Detect VNC in CrowdStrike LogScale
Adversaries may use Valid Accounts to remotely control machines using Virtual Network Computing (VNC). VNC uses the Remote Framebuffer (RFB) protocol to relay screen, mouse, and keyboard inputs over the network. Unlike RDP, VNC provides screen-sharing rather than resource-sharing, making it useful for interactive control. Threat actors including Gamaredon Group, FIN7, and APT groups have used VNC tools including UltraVNC, TightVNC, TigerVNC, and RealVNC for lateral movement and remote access. VNC communicates on TCP port 5900+ by default and can be used with or without password authentication, with some implementations historically vulnerable to authentication bypasses.
MITRE ATT&CK
- Tactic
- Lateral Movement
- Technique
- T1021 Remote Services
- Sub-technique
- T1021.005 VNC
- Canonical reference
- https://attack.mitre.org/techniques/T1021/005/
LogScale Detection Query
#event_simpleName = /^(ProcessRollup2|SyntheticProcessRollup2|NetworkConnectIP4|NetworkConnectIP6|RegistryOperationCreate|RegistryOperationUpdate|RegGenericValueUpdate)$/
| FileName = /(?i)(vncserver|vncviewer|vncconfig|vnc4server|x0vncserver|tightvncserver|tightvncviewer|ultravnc|uvnc_service|tvnserver|tvnviewer|winvnc|winvnc4|rfbdrv)\.exe/
OR (RemotePort >= 5900 AND RemotePort <= 5910)
OR RegObjectName = /(?i)(RealVNC|TightVNC|UltraVNC|TigerVNC|WinVNC|RFBDRV)/
| AlertType := if(
#event_simpleName = /ProcessRollup2|SyntheticProcessRollup2/,
"VNC_ProcessExecution",
if(
#event_simpleName = /NetworkConnectIP/,
"VNC_NetworkConnection",
if(
#event_simpleName = /Registry/,
"VNC_RegistryChange",
"VNC_Other"
)
)
)
| IsVncServer := if(
FileName = /(?i)(vncserver|tvnserver|winvnc|uvnc_service|rfbdrv)\.exe/,
true,
false
)
| IsVncClient := if(
FileName = /(?i)(vncviewer|tvnviewer)\.exe/,
true,
false
)
| table(
[_time, ComputerName, UserName, UserSid, FileName, ImageFileName,
CommandLine, ParentProcessId, RemoteIP, RemotePort, LocalPort,
RegObjectName, RegValueName, AlertType, IsVncServer, IsVncClient]
)
| sort(field=_time, order=desc)Detects VNC-related activity in CrowdStrike Falcon LogScale using Falcon telemetry event types: ProcessRollup2 and SyntheticProcessRollup2 for VNC binary execution (matching FileName against known VNC process names), NetworkConnectIP4/IPv6 for TCP connections to remote ports 5900-5910, and registry operation events (RegistryOperationCreate, RegistryOperationUpdate, RegGenericValueUpdate) for VNC software registry key activity. Enriches each match with IsVncServer and IsVncClient classifications. Uses standard Falcon EDR fields: ComputerName, UserName, FileName, ImageFileName, CommandLine, RemoteIP, RemotePort, and RegObjectName.
Data Sources
Required Tables
False Positives & Tuning
- Falcon-managed endpoints with approved VNC deployment for helpdesk support where the VNC process SHA256 hashes are added to the Falcon allowlist and RemoteIP values correspond to documented support infrastructure
- DevOps and QA automation pipelines executing acceptance tests via VNC-connected virtual machines — typically identifiable by machine accounts, consistent CommandLine patterns, and connections from known CI/CD agent IPs
- Network monitoring or asset discovery agents that include port 5900 in TCP service probes — identify by consistent RemotePort=5900 from a single source ComputerName with high connection volume but no matching ProcessRollup2 VNC binary events
Other platforms for T1021.005
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 1Install TightVNC Server as Windows Service
Expected signal: Sysmon Event ID 1: tvnserver.exe with -install flag. Windows Security Event ID 7045: new service tvnservice created. Registry key created at HKLM\SOFTWARE\TightVNC. Sysmon Event ID 12 (registry key create) for TightVNC registry entries.
- Test 2VNC Connection to Remote Host via vncviewer
Expected signal: Sysmon Event ID 1: tvnviewer.exe process creation with target IP. Sysmon Event ID 3: outbound TCP connection to 127.0.0.1:5900. Windows firewall log entry for port 5900 connection.
- Test 3Configure UltraVNC with No Authentication (Backdoor Setup)
Expected signal: Sysmon Event ID 13 (Registry Value Set) for HKLM\SOFTWARE\UltraVNC\Password and SecurityIdentifier. Security Event ID 4657 (registry value modified). Parent process visible as cmd.exe.
- Test 4Start VNC Server on Non-Standard Port
Expected signal: Linux auditd EXECVE for vncserver with -rfbport 5901 and -SecurityTypes None. Network socket opened on port 5901. Process creation for Xvnc child process.
References (7)
- https://attack.mitre.org/techniques/T1021/005/
- https://www.tightvnc.com/
- https://www.realvnc.com/en/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1021.005/T1021.005.md
- https://symantec-enterprise-blogs.security.com/blogs/threat-intelligence/shuckworm-ukraine-gamaredon
- https://www.crowdstrike.com/blog/carbon-spider-embraces-big-game-hunting/
- https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml
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