Hunting EVTX with Chainsaw

Overview

Chainsaw is a fast, Rust-based forensic artifact search and hunting tool from WithSecure Labs. It provides first-response capability to rapidly identify threats within Windows Event Logs (.evtx) and other artifacts. Chainsaw can hunt with the full SigmaHQ rule corpus (translating Sigma to its internal Tau engine), run its own built-in detection rules, perform high-speed keyword/regex search across logs, and analyse specialized artifacts such as the AppCompatCache (shimcache), SRUM database, and event-log gaps. Output can be a colorized table, CSV, or JSON for downstream tooling.

Chainsaw's strength is speed and flexibility during initial triage: an analyst can drop a folder of collected EVTX onto the tool and get back a prioritized set of detections in seconds, then pivot with targeted search queries to confirm a hypothesis. Unlike a SIEM, it needs no ingestion pipeline, runs as a single binary, and works fully offline against acquired evidence — ideal for the field or an air-gapped analysis VM. The --mapping file tells Chainsaw how Sigma fields translate to Windows event fields, which is what enables broad Sigma coverage over EVTX.

A common hunt outcome is detecting suspicious PowerShell — MITRE ATT&CK T1059.001 (Command and Scripting Interpreter: PowerShell) — by running Sigma rules against PowerShell operational logs (Event ID 4104 script-block logging) or searching for encoded-command patterns. This skill maps to NIST CSF DE.AE-02 (potentially adverse events are analyzed to better understand associated activities).

When to Use

• During first-response triage to rapidly hunt threats across collected Windows event logs.
• When you need offline Sigma-based detection over .evtx without standing up a SIEM.
• To run fast keyword/regex searches confirming or refuting a hunt hypothesis.
• To analyse shimcache, SRUM, or event-log time gaps for execution evidence and tampering.
• To produce CSV/JSON detection output for reporting or pipeline ingestion.

Prerequisites

• Chainsaw binary. Download a release from GitHub or build from source:

  # Build from source (Rust toolchain required)
  git clone https://github.com/WithSecureLabs/chainsaw.git
  cd chainsaw && cargo build --release
  ./target/release/chainsaw --version
  # or: nix profile install github:WithSecureLabs/chainsaw
  

• The Chainsaw repo ships mappings/ (Sigma field mappings) and rules/ (Chainsaw rules).
• A copy of the SigmaHQ rules for full Sigma coverage:

  git clone https://github.com/SigmaHQ/sigma.git
  

• Collected Windows .evtx files (and registry hives like SYSTEM/Amcache.hve for shimcache analysis).

Objectives

• Hunt collected EVTX with Sigma rules using the correct mapping file.
• Filter detections by rule level, status, and kind to reduce noise.
• Search logs by keyword, regex, and Tau expression for targeted confirmation.
• Output detections as table, CSV, and JSON.
• Analyse shimcache (with Amcache timestamp pairing), SRUM, and event-log gaps.

MITRE ATT&CK Mapping

Workflow

1. Hunt EVTX with Sigma rules

Run the SigmaHQ corpus against collected logs using the bundled mapping file. The mapping translates Sigma fields to EVTX fields.

chainsaw hunt ./collected_evtx \
  -s ./sigma/rules \
  --mapping ./mappings/sigma-event-logs-all.yml

2. Hunt with Chainsaw built-in rules plus Sigma

Combine Chainsaw's own rules (-r) with Sigma (-s) for broader coverage.

chainsaw hunt ./collected_evtx \
  -r ./rules \
  -s ./sigma/rules \
  --mapping ./mappings/sigma-event-logs-all.yml

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