IcedID Malware Suspicious Single Digit DLL Execution Via Rundll32

SIGMA detection rule by Nasreddine Bencherchali (Nextron Systems) · GPL-2.0

Detects RunDLL32.exe executing a single digit DLL named "1.dll" with the export function "DllRegisterServer". This behaviour was often seen used by malware and especially IcedID

title: IcedID Malware Suspicious Single Digit DLL Execution Via Rundll32
id: 28ffa72a-4fdf-40aa-4912-e53083a61f96
related:
    - id: 2bd8e100-5b3b-4b6a-bbb5-b129d3ddddc5
      type: derived
status: test
description: Detects RunDLL32.exe executing a single digit DLL named "1.dll" with the export function "DllRegisterServer". This behaviour was often seen used by malware and especially IcedID
references:
    - https://thedfirreport.com/2023/05/22/icedid-macro-ends-in-nokoyawa-ransomware/
    - https://thedfirreport.com/2023/08/28/html-smuggling-leads-to-domain-wide-ransomware/
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-08-31
tags:
    - attack.stealth
    - attack.t1218.011
    - detection.emerging-threats
logsource:
    category: process_creation
    product: windows
detection:
    process_creation:
        EventID: 4688
        Channel: Security
    selection:
        CommandLine|endswith:
            - \1.dll, DllRegisterServer   # In case of full path exec
            - ' 1.dll, DllRegisterServer' # In case of direct exec
        NewProcessName|endswith: \rundll32.exe
    condition: process_creation and selection
falsepositives:
    - Unknown
level: high
ruletype: Sigma

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

ea1194a5-b213-41ab-9274-895f3234e2cc

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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.

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

[[ currentRule.title ]]

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