Detection rules › Splunk
Potential Executable Masquerading as Document - Windows (Sysmon)
Adversaries may abuse double extensions to attempt to conceal dangerous file types of payloads. This use case detects executions of files that appear to be document or image files but execute as script or executable files, or executable/script files with 3 or more spaces or underscores preceding the extension in an effort to obscure the file's actual extension. CVE-2023-38831
MITRE ATT&CK coverage
| Tactic | Techniques |
|---|---|
| Stealth | T1036.007 Masquerading: Double File Extension |
References
Event coverage
| Provider | Event | Title |
|---|---|---|
| Sysmon | Event ID 1 | Process creation |
Rule body yaml
id: '21579.38122'
title: Potential Executable Masquerading as Document - Windows
description: 'Adversaries may abuse double extensions to attempt to conceal dangerous
file types of payloads. This use case detects executions of files that appear to
be document or image files but execute as script or executable files, or executable/script
files with 3 or more spaces or underscores preceding the extension in an effort
to obscure the file''s actual extension. CVE-2023-38831 -- Threat Actor Association:
APT28, APT40, Sandworm Team, UAC-0057 (GhostWriter)'
logic_format: Splunk
logic: '`get_endpoint_data` `get_endpoint_data_sysmon` (TERM(EventCode=1) OR "<EventID>1<")
(".pdf" OR ".jpg" OR ".jpeg" OR "gif" OR ".ico" OR ".doc" OR ".xls" OR ".docx" OR
".xlsx" OR ".png" OR ".xml" OR ".csv" OR ".ppt" OR ".pptx" OR ".rtf" OR ".txt")
(".cmd" OR ".bat" OR ".ps1" OR ".vbs" OR ".js" OR ".py" OR ".exe" OR ".msi" OR ".vbe"
OR ".lnk" OR ".scr") | regex process="(?i)((\s{3,}|_{3,})|(\.(pdf|jpg|jpeg|gif|ico|doc|xls|docx|xlsx|png|xml|csv|ppt|pptx|rtf|txt)(\s+)?))\.(cmd|bat|ps1|vbs|js|py|exe|msi|vbe|lnk|scr)"
| table _time, host, user, process, process_*, parent_process_* | bin span=1s |
stats values(*) as * by _time, host '
techniques:
- defense-evasion:masquerading:double file extension
technique_id:
- T1036.007
data_category:
- Windows Sysmon
- Process command-line parameters
references:
- https://www.group-ib.com/blog/cve-2023-38831-winrar-zero-day/
- https://www.microsoft.com/en-us/security/blog/2024/01/17/new-ttps-observed-in-mint-sandstorm-campaign-targeting-high-profile-individuals-at-universities-and-research-orgs/
Stages and Predicates
Stage 1: search
`get_endpoint_data` `get_endpoint_data_sysmon` (TERM(EventCode=1) OR "<EventID>1<") (".pdf" OR ".jpg" OR ".jpeg" OR "gif" OR ".ico" OR ".doc" OR ".xls" OR ".docx" OR ".xlsx" OR ".png" OR ".xml" OR ".csv" OR ".ppt" OR ".pptx" OR ".rtf" OR ".txt") (".cmd" OR ".bat" OR ".ps1" OR ".vbs" OR ".js" OR ".py" OR ".exe" OR ".msi" OR ".vbe" OR ".lnk" OR ".scr")
Stage 2: regex
| regex process="(?i)((\s{3,}|_{3,})|(\.(pdf|jpg|jpeg|gif|ico|doc|xls|docx|xlsx|png|xml|csv|ppt|pptx|rtf|txt)(\s+)?))\.(cmd|bat|ps1|vbs|js|py|exe|msi|vbe|lnk|scr)"
Stage 3: table
| table _time, host, user, process, process_*, parent_process_*
Stage 4: bucket
| bin span=1s
Stage 5: stats
| stats values(*) as * by _time, host
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.
Search terms
Bare-string tokens in the SPL search body. Splunk matches each token against _raw (the untyped raw event text) anywhere it appears, not against a specific field. These don't surface in the Indicators table because they aren't predicates on a known field.
| Stage | Term |
|---|---|
| 1 | TERM |
| 1 | "<EventID>1<" |
| 1 | ".pdf" |
| 1 | ".jpg" |
| 1 | ".jpeg" |
| 1 | "gif" |
| 1 | ".ico" |
| 1 | ".doc" |
| 1 | ".xls" |
| 1 | ".docx" |
| 1 | ".xlsx" |
| 1 | ".png" |
| 1 | ".xml" |
| 1 | ".csv" |
| 1 | ".ppt" |
| 1 | ".pptx" |
| 1 | ".rtf" |
| 1 | ".txt" |
| 1 | ".cmd" |
| 1 | ".bat" |
| 1 | ".ps1" |
| 1 | ".vbs" |
| 1 | ".js" |
| 1 | ".py" |
| 1 | ".exe" |
| 1 | ".msi" |
| 1 | ".vbe" |
| 1 | ".lnk" |
| 1 | ".scr" |