Detection rules › Splunk
DLL Execution from Uncommon Process (PowerShell)
During an attack using Ursnif malware, threat actors were observed executing DLLs with a renamed rundll32 binary. This use case detects execution of the DLL functions DllRegisterServer, DllMain, DllUnregisterServer, DllInstall, or DllCanUnloadNow from processes other than rundll32.exe
MITRE ATT&CK coverage
| Tactic | Techniques |
|---|---|
| Stealth | T1218.011 System Binary Proxy Execution: Rundll32 |
References
Event coverage
| Provider | Event | Title |
|---|---|---|
| PowerShell | Event ID 4104 | Creating Scriptblock text (MessageNumber of MessageTotal). |
Rule body yaml
id: '27341.49804'
title: DLL Execution from Uncommon Process
description: During an attack using Ursnif malware, threat actors were observed executing
DLLs with a renamed rundll32 binary. This use case detects execution of the DLL
functions DllRegisterServer, DllMain, DllUnregisterServer, DllInstall, or DllCanUnloadNow
from processes other than rundll32.exe. Living Off the Land Binary and Scripts (LOLBAS)
(LOLBIN)
logic_format: Splunk
logic: '`get_endpoint_data` `get_endpoint_data_powershell` (TERM(EventCode=4104) OR
"<EventID>4104<") ",DllRegisterServer" OR ",DllMain" OR ",DllUnregisterServer" OR
",DllInstall" OR ",DllCanUnloadNow" | rex field=process ",(?<dll_function>\S+)"|
where not match(process, "(?i)rundll32") | table _time, host, user, process, process_*,
parent_process*, dll_function | bin span=1s | stats values(*) as * by _time, host '
techniques:
- defense-evasion:system binary proxy execution:rundll32
technique_id:
- T1218.011
data_category:
- PowerShell logs
references:
- https://thedfirreport.com/2023/01/09/unwrapping-ursnifs-gifts/
Stages and Predicates
Stage 1: search
`get_endpoint_data` `get_endpoint_data_powershell` (TERM(EventCode=4104) OR "<EventID>4104<") ",DllRegisterServer" OR ",DllMain" OR ",DllUnregisterServer" OR ",DllInstall" OR ",DllCanUnloadNow"
Stage 2: rex
| rex field=process ",(?<dll_function>\S+)"
Stage 3: where
| where not match(process, "(?i)rundll32")
Stage 4: table
| table _time, host, user, process, process_*, parent_process*, dll_function
Stage 5: bucket
| bin span=1s
Stage 6: stats
| stats values(*) as * by _time, host
Exclusions
Top-level NOT(...) conjuncts: predicates this rule actively suppresses.
| Field | Kind | Excluded values |
|---|---|---|
process | match | "(?i)rundll32" |
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.
| Field | Kind | Values |
|---|---|---|
EventCode | eq |
|
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>4104<" |
| 1 | ",DllRegisterServer" |
| 1 | ",DllMain" |
| 1 | ",DllUnregisterServer" |
| 1 | ",DllInstall" |
| 1 | ",DllCanUnloadNow" |