shellcode that deploys an AdaptixC2 Beacon agent directly in memory. The attacker then uses VS Code tunnels (normally a remote development feature) to create encrypted, outbound‑only connections that bypass firewalls and appear as benign developer traffic.
Step‑by‑step guide to emulate detection (Linux/macOS):
Monitor process ancestry for suspicious PDF readers ps auxf | grep -i sumatra Check for unusual outbound connections on common VS Code tunnel ports sudo netstat -tunap | grep -E ':(8000|8080|443|22).ESTABLISHED' Simulate shellcode extraction from a suspicious binary (forensic) strings suspicious_sumatra.exe | grep -E 'http|https|shellcode|beacon'
For Windows (PowerShell as Admin):
Get-Process -Name SumatraPDF | Select-Object Id, ParentProcessId, Path
Get-NetTCPConnection -State Established | Where-Object {$<em>.RemotePort -in (8000,8080,443,22)}
Check for VS Code tunnel processes
Get-Process | Where-Object {$</em>.ProcessName -like "code-tunnel" -or $_.ProcessName -like "vscode"}
2. Detecting and Blocking Abused VS Code Tunnels
VS Code tunnels create secure, authenticated websocket connections to .tunnels.visualstudio.com. Attackers use them to relay C2 traffic without opening inbound ports. Defenders must monitor for unauthorized tunnel creation.
Step‑by‑step guide:
- Network detection: Create Snort/Suricata rules to alert on DNS queries or TLS SNI for
.tunnels.visualstudio.com. - Endpoint detection (Windows – group policy): Disable VS Code tunnel service if not required.
List existing tunnels (run as user) code tunnel list Kill all active tunnels code tunnel kill Prevent tunnel creation via registry (Windows) reg add "HKLM\SOFTWARE\Policies\Microsoft\Windows\CloudContent" /v DisableVSCODEtunnels /t REG_DWORD /d 1 /f
- Linux: Remove `code-tunnel` binary or restrict execution via AppArmor/SELinux.
sudo apt remove code-tunnel Debian/Ubuntu sudo setfacl -m u:www-data: /usr/bin/code-tunnel restrict specific users
- Cloud hardening (AWS): Use VPC Flow Logs to detect outbound traffic to Microsoft tunnel domains; create AWS Network Firewall rule group to block
.tunnels.visualstudio.com.
3. Analyzing AdaptixC2 Beacon Traffic
AdaptixC2 is a modular C2 framework using custom beaconing patterns. The Tropic Trooper variant uses encrypted HTTP/HTTPS with JARM fingerprints and specific TLS ciphers.
Step‑by‑step guide to extract indicators:
- PCAP analysis with tshark:
tshark -r capture.pcap -Y "tls.handshake.cipher_suite" -T fields -e ip.src -e ip.dst -e tls.handshake.ciphersuite Look for uncommon cipher suite 0x1301 (TLS_AES_128_GCM_SHA256) used by AdaptixC2
- JARM fingerprinting: Use `jarm-py` to scan suspicious hosts.
git clone https://github.com/salesforce/jarm python3 jarm.py 2>/dev/null | grep "0000000000000000000000000000000" typical AdaptixC2 signature
- YARA rule for memory detection (AdaptixC2 beacon string):
rule AdaptixC2_Beacon { strings: $a = "beacon_id=" ascii wide $b = "AdaptixC2" ascii condition: any of them and filesize < 2MB }
4. Mitigating Trojanized Applications (Application Control)
Attackers distribute trojanized SumatraPDF via phishing or drive‑by downloads. Implement application whitelisting and hash verification.
Step‑by‑step guide:
- Windows AppLocker: Create a rule to only allow signed SumatraPDF from
%ProgramFiles%\SumatraPDF.Generate default rules New-AppLockerPolicy -RuleType Exe -User Everyone -RuleNamePrefix "AllowSignedOnly" -Xml C:\AppLocker.xml Set-AppLockerPolicy -PolicyXml C:\AppLocker.xml -Merge
- Linux (DIGITAL signature verification): Always verify GPG signatures or sha256sums from official repositories.
sha256sum suspicious_installer.exe | grep -i "official_hash_here" Use debian's debsig-verify for .deb packages
- Endpoint Detection & Response (EDR) custom rule: Alert when `SumatraPDF.exe` creates child processes (
cmd.exe,powershell.exe,regsvr32.exe) or writes to `%TEMP%` and then executes.
