Detection rules › Splunk
DLL Execution from Uncommon Process (Sysmon)
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 |
References
Telemetry coverage
| Provider | Record / event type |
|---|---|
| Sysmon | Event ID 1: Process creation |
Rule body
id: '27341.49803'
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_sysmon` (TERM(EventCode=1) OR "<EventID>1<")
",DllRegisterServer" OR ",DllMain" OR ",DllUnregisterServer" OR ",DllInstall" OR
",DllCanUnloadNow" | rex field=process ",(?<dll_function>\S+)"| where not match(process_name,
"(?i)rundll32\.exe") | 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:
- Windows Sysmon
references:
- https://thedfirreport.com/2023/01/09/unwrapping-ursnifs-gifts/
Stages and Predicates
Stage 1: search
`get_endpoint_data` `get_endpoint_data_sysmon` (TERM(EventCode=1) OR "<EventID>1<") ",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_name, "(?i)rundll32\.exe")
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
The rule actively suppresses these predicates.
| Field | Kind | Excluded values | Search |
|---|---|---|---|
process_name | regex_match | "(?i)rundll32.exe" | excludes:process_name |
Indicators
These rows show field, operator, and value matches.
| Field | Kind | Values | Search |
|---|---|---|---|
EventCode | eq |
| field:"EventID" kind:eq value:"1" |
Search terms
These SPL tokens match against raw event text.
| Stage | Term |
|---|---|
| 1 | "<EventID>1<" |
| 1 | ",DllRegisterServer" |
| 1 | ",DllMain" |
| 1 | ",DllUnregisterServer" |
| 1 | ",DllInstall" |
| 1 | ",DllCanUnloadNow" |