Linux Base64 Encoded Shebang In CLI

SIGMA detection rule by Nasreddine Bencherchali (Nextron Systems) · NOASSERTION

Detects the presence of a base64 version of the shebang in the commandline, which could indicate a malicious payload about to be decoded

title: Linux Base64 Encoded Shebang In CLI
id: fe2f9663-41cb-47e2-b954-8a228f3b9dff
status: test
description: Detects the presence of a base64 version of the shebang in the commandline, which could indicate a malicious payload about to be decoded
references:
    - https://www.trendmicro.com/pl_pl/research/20/i/the-evolution-of-malicious-shell-scripts.html
    - https://github.com/carlospolop/PEASS-ng/tree/master/linPEAS
author: Nasreddine Bencherchali (Nextron Systems)
date: 2022-09-15
tags:
    - attack.stealth
    - attack.t1140
logsource:
    product: linux
    category: process_creation
detection:
    selection:
        CommandLine|contains:
            - "IyEvYmluL2Jhc2" # Note: #!/bin/bash"
            - "IyEvYmluL2Rhc2" # Note: #!/bin/dash"
            - "IyEvYmluL3pza" # Note: #!/bin/zsh"
            - "IyEvYmluL2Zpc2" # Note: #!/bin/fish
            - "IyEvYmluL3No" # Note: # !/bin/sh"
    condition: selection
falsepositives:
    - Legitimate administration activities
level: medium

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

2c56f51a-3a5d-4952-9708-1583ba920c13

Propose edits, view related rules, and engage with the community through comments.

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

[[ reason ]]

What does this mean?

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 ]]

What do these messages mean?

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

[[ currentRule.title ]]

Community Discussion