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File Encoded To Base64 Via Certutil.EXE

SIGMA detection rule by Florian Roth (Nextron Systems), Jonhnathan Ribeiro, oscd.community, Nasreddine Bencherchali (Nextron Systems) · NOASSERTION

Detects the execution of certutil with the "encode" flag to encode a file to base64. This can be abused by threat actors and attackers for data exfiltration

title: File Encoded To Base64 Via Certutil.EXE
id: e62a9f0c-ca1e-46b2-85d5-a6da77f86d1a
status: test
description: Detects the execution of certutil with the "encode" flag to encode a file to base64. This can be abused by threat actors and attackers for data exfiltration
references:
    - https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/certutil
    - https://unit42.paloaltonetworks.com/new-babyshark-malware-targets-u-s-national-security-think-tanks/
    - https://lolbas-project.github.io/lolbas/Binaries/Certutil/
author: Florian Roth (Nextron Systems), Jonhnathan Ribeiro, oscd.community, Nasreddine Bencherchali (Nextron Systems)
date: 2019-02-24
modified: 2024-03-05
tags:
    - attack.stealth
    - attack.t1027
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith: '\certutil.exe'
        - OriginalFileName: 'CertUtil.exe'
    selection_cli:
        CommandLine|contains|windash: '-encode'
    condition: all of selection_*
falsepositives:
    - As this is a general purpose rule, legitimate usage of the encode functionality will trigger some false positives. Apply additional filters accordingly
level: medium
regression_tests_path: regression_data/rules/windows/process_creation/proc_creation_win_certutil_encode/info.yml

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11e8738b-f6fb-41bf-9d14-f0f9e321041f

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Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

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Format & documentation suggestions

Not a security issue — this rule is valid and was accepted as-is. These are optional fields/conventions its own format recommends, mainly affecting its quality score and how well it documents itself for other reviewers.

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