Detect Fallback Channels in Google Chronicle
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/
YARA-L Detection Query
rule t1008_fallback_channels_multi_dest {
meta:
author = "Argus Detection Engineering"
description = "Detects non-browser processes connecting to 3+ distinct external IPs or 3+ distinct ports on C2 fallback ports within 1 hour — indicative of T1008 Fallback Channels behavior."
mitre_attack_tactic = "Command and Control"
mitre_attack_technique = "T1008"
severity = "HIGH"
confidence = "MEDIUM"
events:
$e.metadata.event_type = "NETWORK_CONNECTION"
$e.network.direction = "OUTBOUND"
$e.target.port in (53, 80, 443, 4443, 8080, 8443, 8888, 1194, 4444, 9443, 2222, 3128)
not $e.target.ip_subnet = "10.0.0.0/8"
not $e.target.ip_subnet = "172.16.0.0/12"
not $e.target.ip_subnet = "192.168.0.0/16"
not $e.target.ip_subnet = "127.0.0.0/8"
not $e.target.ip_subnet = "169.254.0.0/16"
not $e.principal.process.file.full_path = /(?i)(chrome|firefox|msedge|iexplore|Teams|Slack|Zoom|outlook|MsMpEng|OneDrive|brave|SearchApp|msedgewebview2)\.exe$/
$host = $e.principal.hostname
$proc = $e.principal.process.file.full_path
$dest_ip = $e.target.ip
$dest_port = $e.target.port
match:
$host, $proc over 1h
outcome:
$unique_dest_ips = count_distinct($dest_ip)
$unique_dest_ports = count_distinct($dest_port)
$total_connections = count()
$risk_score = if(
$unique_dest_ips >= 5 and $unique_dest_ports >= 3, "Critical",
if($unique_dest_ips >= 4 or ($unique_dest_ports >= 3 and $total_connections >= 8), "High",
if($unique_dest_ips >= 3, "Medium", "Low")))
condition:
$e and ($unique_dest_ips >= 3 or ($unique_dest_ports >= 3 and $total_connections >= 5))
}Chronicle YARA-L 2.0 rule detecting T1008 Fallback Channels. Matches outbound network connections from non-browser processes to known C2 fallback ports, grouped by host and process over 1-hour windows. Fires when 3+ distinct external IPs or 3+ distinct ports are contacted, with risk scoring aligned to the KQL/SPL reference logic.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate software that performs load balancing by rotating through multiple server IPs (e.g., cloud-native apps using service mesh)
- Patch management tools contacting multiple Windows Update or vendor distribution servers during patch cycles
- Helpdesk or remote access tools (e.g., ConnectWise, TeamViewer) with multi-region relay infrastructure may trigger port diversity conditions
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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