Detection rules › Splunk
Zscaler Potentially Abused File Download
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.
MITRE ATT&CK coverage
| Tactic | Techniques |
|---|---|
| Initial Access | T1566 Phishing |
Rule body splunk
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
Stages and Predicates
Stage 1: search
`zscaler_proxy` url IN ("*.scr", "*.dll", "*.bat", "*.lnk")
Stage 2: stats
| stats count min(_time) as firstTime max(_time) as lastTime
BY deviceowner user urlcategory
url src filename
dest
Stage 3: search
| `security_content_ctime(firstTime)`
Stage 4: search
| `security_content_ctime(lastTime)`
Stage 5: search
| `zscaler_potentially_abused_file_download_filter`
Indicators
Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.
| Field | Kind | Values |
|---|---|---|
sourcetype | eq |
|
url | in |
|