Linux Proxy Socks Curl

SPLUNK detection rule by Michael Haag, Splunk, 0xC0FFEEEE, Github Community · Apache-2.0

The following analytic detects the use of the `curl` command with proxy-related arguments such as `-x`, `socks`, `--preproxy`, and `--proxy`. This detection leverages data from Endpoint Detection and Response (EDR) agents, focusing on command-line executions and process details. This activity is significant as it may indicate an adversary attempting to use a proxy to evade network monitoring and obscure their actions. If confirmed malicious, this behavior could allow attackers to bypass security controls, making it difficult to track their activities and potentially leading to unauthorized data access or exfiltration.

name: Linux Proxy Socks Curl
id: bd596c22-ad1e-44fc-b242-817253ce8b08
version: 14
creation_date: '2022-07-29'
modification_date: '2026-06-04'
author: Michael Haag, Splunk, 0xC0FFEEEE, Github Community
status: production
type: TTP
description: |-
    The following analytic detects the use of the `curl` command with proxy-related arguments such as `-x`, `socks`, `--preproxy`, and `--proxy`.
    This detection leverages data from Endpoint Detection and Response (EDR) agents, focusing on command-line executions and process details.
    This activity is significant as it may indicate an adversary attempting to use a proxy to evade network monitoring and obscure their actions.
    If confirmed malicious, this behavior could allow attackers to bypass security controls, making it difficult to track their activities and potentially leading to unauthorized data access or exfiltration.
data_source:
    - Sysmon for Linux EventID 1
search: |-
    | tstats `security_content_summariesonly`
      count min(_time) as firstTime
            max(_time) as lastTime
    FROM datamodel=Endpoint.Processes WHERE

    Processes.process_name=curl
    Processes.process IN (
        "*--preproxy *",
        "*--proxy*",
        "*-x *",
        "*socks4://*",
        "*socks4a://*",
        "*socks5://*",
        "*socks5h://*"
    )

    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)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | where match(process, "-x\s") OR match(process, "(?i)(socks\d\w?:\/\/|--(pre)?proxy)")
    | `linux_proxy_socks_curl_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: False positives may be present based on proxy usage internally. Filter as needed.
references:
    - https://www.offensive-security.com/metasploit-unleashed/proxytunnels/
    - https://curl.se/docs/manpage.html
    - https://en.wikipedia.org/wiki/SOCKS
    - https://oxylabs.io/blog/curl-with-proxy
    - https://reqbin.com/req/c-ddxflki5/curl-proxy-server#:~:text=To%20use%20a%20proxy%20with,be%20URL%20decoded%20by%20Curl.
    - https://gtfobins.github.io/gtfobins/curl/
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 ("$user$", "$dest$") | 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"
finding:
    title: An instance of $process_name$ was identified on endpoint $dest$ by user $user$ utilizing a proxy. Review activity for further details.
    entity:
        field: user
        type: user
        score: 50
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 50
          message: An instance of $process_name$ was identified on endpoint $dest$ by user $user$ utilizing a proxy. Review activity for further details.
threat_objects:
    - field: process_name
      type: process_name
analytic_story:
    - Linux Living Off The Land
    - Ingress Tool Transfer
asset_type: Endpoint
mitre_attack_id:
    - T1090
    - T1095
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1105/atomic_red_team/curl-linux-sysmon.log
          source: Syslog:Linux-Sysmon/Operational
          sourcetype: sysmon:linux
      test_type: unit

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