Invocation Of Crypto-Classes From The "Cryptography" PowerShell Namespace

SIGMA detection rule by Andreas Braathen (mnemonic.io) · NOASSERTION

Detects the invocation of PowerShell commands with references to classes from the "System.Security.Cryptography" namespace. The PowerShell namespace "System.Security.Cryptography" provides classes for on-the-fly encryption and decryption. These can be used for example in decrypting malicious payload for defense evasion.

title: Invocation Of Crypto-Classes From The "Cryptography" PowerShell Namespace
id: ad856965-f44d-42a8-945e-bbf7bd03d05a
status: test
description: |
    Detects the invocation of PowerShell commands with references to classes from the "System.Security.Cryptography" namespace.
    The PowerShell namespace "System.Security.Cryptography" provides classes for on-the-fly encryption and decryption.
    These can be used for example in decrypting malicious payload for defense evasion.
references:
    - https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography?view=net-8.0
    - https://blogs.vmware.com/security/2023/11/jupyter-rising-an-update-on-jupyter-infostealer.html
    - https://www.virustotal.com/gui/file/39102fb7bb6a74a9c8cb6d46419f9015b381199ea8524c1376672b30fffd69d2
author: Andreas Braathen (mnemonic.io)
date: 2023-12-01
tags:
    - attack.execution
    - attack.stealth
    - attack.t1059.001
    - attack.t1027.010
    - detection.threat-hunting
logsource:
    product: windows
    category: process_creation
detection:
    selection_img:
        - Image|endswith:
              - '\powershell.exe'
              - '\pwsh.exe'
        - OriginalFileName:
              - 'PowerShell.EXE'
              - 'pwsh.dll'
    selection_cmdlet_namespace:
        CommandLine|contains: 'System.Security.Cryptography.'
    selection_cmdlet_classes:
        CommandLine|contains:
            - '.AesCryptoServiceProvider'
            - '.DESCryptoServiceProvider'
            - '.DSACryptoServiceProvider'
            - '.RC2CryptoServiceProvider'
            - '.Rijndael'
            - '.RSACryptoServiceProvider'
            - '.TripleDESCryptoServiceProvider'
    condition: all of selection_*
falsepositives:
    - Classes are legitimately used, but less so when e.g. parents with low prevalence or decryption of content in temporary folders.
level: medium

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

515ddf23-4feb-4d77-b9a5-da1f911ea7e1

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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.

Rejected pattern present

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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.

[[ reason ]]

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[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

[[ currentRule.title ]]

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