Windows: Small Sieve IoC File Creation via AppData and Typo Filename Indicators
Alerts on Windows file events with Small Sieve filename typo/path indicators or the OutlookDataPlus.txt IOCs.
- Product
- windows
- Category
- file_event
- Author
- Nasreddine Bencherchali (Nextron Systems), X__Junior (Nextron Systems) (SigmaHQ), DRL 1.1
- Published
- 2023-05-19
- Updated
- 2026-07-31
ATT&CK techniques
Defense EvasionRecon
Resource Dev
Initial Access
Execution
Persistence
Priv Esc
Defense Evasion
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact
What it detects
This rule identifies Windows file creation events where the target filename contains specific AppData path fragments indicative of the Small Sieve malware’s naming patterns, including a typo in “Microsift.” It also matches a known filename IoC ending with “\AppData\Local\MicrosoftWindowsOutlookDataPlus.txt.” These behaviors matter because malware often writes staged or configuration files into user AppData locations using characteristic filenames; the detection relies on Windows file event telemetry that includes the TargetFilename field.
Reporting behind it
Changelog
v5- v5Candidate ingested via manual entry.2026-07-31
- v4Candidate ingested via manual entry.2026-07-31
- v3Candidate ingested via manual entry.2026-07-31
- v2Candidate ingested via manual entry.2026-07-31
- v1No changelog recorded for this version.2026-07-30
Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.
title: "Windows: Small Sieve IoC File Creation via AppData and Typo Filename Indicators"
id: e54bd9ec-b3ab-4254-b437-13c07c835b38
status: test
description: This rule identifies Windows file creation events where the target filename contains specific AppData path fragments indicative of the Small Sieve malware’s naming patterns, including a typo in “Microsift.” It also matches a known filename IoC ending with “\AppData\Local\MicrosoftWindowsOutlookDataPlus.txt.” These behaviors matter because malware often writes staged or configuration files into user AppData locations using characteristic filenames; the detection relies on Windows file event telemetry that includes the TargetFilename field.
references:
- https://www.ncsc.gov.uk/static-assets/documents/malware-analysis-reports/small-sieve/NCSC-MAR-Small-Sieve.pdf
- https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Malware/Small-Sieve/file_event_win_malware_small_sieve_evasion_typo.yml
author: Nasreddine Bencherchali (Nextron Systems), X__Junior (Nextron Systems), Huntrule Team
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
license: DRL-1.1
related:
- id: 39466c42-c189-476a-989f-8cdb135c163a
type: derived