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Suspicious Process Access via Direct System Call

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Suspicious Process Access via Direct System Call

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Identifies suspicious process access events from an unknown memory region. Endpoint security solutions usually hook userland Windows APIs in order to decide if the code that is being executed is malicious or not. It’s possible to bypass hooked functions by writing malicious functions that call syscalls directly.

Rule type: eql

Rule indices:

  • winlogbeat-*
  • logs-windows.sysmon_operational-*

Severity: high

Risk score: 73

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: Defense Evasion
  • Tactic: Execution
  • Resources: Investigation Guide
  • Data Source: Sysmon

Version: 311

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Investigating Suspicious Process Access via Direct System Call

Endpoint security solutions usually hook userland Windows APIs in order to decide if the code that is being executed is malicious or not. It’s possible to bypass hooked functions by writing malicious functions that call syscalls directly.

More context and technical details can be found in this research blog.

This rule identifies suspicious process access events from an unknown memory region. Attackers can use direct system calls to bypass security solutions that rely on hooks.

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.
  • 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

  • This detection may be triggered by certain applications that install root certificates for the purpose of inspecting SSL traffic. Benign true positives (B-TPs) can be added as exceptions if necessary.

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 the malicious certificate from the root certificate store.
  • 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.
  • Investigate credential exposure on systems compromised or used by the attacker to ensure all compromised accounts are identified. Reset passwords for these accounts and other potentially compromised credentials, such as email, business systems, and web services.
  • 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).

Setup

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Setup

If enabling an EQL rule on a non-elastic-agent index (such as beats) for versions <8.2, events will not define event.ingested and default fallback for EQL rules was not added until version 8.2. Hence for this rule to work effectively, users will need to add a custom ingest pipeline to populate event.ingested to @timestamp. For more details on adding a custom ingest pipeline refer - https://www.elastic.co/guide/en/fleet/current/data-streams-pipeline-tutorial.html

Rule query

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process where host.os.type == "windows" and event.code == "10" and
 length(winlog.event_data.CallTrace) > 0 and

 /* Sysmon CallTrace starting with unknown memory module instead of ntdll which host Windows NT Syscalls */
 not winlog.event_data.CallTrace :
            ("?:\\WINDOWS\\SYSTEM32\\ntdll.dll*",
             "?:\\WINDOWS\\SysWOW64\\ntdll.dll*",
             "?:\\Windows\\System32\\wow64cpu.dll*",
             "?:\\WINDOWS\\System32\\wow64win.dll*",
             "?:\\Windows\\System32\\win32u.dll*") and

 not winlog.event_data.TargetImage :
            ("?:\\Program Files (x86)\\Malwarebytes Anti-Exploit\\mbae-svc.exe",
             "?:\\Program Files\\Cisco\\AMP\\*\\sfc.exe",
             "?:\\Program Files (x86)\\Microsoft\\EdgeWebView\\Application\\*\\msedgewebview2.exe",
             "?:\\Program Files\\Adobe\\Acrobat DC\\Acrobat\\*\\AcroCEF.exe") and

 not (process.executable : ("?:\\Program Files\\Adobe\\Acrobat DC\\Acrobat\\Acrobat.exe",
                            "?:\\Program Files (x86)\\World of Warcraft\\_classic_\\WowClassic.exe") and
      not winlog.event_data.TargetImage : "?:\\WINDOWS\\system32\\lsass.exe")

Framework: MITRE ATT&CKTM

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