Explorer NOUACCHECK Flag

Severity: high
Author: Florian Roth (Nextron Systems)
Source: upstream

Detects suspicious starts of explorer.exe that use the /NOUACCHECK flag that allows to run all sub processes of that newly started explorer.exe without any UAC checks

MITRE ATT&CK coverage

Tactic	Techniques
Privilege Escalation	`T1548.002` Abuse Elevation Control Mechanism: Bypass User Account Control
Defense Evasion	`T1548.002` Abuse Elevation Control Mechanism: Bypass User Account Control

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `selection`

CommandLine|contains: '/NOUACCHECK'
Image|endswith: '\explorer.exe'

Stage 2: `not 1 of filter_dc_logon`

or:
ParentCommandLine: 'C:\Windows\system32\svchost.exe -k netsvcs -p -s Schedule'
ParentImage: 'C:\Windows\System32\svchost.exe'

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
`CommandLine`	match	`/NOUACCHECK`
`Image`	ends_with	`\explorer.exe` corpus 12 (sigma 12)
`ParentCommandLine`	eq	`C:\Windows\system32\svchost.exe -k netsvcs -p -s Schedule` corpus 2 (sigma 2)
`ParentImage`	eq	`C:\Windows\System32\svchost.exe` corpus 3 (sigma 3)