Small Sieve Malware File Indicator Creation

Status: test
Severity: high
Log source: category file_event, product windows
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.

Known false positives

Unlikely

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1036.005` Masquerading: Match Legitimate Resource Name or Location

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 11: FileCreate

Rule body

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

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`TargetFilename`	ends_with	`\AppData\Local\MicrosoftWindowsOutlookDataPlus.txt`	`field:"TargetFilename" kind:ends_with value:"\AppData\Local\MicrosoftWindowsOutlookDataPlus.txt"`
`TargetFilename`	match	`:\Users\` corpus 4 (sigma 4) `Microsift` `\AppData\` corpus 4 (sigma 4) `\Local\` `\Roaming\`	`field:"TargetFilename" kind:match`

Small Sieve Malware File Indicator Creation

Known false positives

MITRE ATT&CK coverage

Telemetry coverage

Rule body

Stages and Predicates

Stage 0: condition

Stage 1: selection_typo_path

Stage 2: selection_typo_keyword

Stage 3: selection_ioc

Indicators

Stage 0: `condition`

Stage 1: `selection_typo_path`

Stage 2: `selection_typo_keyword`

Stage 3: `selection_ioc`