Curl Execution with Percent Encoded URL

name: Curl Execution with Percent Encoded URL
id: 9a8d5516-4c5e-11ef-9d42-acde48001122
version: 2
date: '2026-03-10'
author: Nasreddine Bencherchali, Splunk
status: production
type: Anomaly
description: |
    The following analytic detects the execution of the curl utility where the command line includes percent-encoded characters and explicit file output options (such as -o or --output).
    It leverages process execution telemetry from Endpoint Detection and Response (EDR) data sources to identify curl commands that may be using URL encoding to obfuscate download locations or payload paths.
    This behavior is notable because percent-encoded URLs are commonly used by adversaries to evade simple string-based detections, hide malicious infrastructure, or bypass network security controls.
    When combined with file download behavior, this activity may indicate malware staging, payload retrieval, or secondary tool deployment.
    Analysts should review the decoded URL, destination host, parent process, and downloaded file to determine whether the activity is authorized or malicious.
    The analytic calculates the number of percent (%) characters in the curl command line and triggers when a threshold of three or more is met, indicating potential URL encoding.
    Adjust the threshold as needed based on your environment and tuning requirements.
data_source:
    - CrowdStrike ProcessRollup2
    - Sysmon EventID 1
    - Sysmon for Linux EventID 1
    - Windows Event Log Security 4688
search: |
    | tstats `security_content_summariesonly`
      count min(_time) as firstTime
            max(_time) as lastTime
    from datamodel=Endpoint.Processes where
    (
      Processes.process_name IN ("curl.exe", "curl")
      OR
      Processes.original_file_name="curl.exe"
    )
    Processes.process IN (
      "* --output *",
      "* -o *" /* Covers both options since the search is case insensitive */,
    )
    Processes.process IN ("*%*")
    by Processes.action Processes.dest Processes.original_file_name
       Processes.parent_process Processes.parent_process_exec
       Processes.parent_process_guid Processes.parent_process_id
       Processes.parent_process_name Processes.parent_process_path
       Processes.process Processes.process_exec Processes.process_guid
       Processes.process_hash Processes.process_id
       Processes.process_integrity_level Processes.process_name
       Processes.process_path Processes.user
       Processes.user_id Processes.vendor_product

    | `drop_dm_object_name(Processes)`

    ```
      Count the number of % characters in the process command line.
      Change this threshold based on your environment and tuning needs.
    ```
    | eval percent_count = mvcount(split(process, "%")) - 1
    | where percent_count >= 3

    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `curl_execution_with_percent_encoded_url_filter`
how_to_implement: |
    The detection is based on data that originates from Endpoint Detection
    and Response (EDR) agents. These agents are designed to provide security-related
    telemetry from the endpoints where the agent is installed. To implement this search,
    you must ingest logs that contain the process GUID, process name, and parent process.
    Additionally, you must ingest complete command-line executions. These logs must
    be processed using the appropriate Splunk Technology Add-ons that are specific to
    the EDR product. The logs must also be mapped to the `Processes` node of the `Endpoint`
    data model. Use the Splunk Common Information Model (CIM) to normalize the field
    names and speed up the data modeling process.
known_false_positives: |
    No false positives have been identified at this time.
references:
    - https://github.com/nasbench/Misc-Research/blob/main/LOLBINs/Curl.md
    - https://attack.mitre.org/techniques/T1027/
    - https://attack.mitre.org/techniques/T1105/
    - https://curl.se/docs/manpage.html
drilldown_searches:
    - name: View the detection results for - "$user$" and "$dest$"
      search: '%original_detection_search% | search  user = "$user$" dest = "$dest$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$user$" and "$dest$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$", "$dest$") starthoursago=168  | 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: $info_min_time$
      latest_offset: $info_max_time$
rba:
    message: An instance of $parent_process_name$ spawning $process_name$ was identified on endpoint $dest$ by user $user$ with URL-encoded parameters $process$.
    risk_objects:
        - field: user
          type: user
          score: 20
        - field: dest
          type: system
          score: 20
    threat_objects:
        - field: parent_process_name
          type: parent_process_name
        - field: process_name
          type: process_name
        - field: process
          type: process
tags:
    analytic_story:
        - Compromised Windows Host
        - Ingress Tool Transfer
        - Living Off The Land
    asset_type: Endpoint
    mitre_attack_id:
        - T1027
        - T1105
    product:
        - Splunk Enterprise
        - Splunk Enterprise Security
        - Splunk Cloud
    security_domain: endpoint
tests:
    - name: True Positive Test - Sysmon Linux
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1027/url_encoded_curl/linux-sysmon.log
          source: Syslog:Linux-Sysmon/Operational
          sourcetype: sysmon:linux
    - name: True Positive Test - Sysmon Windows
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1027/url_encoded_curl/windows-sysmon.log
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          sourcetype: XmlWinEventLog