Detect Fallback Channels in Microsoft Sentinel
Adversaries may use fallback or alternate communication channels if the primary channel is compromised or inaccessible in order to maintain reliable command and control and to avoid data transfer thresholds. Malware families such as HOPLIGHT, InvisiMole, TrickBot, and BISCUIT implement hard-coded primary and secondary C2 addresses, while others like OilRig's ISMAgent dynamically fall back from HTTP to DNS tunneling. Detection focuses on processes establishing connections to multiple distinct external destinations in sequence — particularly where port diversity (80→443→8080) or protocol switching (HTTP→DNS) is observed — which is anomalous for non-browser processes.
MITRE ATT&CK
- Tactic
- Command and Control
- Technique
- T1008 Fallback Channels
- Canonical reference
- https://attack.mitre.org/techniques/T1008/
KQL Detection Query
// Primary detection: non-browser processes connecting to 3+ distinct external IPs
// or 3+ distinct ports within a 1-hour window, indicating fallback C2 behavior
let ExcludedProcesses = dynamic([
"chrome.exe", "firefox.exe", "msedge.exe", "iexplore.exe", "opera.exe", "brave.exe",
"MicrosoftEdgeUpdate.exe", "MsMpEng.exe", "OneDrive.exe", "Teams.exe", "Slack.exe",
"Zoom.exe", "Skype.exe", "outlook.exe", "lync.exe", "SearchApp.exe",
"msedgewebview2.exe", "WINWORD.EXE", "EXCEL.EXE", "POWERPNT.EXE"
]);
let C2FallbackPorts = dynamic([53, 80, 443, 4443, 8080, 8443, 8888, 1194, 4444, 9443, 2222, 3128]);
DeviceNetworkEvents
| where Timestamp > ago(24h)
| where RemoteIPType == "Public"
| where RemotePort in (C2FallbackPorts)
| where not(InitiatingProcessFileName has_any (ExcludedProcesses))
| summarize
UniqueDestIPs = dcount(RemoteIP),
UniqueDestPorts = dcount(RemotePort),
TotalConnections = count(),
DestinationIPs = make_set(RemoteIP, 10),
DestinationPorts = make_set(RemotePort, 10),
FirstSeen = min(Timestamp),
LastSeen = max(Timestamp)
by DeviceName, AccountName, InitiatingProcessFileName, InitiatingProcessCommandLine,
InitiatingProcessId, InitiatingProcessParentFileName, bin(Timestamp, 1h)
| where UniqueDestIPs >= 3 or (UniqueDestPorts >= 3 and TotalConnections >= 5)
| extend ConnectionSpanMinutes = datetime_diff('minute', LastSeen, FirstSeen)
| extend RiskScore = case(
UniqueDestIPs >= 5 and UniqueDestPorts >= 3, "Critical",
UniqueDestIPs >= 4 or (UniqueDestPorts >= 3 and TotalConnections >= 8), "High",
UniqueDestIPs >= 3, "Medium",
"Low"
)
| project FirstSeen, LastSeen, ConnectionSpanMinutes, DeviceName, AccountName,
InitiatingProcessFileName, InitiatingProcessCommandLine, InitiatingProcessParentFileName,
UniqueDestIPs, UniqueDestPorts, TotalConnections, DestinationIPs, DestinationPorts, RiskScore
| sort by UniqueDestIPs desc, UniqueDestPorts descDetects potential C2 fallback channel behavior using Microsoft Defender for Endpoint DeviceNetworkEvents. Identifies non-browser processes that connect to three or more distinct external IP addresses, or use three or more distinct ports within a one-hour window. This pattern is characteristic of malware that hard-codes multiple C2 servers (BISCUIT, HOPLIGHT, SslMM) or dynamically tries alternate ports (S-Type: 80→443→8080) when the primary channel fails. Browser and known-legitimate processes are excluded. A risk score is assigned based on the breadth of destination diversity.
Data Sources
Required Tables
False Positives & Tuning
- Software update clients and package managers (e.g., Windows Update components, npm, pip) that contact multiple CDN endpoints or mirror servers during downloads
- IT monitoring and management agents (SCCM, Qualys, Tenable) that beacon to multiple management servers or cloud endpoints
- Backup agents and cloud sync clients (Veeam, Backblaze, Crashplan) contacting multiple storage endpoints
- Custom business applications with built-in load-balancing or geographic failover logic connecting to multiple cloud provider IPs
- Security scanning tools and vulnerability assessment agents that make broad outbound connections as part of their normal operation
Other platforms for T1008
Testing Methodology
Validate this detection against 4 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 1Sequential HTTP Fallback Simulation (Windows)
Expected signal: Sysmon Event ID 3: Three sequential network connection events from powershell.exe to 192.0.2.10:80, 192.0.2.11:443, and 192.0.2.12:8080 within seconds of each other. Sysmon Event ID 1: Process creation for powershell.exe with Net.WebClient in the command line. All three connections will fail (no listener), but Sysmon logs all outbound connection attempts.
- Test 2DNS Fallback Simulation After HTTP Failure (Linux/macOS)
Expected signal: Syslog/auditd: curl process creation with failed connections to 192.0.2.50 and 192.0.2.51. dig process creation events for 20 sequential DNS queries to 8.8.8.8 (external resolver). If Sysmon for Linux is deployed: Event ID 3 for curl network connections and dig DNS queries. Network capture shows failed TCP SYN to RFC 5737 IPs followed by UDP/53 query burst to 8.8.8.8.
- Test 3Multi-Port C2 Fallback via Netcat (Windows)
Expected signal: Sysmon Event ID 3: Three network connection events from powershell.exe to 192.0.2.100 on ports 80, 443, and 8080. Connections will time out (no listener). Sysmon Event ID 1: Process creation with TcpClient and multiple ports visible in command line.
- Test 4Proxy-Aware Fallback (JHUHUGIT Pattern, Windows)
Expected signal: Sysmon Event ID 1: powershell.exe with registry access command in arguments. Sysmon Event ID 3: Two outbound network connections — first to 192.0.2.200:443, then to 192.0.2.201:8080. Sysmon Event ID 12/13: Registry read from HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings (proxy settings access). Security Event ID 4663 if object access auditing is enabled.
References (10)
- https://attack.mitre.org/techniques/T1008/
- https://www.mandiant.com/resources/apt1-exposing-one-of-chinas-cyber-espionage-units
- https://www.welivesecurity.com/2017/05/09/sednit-adds-two-zero-day-exploits-using-trump-decoy/
- https://us-cert.cisa.gov/ncas/analysis-reports/AR19-100A
- https://researchcenter.paloaltonetworks.com/2017/07/unit42-oilrig-uses-ismdoor-variant-possibly-linked-greenbug-threat-group/
- https://www.welivesecurity.com/wp-content/uploads/2018/06/ESET_InvisiMole.pdf
- https://www.welivesecurity.com/2017/10/30/ebury-is-alive-but-unseen/
- https://learn.microsoft.com/en-us/defender-endpoint/advanced-hunting-devicenetworkevents-table
- https://docs.splunk.com/Documentation/SplunkCloud/latest/SearchReference/CommonStatsFunctions
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1008/T1008.md
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