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Detection rules › Sigma

Small Sieve Malware File Indicator Creation

Status
test
Severity
high
Log source
product windows, category file_event
Author
Nasreddine Bencherchali (Nextron Systems), X__Junior (Nextron Systems)
Source
github.com/SigmaHQ/sigma

Detects filename indicators that contain a specific typo seen used by the Small Sieve malware.

MITRE ATT&CK coverage

TacticTechniques
StealthT1036.005 Masquerading: Match Legitimate Resource Name or Location

Event coverage

ProviderEventTitle
SysmonEvent ID 11FileCreate

Rule body yaml

title: Small Sieve Malware File Indicator Creation
id: 39466c42-c189-476a-989f-8cdb135c163a
status: test
description: Detects filename indicators that contain a specific typo seen used by the Small Sieve malware.
references:
    - https://www.ncsc.gov.uk/static-assets/documents/malware-analysis-reports/small-sieve/NCSC-MAR-Small-Sieve.pdf
author: Nasreddine Bencherchali (Nextron Systems), X__Junior (Nextron Systems)
date: 2023-05-19
tags:
    - attack.stealth
    - attack.t1036.005
    - detection.emerging-threats
logsource:
    product: windows
    category: file_event
detection:
    selection_typo_path:
        TargetFilename|contains|all:
            - ':\Users\'
            - '\AppData\'
        TargetFilename|contains:
            - '\Roaming\'
            - '\Local\'
    selection_typo_keyword:
        TargetFilename|contains: 'Microsift'
    selection_ioc:
        TargetFilename|endswith: '\AppData\Local\MicrosoftWindowsOutlookDataPlus.txt'
    condition: all of selection_typo_* or selection_ioc
falsepositives:
    - Unlikely
level: high

Stages and Predicates

Stage 0: condition

all of selection_typo_* or selection_ioc

Stage 1: selection_typo_path

selection_typo_path:
    TargetFilename|contains|all:
        - ':\Users\'
        - '\AppData\'
    TargetFilename|contains:
        - '\Roaming\'
        - '\Local\'

Stage 2: selection_typo_keyword

selection_typo_keyword:
    TargetFilename|contains: 'Microsift'

Stage 3: selection_ioc

selection_ioc:
    TargetFilename|endswith: '\AppData\Local\MicrosoftWindowsOutlookDataPlus.txt'

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
TargetFilenameends_with
  • \AppData\Local\MicrosoftWindowsOutlookDataPlus.txt
TargetFilenamematch
  • :\Users\ corpus 4 (sigma 4)
  • Microsift
  • \AppData\ corpus 4 (sigma 4)
  • \Local\
  • \Roaming\