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Linux Base64 Utility Decoding via base64 -d Command-Line

Identifies Linux base64 decoding activity using base64 with the -d/--decode flag.

FreeUnreviewedSigmalowv1
title: Linux Base64 Utility Decoding via base64 -d Command-Line
id: 084fd3dc-2248-4df7-9124-118626c8b59d
status: test
description: This rule flags Linux process executions of the base64 utility where the binary path ends with /base64 and the command line includes the decode flag (-d or --decode). Attackers commonly use base64 decoding to transform encoded payloads into readable content during execution, aiding stealth and evasion. It relies on process creation telemetry, specifically the executable path and command-line arguments.
references:
  - https://github.com/redcanaryco/atomic-red-team/blob/f339e7da7d05f6057fdfcdd3742bfcf365fee2a9/atomics/T1027/T1027.md
  - https://github.com/SigmaHQ/sigma/blob/master/rules/linux/process_creation/proc_creation_lnx_base64_decode.yml
author: Daniil Yugoslavskiy, oscd.community, Huntrule Team
date: 2020-10-19
modified: 2021-11-27
tags:
  - attack.stealth
  - attack.t1027
logsource:
  category: process_creation
  product: linux
detection:
  selection:
    Image|endswith: /base64
    CommandLine|contains: -d
  condition: selection
falsepositives:
  - Legitimate activities
level: low
license: DRL-1.1
related:
  - id: e2072cab-8c9a-459b-b63c-40ae79e27031
    type: derived

What it detects

This rule flags Linux process executions of the base64 utility where the binary path ends with /base64 and the command line includes the decode flag (-d or --decode). Attackers commonly use base64 decoding to transform encoded payloads into readable content during execution, aiding stealth and evasion. It relies on process creation telemetry, specifically the executable path and command-line arguments.

Known false positives

  • Legitimate activities

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.