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MsBuild Making Network Connections

Identifies MsBuild.exe making outbound network connections. This may indicate adversarial activity as MsBuild is often leveraged by adversaries to execute code and evade detection.

Rule type: eql
Rule indices:

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

Rule Severity: medium
Risk Score: 47
Runs every:
Searches indices from: now-9m
Maximum alerts per execution: ?
References:

Tags:

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

Version: ?
Rule authors:

  • Elastic

Rule license: Elastic License v2

The performance impact of this rule is expected to be low to medium because of the first sequence, which looks for MsBuild.exe process execution. The events for this first sequence may be noisy, consider adding exceptions.

By examining the specific traits of Windows binaries (such as process trees, command lines, network connections, registry modifications, and so on) it's possible to establish a baseline of normal activity. Deviations from this baseline can indicate malicious activity, such as masquerading and deserve further investigation.

The Microsoft Build Engine, also known as MSBuild, is a platform for building applications. This engine provides an XML schema for a project file that controls how the build platform processes and builds software, and can be abused to proxy code execution.

This rule looks for the Msbuild.exe utility execution, followed by a network connection to an external address. Attackers can abuse MsBuild to execute malicious files or masquerade as those utilities in order to bypass detections and evade defenses.

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.

  • Investigate other alerts associated with the user/host during the past 48 hours.
  • 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 any abnormal behavior by the subject process such as network connections, registry or file modifications, and any spawned child processes.
    • Investigate the file digital signature and process original filename, if suspicious, treat it as potential malware.
  • Investigate the target host that the signed binary is communicating with.
    • Check if the domain is newly registered or unexpected.
    • Check the reputation of the domain or IP address.
  • Assess whether this behavior is prevalent in the environment by looking for similar occurrences across hosts.
  • 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_0
      • 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_1
        • $osquery_2
        • $osquery_3
    • 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.
  • If this activity is expected and noisy in your environment, consider adding exceptions — preferably with a combination of destination IP address and command line conditions.
  • 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).
sequence by process.entity_id with maxspan=30s

  /* Look for MSBuild.exe process execution */
  /* The events for this first sequence may be noisy, consider adding exceptions */
  [process where host.os.type == "windows" and event.type == "start" and
    (
      process.pe.original_file_name: "MSBuild.exe" or
      process.name: "MSBuild.exe"
    ) and
    not user.id == "S-1-5-18"]

  /* Followed by a network connection to an external address */
  /* Exclude domains that are known to be benign */
  [network where host.os.type == "windows" and
    event.action: ("connection_attempted", "lookup_requested") and
    (
      process.pe.original_file_name: "MSBuild.exe" or
      process.name: "MSBuild.exe"
    ) and
    not user.id == "S-1-5-18" and
    not cidrmatch(destination.ip, "127.0.0.1", "::1") and
    not dns.question.name : (
      "localhost",
      "dc.services.visualstudio.com",
      "vortex.data.microsoft.com",
      "api.nuget.org")]

Framework: MITRE ATT&CK