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SMB Connections via LOLBin or Untrusted Process

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SMB Connections via LOLBin or Untrusted Process

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Identifies potentially suspicious processes that are not trusted or living-off-the-land binaries (LOLBin) making Server Message Block (SMB) network connections over port 445. Windows File Sharing is typically implemented over SMB, which communicates between hosts using port 445. Legitimate connections are generally established by the kernel (PID 4). This rule helps to detect processes that might be port scanners, exploits, or user-level processes attempting lateral movement within the network by leveraging SMB connections.

Rule type: eql

Rule indices:

  • logs-endpoint.events.process-*
  • logs-endpoint.events.network-*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Domain: Endpoint
  • OS: Windows
  • Use Case: Threat Detection
  • Tactic: Lateral Movement
  • Resources: Investigation Guide
  • Data Source: Elastic Defend

Version: 112

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Performance

This rule may have low to medium performance impact due to filtering for LOLBins processes starting, followed by network connections over port 445. Additional filtering is applied to reduce the volume of matching events and improve performance.

Investigating Untrusted Non-Microsoft or LOLBin SMB Connections

This rule looks for unexpected processes or LOLBins making network connections over port 445. Windows file sharing is typically implemented over Server Message Block (SMB), which communicates between hosts using port 445. When legitimate, these network connections are established by the kernel (PID 4). Occurrences of non-system processes using this port can indicate port scanners, exploits, and tools used to move laterally on the environment.

Note: This investigation guide uses the Osquery Markdown Plugin introduced in Elastic Stack version 8.5.0. Older Elastic Stack versions will display unrendered Markdown in this guide.

Possible investigation steps

  • Investigate the process execution chain (parent process tree) for unknown processes. Examine their executable files for prevalence, whether they are located in expected locations, and if they are signed with valid digital signatures.
  • Investigate other alerts associated with the user/host during the past 48 hours.
  • Contact the account owner and confirm whether they are aware of this activity.
  • Investigate any abnormal behavior by the subject process such as network connections, registry or file modifications, and any spawned child processes.
  • Examine the host for derived artifacts that indicate suspicious activities:
  • Analyze the process executable using a private sandboxed analysis system.
  • Observe and collect information about the following activities in both the sandbox and the alert subject host:
  • Attempts to contact external domains and addresses.
  • Use the Elastic Defend network events to determine domains and addresses contacted by the subject process by filtering by the process' process.entity_id.
  • Examine the DNS cache for suspicious or anomalous entries.
  • !{osquery{"label":"Osquery - Retrieve DNS Cache","query":"SELECT * FROM dns_cache"}}
  • Use the Elastic Defend registry events to examine registry keys accessed, modified, or created by the related processes in the process tree.
  • Examine the host services for suspicious or anomalous entries.
  • !{osquery{"label":"Osquery - Retrieve All Services","query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services"}}
  • !{osquery{"label":"Osquery - Retrieve Services Running on User Accounts","query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services WHERE\nNOT (user_account LIKE %LocalSystem OR user_account LIKE %LocalService OR user_account LIKE %NetworkService OR\nuser_account == null)\n"}}
  • !{osquery{"label":"Osquery - Retrieve Service Unsigned Executables with Virustotal Link","query":"SELECT concat(https://www.virustotal.com/gui/file/, sha1) AS VtLink, name, description, start_type, status, pid,\nservices.path FROM services JOIN authenticode ON services.path = authenticode.path OR services.module_path =\nauthenticode.path JOIN hash ON services.path = hash.path WHERE authenticode.result != trusted\n"}}
  • Retrieve the files' SHA-256 hash values using the PowerShell Get-FileHash cmdlet and search for the existence and reputation of the hashes in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.
  • Investigate potentially compromised accounts. Analysts can do this by searching for login events (for example, 4624) to the target host after the registry modification.

False positive analysis

  • If this rule is noisy in your environment due to expected activity, consider adding exceptions — preferably with a combination of user and command line conditions.
  • In hybrid environments, SMB may be used for legitimate purposes if operations are performed in Azure. In such cases, consider adding exceptions for known Azure services and operations.

Response and remediation

  • Initiate the incident response process based on the outcome of the triage.
  • Isolate the involved host to prevent further post-compromise behavior.
  • If the triage identified malware, search the environment for additional compromised hosts.
  • Implement temporary network rules, procedures, and segmentation to contain the malware.
  • Stop suspicious processes.
  • Immediately block the identified indicators of compromise (IoCs).
  • Inspect the affected systems for additional malware backdoors like reverse shells, reverse proxies, or droppers that attackers could use to reinfect the system.
  • Remove and block malicious artifacts identified during triage.
  • Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and malware components.
  • Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
  • Using the incident response data, update logging and audit policies to improve the mean time to detect (MTTD) and the mean time to respond (MTTR).

Rule query

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sequence by process.entity_id with maxspan=1m

  /* first sequence to capture the start of Windows processes */
  [process where host.os.type == "windows" and event.type == "start" and process.pid != 4 and

    /* ignore NT Authority and Network Service accounts */
    not user.id : ("S-1-5-19", "S-1-5-20") and

    /* filter out anything trusted but not from Microsoft */
    /* LOLBins will be inherently trusted and signed, so ignore everything else trusted */
    not (process.code_signature.trusted == true and not startsWith(process.code_signature.subject_name, "Microsoft")) and

    /* filter out PowerShell scripts from Windows Defender ATP */
    not (
      process.name : "powershell.exe" and
      process.args :"?:\\ProgramData\\Microsoft\\Windows Defender Advanced Threat Protection\\Downloads\\PSScript_*.ps1")]

  /* second sequence to capture network connections over port 445 related to SMB */
  [network where host.os.type == "windows" and destination.port == 445 and process.pid != 4]

/* end the sequence when the process ends where joining was on process.entity_id */
until [process where host.os.type == "windows" and event.type == "end"]

Framework: MITRE ATT&CKTM

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