Bypass UAC via CMSTP

Severity: high
Author: E.M. Anhaus (originally from Atomic Blue Detections, Endgame), oscd.community
Source: upstream

Detect commandline usage of Microsoft Connection Manager Profile Installer (cmstp.exe) to install specially formatted local .INF files

MITRE ATT&CK coverage

Tactic	Techniques
Privilege Escalation	`T1548.002` Abuse Elevation Control Mechanism: Bypass User Account Control
Defense Evasion	`T1218.003` System Binary Proxy Execution: CMSTP, `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: `all of selection_img`

or:
Image|endswith: '\cmstp.exe'
OriginalFileName: CMSTP.EXE

Stage 2: `all of selection_cli`

or:
CommandLine|contains: -au
CommandLine|contains: -ni
CommandLine|contains: -s
CommandLine|contains: '/au'
CommandLine|contains: '/ni'
CommandLine|contains: '/s'

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	`-au` `-ni` `-s` corpus 2 (sigma 2) `/au` `/ni` `/s` corpus 3 (sigma 3)
`Image`	ends_with	`\cmstp.exe` corpus 9 (sigma 9)
`OriginalFileName`	eq	`CMSTP.EXE` corpus 4 (sigma 4)