Detectionhightest

Suspicious File Encoded To Base64 Via Certutil.EXE

Detects the execution of certutil with the "encode" flag to encode a file to base64 where the extensions of the file is suspicious

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Nasreddine Bencherchali (Nextron Systems)Created Mon May 15Updated Tue Mar 05ea0cdc3e-2239-4f26-a947-4e8f8224e464windows

Log Source

WindowsProcess Creation

ProductWindows← raw: windows

CategoryProcess Creation← raw: process_creation

Events generated when a new process is spawned on the system. Covers command-line arguments, parent/child relationships, and process metadata.

Detection Logic

detection:
    selection_img:
        - Image|endswith: '\certutil.exe'
        - OriginalFileName: 'CertUtil.exe'
    selection_cli:
        CommandLine|contains|windash: '-encode'
    selection_extension:
        CommandLine|contains:
            - '.acl'
            - '.bat'
            - '.doc'
            - '.gif'
            - '.jpeg'
            - '.jpg'
            - '.mp3'
            - '.pdf'
            - '.png'
            - '.ppt'
            - '.tmp'
            - '.xls'
            - '.xml'
    condition: all of selection_*

False Positives

Unknown

False positive likelihood has not been assessed. Additional context may be needed during triage.

References

Testing & Validation

Regression Tests

by SigmaHQ Team

Positive Detection Test1 matchevtx

Microsoft-Windows-Sysmon

EVTX JSON

MITRE ATT&CK

Tactics

TA0005 · Defense Evasion

Techniques

T1027 · Obfuscated Files or Information

Related Rules

DerivedDetectionmedium

File Encoded To Base64 Via Certutil.EXE

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

This rule was derived from the related rule - both detect similar activity with different scope.

Rule Metadata

Rule ID

ea0cdc3e-2239-4f26-a947-4e8f8224e464

Status

test

Level

high

Type

Detection

Created

Mon May 15

Modified

Tue Mar 05

Author

Nasreddine Bencherchali (Nextron Systems)

Path

rules/windows/process_creation/proc_creation_win_certutil_encode_susp_extensions.yml

Raw Tags

attack.defense-evasionattack.t1027

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