Windows TinyCC Shellcode Execution

SPLUNK detection rule by Michael Haag, Splunk · Apache-2.0

Detects abuse of Tiny-C-Compiler (TinyCC) for shellcode execution, where tcc.exe is renamed to masquerade as svchost.exe and used to compile and execute C source files containing shellcode. This technique was observed in the Lotus Blossom Chrysalis backdoor campaign, where attackers renamed "tcc.exe" to "svchost.exe", and executed a file named "conf.c" containing Metasploit block_api shellcode with the flags -nostdlib -run. TinyCC is a legitimate C compiler, but its ability to compile and execute code on-the-fly makes it attractive to attackers seeking to evade detection. The combination of a renamed compiler binary executing from non-standard locations with suspicious flags is a strong indicator of malicious activity.

name: Windows TinyCC Shellcode Execution
id: fdb6774e-e465-4912-86e3-63cf9ab91491
version: 4
creation_date: '2026-03-16'
modification_date: '2026-09-01'
author: Michael Haag, Splunk
status: production
type: TTP
description: |
    Detects abuse of Tiny-C-Compiler (TinyCC) for shellcode execution, where tcc.exe is renamed to masquerade as svchost.exe and used to compile and execute C source files containing shellcode.
    This technique was observed in the Lotus Blossom Chrysalis backdoor campaign, where attackers renamed "tcc.exe" to "svchost.exe", and executed a file named "conf.c" containing Metasploit block_api shellcode with the flags -nostdlib -run.
    TinyCC is a legitimate C compiler, but its ability to compile and execute code on-the-fly makes it attractive to attackers seeking to evade detection.
    The combination of a renamed compiler binary executing from non-standard locations with suspicious flags is a strong indicator of malicious activity.
data_source:
    - Sysmon 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 ("svchost.exe", "tcc.exe")
    Processes.process="* -nostdlib*"
    Processes.process="* -run*"
    Processes.process IN ("*.c", "*conf.c*")
    NOT Processes.process_path IN (
        "*:\\Windows\\System32\\*",
        "*:\\Windows\\SysWOW64\\*"
    )

    By Processes.action Processes.dest Processes.user Processes.process 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_name Processes.process_id
       Processes.process_exec Processes.process_guid Processes.process_hash
       Processes.process_integrity_level Processes.process_path
       Processes.original_file_name Processes.user_id Processes.vendor_product

    | `drop_dm_object_name(Processes)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_tinycc_shellcode_execution_filter`
how_to_implement: |
    To successfully implement this search, you need to be ingesting logs with process creation information from your endpoints. If you are using Sysmon, you must have at least version 6.0.4 of the Sysmon TA and have enabled EventCode 1 (Process Create). Ensure that command-line arguments are being captured in your Sysmon configuration.
    The detection relies on the OriginalFileName field being populated to distinguish between legitimate svchost.exe and renamed tcc.exe binaries. Ensure Sysmon is configured to capture full command-line arguments.
known_false_positives: |
    Legitimate TinyCC usage by developers may trigger this detection if executed from non-standard locations. However, the combination of:
    1. Renamed binary (svchost.exe with tcc.exe OriginalFileName)
    2. Execution from user-writable directories (AppData, Temp, ProgramData)
    3. Suspicious flags (-nostdlib -run) with .c file execution

    is highly suspicious and warrants investigation. Legitimate TinyCC usage typically occurs from Program Files or developer directories with standard compilation workflows.

    Allowlist known development environments if needed.
references:
    - https://attack.mitre.org/techniques/T1059/003/
    - https://attack.mitre.org/techniques/T1027/
    - https://www.rapid7.com/blog/post/tr-chrysalis-backdoor-dive-into-lotus-blossoms-toolkit/
    - https://github.com/phoenixthrush/Tiny-C-Compiler
drilldown_searches:
    - name: View the detection results for - "$dest$" and "$user$"
      search: '%original_detection_search% | search  dest = "$dest$" user = "$user$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$dest$" and "$user$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$dest$", "$user$") | 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"
finding:
    title: TinyCC compiler execution on [$dest$] by user [$user$] from [$process_path$] via the CommandLine [$process$] from the ParentProcess [$parent_process$], indicating potential malicious code execution.
    entity:
        field: user
        type: user
        score: 50
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 50
          message: TinyCC compiler execution on [$dest$] by user [$user$] from [$process_path$] via the CommandLine [$process$] from the ParentProcess [$parent_process$], indicating potential malicious code execution.
threat_objects:
    - field: process
      type: process
    - field: process_name
      type: process_name
    - field: process_path
      type: file_path
analytic_story:
    - Lotus Blossom Chrysalis Backdoor
asset_type: Endpoint
mitre_attack_id:
    - T1059.003
    - T1027
    - T1036
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/T1059.005/lotus_blossom_chrysalis/windows-sysmon.log
          sourcetype: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
      test_type: unit

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