Detect Internal Proxy in Splunk
Adversaries may use an internal proxy to direct command and control traffic between two or more systems in a compromised environment. Tools such as HTRAN, ZXProxy, ZXPortMap, and Cobalt Strike's peer-to-peer beacon mode enable traffic redirection through proxies or port forwarding. Adversaries use internal proxies to manage C2 communications inside a compromised environment, reduce the number of simultaneous outbound connections, provide resiliency, or ride over existing trusted communications paths between infected systems. Internal proxy connections may use common protocols such as SMB to blend in with normal traffic.
MITRE ATT&CK
- Tactic
- Command and Control
- Technique
- T1090 Proxy
- Sub-technique
- T1090.001 Internal Proxy
- Canonical reference
- https://attack.mitre.org/techniques/T1090/001/
SPL Detection Query
index=wineventlog (sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1)
OR (sourcetype="WinEventLog:Security" EventCode=4688)
| eval Image=coalesce(Image, NewProcessName)
| eval CommandLine=coalesce(CommandLine, ProcessCommandLine, CommandLine)
| eval CommandLineLower=lower(CommandLine)
| eval ImageLower=lower(Image)
| eval IsProxyTool=if(match(ImageLower, "(htran|zxproxy|lcx\.exe|socat|chisel|ligolo|frpc|frps|earthworm|ew\.exe|gost|iox|revsocks|venom)") OR match(CommandLineLower, "(htran|zxproxy|lcx|earthworm|chisel|ligolo|frpc|frps|gost|iox|revsocks|pivotnacci|rpivot)"), 1, 0)
| eval IsNetshProxy=if(match(ImageLower, "netsh\.exe") AND match(CommandLineLower, "portproxy") AND match(CommandLineLower, "add"), 1, 0)
| eval IsSSHTunnel=if(match(ImageLower, "(ssh\.exe|plink\.exe)") AND match(CommandLineLower, "(\s-[lrd]\s|localforward|remoteforward|gatewayports|dynamicforward|-w \d)"), 1, 0)
| eval IsPortForwardCmd=if(match(CommandLineLower, "(v4tov4|listenport=|connectport=|connectaddress=|listenaddress=)"), 1, 0)
| eval SuspicionScore=IsProxyTool + IsNetshProxy + IsSSHTunnel + IsPortForwardCmd
| where SuspicionScore > 0
| eval DetectionType=case(
IsProxyTool=1, "KnownProxyTool",
IsNetshProxy=1, "NetshPortProxy",
IsSSHTunnel=1, "SSHTunneling",
IsPortForwardCmd=1, "PortForwardCommand",
true(), "Unknown"
)
| eval ParentProcess=coalesce(ParentImage, ParentCommandLine, "unknown")
| table _time, host, User, Image, CommandLine, ParentProcess, DetectionType, SuspicionScore, IsProxyTool, IsNetshProxy, IsSSHTunnel, IsPortForwardCmd
| sort - _timeDetects internal proxy and traffic pivoting activity using Sysmon Event ID 1 (Process Create) and Security Event ID 4688. Evaluates process names and command lines against known proxy tool names, netsh portproxy add commands, SSH tunneling flags, and explicit port-forwarding parameter patterns. Assigns a suspicion score across four detection categories. Supports both Sysmon and native Windows Security auditing log sources via coalesce.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate use of netsh portproxy by IT/network teams to redirect traffic for lab or testing environments
- SSH port forwarding by developers or DevOps teams for legitimate access to internal services
- Authorized penetration testing or red team exercises using proxy/pivoting tools
- Network monitoring tools that use SOCKS proxies or port redirection for traffic analysis
- VPN clients and zero-trust network access agents that establish internal proxy connections
Other platforms for T1090.001
Testing Methodology
Validate this detection against 5 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Netsh PortProxy Rule Creation for Internal Traffic Forwarding
Expected signal: Sysmon Event ID 1: Process Create with Image=netsh.exe, CommandLine containing 'portproxy add v4tov4 listenport=8080'. Security Event ID 4688 (with command line auditing). Sysmon Event ID 13: Registry value set in HKLM\SYSTEM\CurrentControlSet\Services\PortProxy\v4tov4\tcp. Windows Firewall may log new listening port.
- Test 2SSH Local Port Forwarding for Internal Proxy Pivot
Expected signal: Sysmon Event ID 1: Process Create with Image=ssh.exe, CommandLine containing '-L 9090:192.168.1.50:445'. Sysmon Event ID 3: Network Connection attempt from ssh.exe to 10.0.0.1:22. Security Event ID 4688 if command line auditing is enabled.
- Test 3Chisel Reverse Proxy Tool Execution
Expected signal: Sysmon Event ID 1: Process Create with Image=chisel.exe (or renamed binary), CommandLine containing 'client' and 'R:socks' or 'socks'. Sysmon Event ID 3: Network Connection attempt from chisel.exe to 10.0.0.99:8443. File hash of chisel.exe will match known threat intelligence indicators.
- Test 4HTRAN-Style Port Forwarding with Netcat
Expected signal: Sysmon Event ID 1: Process Create for cmd.exe spawning nc.exe with '-l -p 4444 -e cmd.exe'. Sysmon Event ID 3: Network Connection showing nc.exe listening on 0.0.0.0:4444. Windows Firewall event for new inbound rule exception (if UAC prompt accepted).
- Test 5Linux SOCKS Proxy via SSH Dynamic Forwarding
Expected signal: Auditd syscall logs showing execve for ssh with -D 1080 arguments. Syslog entry for ssh process. Network connection attempt to 192.168.1.10:22. Process listing shows ssh with -D flag in /proc/<pid>/cmdline.
References (13)
- https://attack.mitre.org/techniques/T1090/001/
- http://blog.trendmicro.com/trendlabs-security-intelligence/in-depth-look-apt-attack-tools-of-the-trade/
- https://sygnia.co/blog/velvet-ant-espionage-campaign/
- https://research.checkpoint.com/2020/pay2key-ransomware-a-new-campaign-by-fox-kitten/
- https://media.defense.gov/2020/Aug/13/2002476465/-1/-1/0/CSA_DROVORUB_RUSSIAN_GRU_MALWARE_AUG_2020.PDF
- https://www.cobaltstrike.com/blog/cobalt-strike-peer-to-peer-c2/
- https://github.com/jpillora/chisel
- https://github.com/fatedier/frp
- https://github.com/nicocha30/ligolo-ng
- https://docs.microsoft.com/en-us/windows-server/networking/technologies/netsh/netsh-interface-portproxy
- https://www.ired.team/offensive-security/lateral-movement/ssh-tunnelling-port-forwarding
- https://sygnia.co/blog/elephant-beetle-uncovering-a-years-long-espionage-campaign/
- https://www.secureworks.com/research/htran
Unlock Pro Content
Get the full detection package for T1090.001 including response playbook, investigation guide, and atomic red team tests.