Zscaler Potentially Abused File Download

Description

The following analytic identifies the download of potentially malicious file types, such as .scr, .dll, .bat, and .lnk, within a network. It leverages web proxy logs from Zscaler, focusing on blocked actions and analyzing fields like deviceowner, user, urlcategory, url, dest, and filename. This activity is significant as these file types are often used to spread malware, posing a threat to network security. If confirmed malicious, this activity could lead to malware execution, data compromise, or further network infiltration.

Query · spl

`zscaler_proxy` url IN ("*.scr", "*.dll", "*.bat", "*.lnk")
  | stats count min(_time) as firstTime max(_time) as lastTime
    BY deviceowner user urlcategory
       url src filename
       dest
  | `security_content_ctime(firstTime)`
  | `security_content_ctime(lastTime)`
  | `zscaler_potentially_abused_file_download_filter`

Implementation guide

You must install the latest version of Zscaler Add-on from Splunkbase. You must be ingesting Zscaler events into your Splunk environment through an ingester. This analytic was written to be used with the "zscalernss-web" sourcetype leveraging the Zscaler proxy data. This enables the integration with Splunk Enterprise Security. Security teams are encouraged to adjust the detection parameters, ensuring the detection is tailored to their specific environment.

Known false positives

  • False positives are limited to Zscaler configuration.

Analyst notes

Known false positives: False positives are limited to Zscaler configuration.

Raw source Zscaler Potentially Abused File Download · SPL
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Published by splunk/security_content ↗, licensed under Apache 2.0 ↗. Reproduced here unmodified. 
name: Zscaler Potentially Abused File Download
id: b0c21379-f4ba-4bac-a958-897e260f964a
version: 10
creation_date: '2024-04-17'
modification_date: '2026-05-13'
author: Gowthamaraj Rajendran, Rod Soto, Splunk
status: production
type: Anomaly
description: The following analytic identifies the download of potentially malicious file types, such as .scr, .dll, .bat, and .lnk, within a network. It leverages web proxy logs from Zscaler, focusing on blocked actions and analyzing fields like deviceowner, user, urlcategory, url, dest, and filename. This activity is significant as these file types are often used to spread malware, posing a threat to network security. If confirmed malicious, this activity could lead to malware execution, data compromise, or further network infiltration.
data_source: []
search: |-
    `zscaler_proxy` url IN ("*.scr", "*.dll", "*.bat", "*.lnk")
      | stats count min(_time) as firstTime max(_time) as lastTime
        BY deviceowner user urlcategory
           url src filename
           dest
      | `security_content_ctime(firstTime)`
      | `security_content_ctime(lastTime)`
      | `zscaler_potentially_abused_file_download_filter`
how_to_implement: You must install the latest version of Zscaler Add-on from Splunkbase. You must be ingesting Zscaler events into your Splunk environment through an ingester. This analytic was written to be used with the "zscalernss-web" sourcetype leveraging the Zscaler proxy data. This enables the integration with Splunk Enterprise Security. Security teams are encouraged to adjust the detection parameters, ensuring the detection is tailored to their specific environment.
known_false_positives: False positives are limited to Zscaler configuration.
references:
    - https://help.zscaler.com/zia/nss-feed-output-format-web-logs
drilldown_searches:
    - name: View the detection results for - "$src$" and "$user$"
      search: '%original_detection_search% | search  src = "$src$" user = "$user$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$src$" and "$user$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$src$", "$user$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
intermediate_findings:
    entities:
        - field: src
          type: system
          score: 20
          message: Potential Abused File Download from dest -[$dest$] on $src$ for user-[$user$].
        - field: user
          type: user
          score: 20
          message: Potential Abused File Download from dest -[$dest$] on $src$ for user-[$user$].
threat_objects:
    - field: url
      type: url
analytic_story:
    - Zscaler Browser Proxy Threats
asset_type: Web Server
mitre_attack_id:
    - T1566
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: web
security_domain: threat
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1566/zscalar_web_proxy/zscalar_web_proxy.json
          source: zscaler
          sourcetype: zscalernss-web
      test_type: unit

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