Adversaries may use base64 decoding to obfuscate malicious payloads or exfiltrate data, making it a potential indicator of command and control activity. Proactively hunting for base64 decoding in Azure Sentinel can help identify covert communication or data exfiltration attempts that evade basic detection mechanisms.
title: Decode Base64 Encoded Text
id: e2072cab-8c9a-459b-b63c-40ae79e27031
status: test
description: Detects usage of base64 utility to decode arbitrary base64-encoded text
references:
- https://github.com/redcanaryco/atomic-red-team/blob/f339e7da7d05f6057fdfcdd3742bfcf365fee2a9/atomics/T1027/T1027.md
author: Daniil Yugoslavskiy, oscd.community
date: 2020-10-19
modified: 2021-11-27
tags:
- attack.defense-evasion
- attack.t1027
logsource:
category: process_creation
product: linux
detection:
selection:
Image|endswith: '/base64'
CommandLine|contains: '-d' # Also covers "--decode"
condition: selection
falsepositives:
- Legitimate activities
level: low
imProcessCreate
| where TargetProcessName endswith "/base64" and TargetProcessCommandLine contains "-d"Scenario: A system administrator uses the base64 command to decode a base64-encoded string as part of a script to verify data integrity.
Filter/Exclusion: Exclude processes where the command line includes base64 -d and the decoded output is less than 100 characters.
Scenario: A scheduled job runs a script that uses base64 to decode a token for API authentication, which is part of a CI/CD pipeline.
Filter/Exclusion: Exclude processes where the command line includes base64 -d and the decoded string matches a known API token pattern (e.g., Bearer <token>).
Scenario: A developer uses the Python base64 module to decode a string during debugging or testing.
Filter/Exclusion: Exclude processes where the command line includes python and the script path contains debug.py or test_*.py.
Scenario: A backup job uses base64 to decode a compressed archive that was previously encoded for storage.
Filter/Exclusion: Exclude processes where the command line includes base64 -d and the file path contains backup_*.tar.gz or similar naming patterns.
Scenario: A security tool or SIEM system uses base64 to decode log entries that were encoded for transmission.
Filter/Exclusion: Exclude processes where the command line includes base64 -d and the process name or parent process is a known security tool (e.g., splunk, logstash, or syslog-ng).