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Network Connection via Certutil

Elastic Stack Serverless Security

Identifies certutil.exe making a network connection. Adversaries could abuse certutil.exe to download a certificate, or malware, from a remote URL.

Rule type: eql

Rule indices:

  • winlogbeat-*
  • logs-endpoint.events.*
  • logs-windows.*

Severity: low

Risk score: 21

Runs every: 5m

Searches indices from: now-9m (https://www.elastic.co/guide/en/elasticsearch/reference/current/common-options.html#date-math[Date Math format], see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Elastic
  • Host
  • Windows
  • Threat Detection
  • Command and Control
  • Investigation Guide

Version: 102

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

Attackers can abuse certutil.exe to download malware, offensive security tools, and certificates from external sources
in order to take the next steps in a compromised environment.

This rule looks for network events where certutil.exe contacts IP ranges other than the ones specified in
IANA IPv4 Special-Purpose Address Registry

Note:
This investigation guide uses the Osquery Markdown Plugin introduced in Elastic stack version 8.5.0. Older Elastic stacks versions will see unrendered markdown in this guide.

  • Investigate the script 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.
  • Investigate if the downloaded file was executed.
  • Determine the context in which certutil.exe and the file were run.
  • Examine the host for derived artifacts that indicates suspicious activities:
    • Analyze the downloaded file 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{"query":"SELECT * FROM dns_cache", "label":"Osquery - Retrieve 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{"query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services","label":"Osquery - Retrieve All Services"}}
        • !{osquery{"query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services WHERE NOT (user_account LIKE "%LocalSystem" OR user_account LIKE "%LocalService" OR user_account LIKE "%NetworkService" OR user_account == null)","label":"Osquery - Retrieve Services Running on User Accounts"}}
        • !{osquery{"query":"SELECT concat('https://www.virustotal.com/gui/file/', sha1) AS VtLink, name, description, start_type, status, pid, services.path FROM services JOIN authenticode ON services.path = authenticode.path OR services.module_path = authenticode.path JOIN hash ON services.path = hash.path WHERE authenticode.result != "trusted"","label":"Osquery - Retrieve Service Unsigned Executables with Virustotal Link"}}
    • 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.
  • This mechanism can be used legitimately. If trusted software uses this command and the triage has not identified
    anything suspicious, this alert can be closed as a false positive.
  • 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.
  • 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

sequence by process.entity_id
  [process where process.name : "certutil.exe" and event.type == "start"]
  [network where process.name : "certutil.exe" and
    not cidrmatch(destination.ip, "10.0.0.0/8", "127.0.0.0/8", "169.254.0.0/16", "172.16.0.0/12", "192.0.0.0/24",
                                  "192.0.0.0/29", "192.0.0.8/32", "192.0.0.9/32", "192.0.0.10/32", "192.0.0.170/32",
                                  "192.0.0.171/32", "192.0.2.0/24", "192.31.196.0/24", "192.52.193.0/24",
                                  "192.168.0.0/16", "192.88.99.0/24", "224.0.0.0/4", "100.64.0.0/10", "192.175.48.0/24",
                                  "198.18.0.0/15", "198.51.100.0/24", "203.0.113.0/24", "240.0.0.0/4", "::1",
                                  "FE80::/10", "FF00::/8")]

Framework: MITRE ATT&CKTM