5. Memory Shellcode Detection & Forensics
The AdaptixC2 Beacon runs entirely in memory without writing to disk. Use memory forensics to detect injection.
Step‑by‑step guide using Volatility 3 (Linux/Windows memory dumps):
Dump memory from running process (Linux) sudo gcore <PID_of_Suspicious_PDF> Analyze with volatility vol3 -f memory.dump windows.malfind.Malfind --pid <PID> Look for PAGE_EXECUTE_READWRITE regions with shellcode patterns vol3 -f memory.dump windows.cmdline.CmdLine
Live detection on Windows with PowerShell:
List processes with executable memory regions not backed by a file
Get-Process | ForEach-Object {
$mod = Get-Process -Id $<em>.Id -Module | Where-Object {$</em>.ModuleName -eq $<em>.ProcessName}
if (-not $mod) { Write-Host "Suspicious memory-only process: $($</em>.Name) PID $($_.Id)" }
}
6. Hardening Remote Access Tools for Enterprise
VS Code tunnels are legitimate; instead of outright blocking, implement conditional access and logging.
Step‑by‑step guide:
- Azure AD / Conditional Access: Require compliant devices and MFA for any VS Code tunnel authentication.
- Microsoft Defender for Endpoint: Create indicator to monitor `vscode-tunnel.exe` network connections.
Add custom detection rule (MDE) New-MdeIndicator -FileHash <hash_of_code_tunnel> -Action AlertAndBlock
- Linux auditd rules to track tunnel execution:
sudo auditctl -w /usr/bin/code-tunnel -p x -k vscode_tunnel_exec sudo ausearch -k vscode_tunnel_exec --format text
- Network segmentation: Isolate developer VMs that require tunnels into a dedicated VLAN with egress filtering.
7. Threat Hunting Queries for AdaptixC2 & Tunnels
Proactively search for the Tropic Trooper campaign using SIEM queries.
Step‑by‑step for Splunk/ELK:
- Find processes with parent SumatraPDF and network connections:
index=windows sourcetype=WinEventLog:Security EventID=4688 (ParentProcess=SumatraPDF.exe) AND ProcessName IN (cmd.exe, powershell.exe, code-tunnel.exe)
- Detect VS Code tunnel authentication logs (Azure AD):
AADNonInteractiveUserSignInLogs | where AppDisplayName contains "Visual Studio Code" | where ResultType == 0
- Linux syslog:
grep "code-tunnel.connected" /var/log/syslog | awk '{print $1,$2,$3,$5,$9}'
What Undercode Say:
- Key Takeaway 1: Tropic Trooper’s use of VS Code tunnels highlights how attackers repurpose legitimate development tools to evade firewalls and EDR. Defenders must extend visibility to outbound TLS tunnels, not just inbound attacks.
- Key Takeaway 2: Memory‑only beacons like AdaptixC2 require behavioral detection (process ancestry, memory region attributes) – traditional file‑based AV is obsolete. Combining JARM fingerprinting, YARA, and anomaly detection on parent‑child relationships is essential.
Analysis: This campaign is a clear evolution of living‑off‑the‑land techniques (LotL). While VS Code tunnels are designed for remote development, they inadvertently create a covert channel. The attacker’s choice of a trojanized PDF reader also shows persistence in social engineering. Organizations should treat any outbound connection to `.tunnels.visualstudio.com` as suspicious unless explicitly approved, and implement strict application control to block unsigned or unexpected binaries.
Prediction:
In the next 6–12 months, similar advanced threat actors will increasingly abuse other development tools (e.g., GitHub Codespaces, JetBrains Projector, Jupyter notebooks) for stealthy C2. We will also see the rise of “tunnel‑as‑a‑service” offerings on the dark web, enabling low‑skill attackers to bypass network controls. Security vendors will respond with ML models that profile normal tunnel traffic vs. beacon patterns, but proactive micro‑segmentation and outbound TLS inspection will become mandatory for regulated industries.
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