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Detection rules › Sigma

File Encoded To Base64 Via Certutil.EXE

Status
test
Severity
medium
Log source
product windows, category process_creation
Author
Florian Roth (Nextron Systems), Jonhnathan Ribeiro, oscd.community, Nasreddine Bencherchali (Nextron Systems)
Source
github.com/SigmaHQ/sigma

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

MITRE ATT&CK coverage

TacticTechniques
StealthT1027 Obfuscated Files or Information

Event coverage

ProviderEventTitle
SysmonEvent ID 1Process creation

Rule body yaml

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

Stages and Predicates

Stage 0: condition

all of selection_*

Stage 1: selection_img

selection_img:
    - Image|endswith: '\certutil.exe'
    - OriginalFileName: 'CertUtil.exe'

Stage 2: selection_cli

selection_cli:
    CommandLine|contains|windash: '-encode'

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

FieldKindValues
CommandLinematch
  • -encode transforms: windash corpus 3 (sigma 3)
Imageends_with
  • \certutil.exe corpus 43 (sigma 43)
OriginalFileNameeq
  • CertUtil.exe corpus 21 (sigma 14, splunk 6, elastic 1)