Suspicious Workstation Locking via Rundll32

Severity: medium
Author: frack113
Source: upstream

Detects a suspicious call to the user32.dll function that locks the user workstation

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `all of selection_call_img`

or:
Image|endswith: '\rundll32.exe'
OriginalFileName: RUNDLL32.EXE

Stage 2: `all of selection_call_parent`

ParentImage|endswith: '\cmd.exe'

Stage 3: `all of selection_call_cli`

CommandLine|contains: 'user32.dll,'

Stage 4: `all of selection_function`

CommandLine|contains: LockWorkStation

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	`LockWorkStation` `user32.dll,`
`Image`	ends_with	`\rundll32.exe` corpus 76 (sigma 76)
`OriginalFileName`	eq	`RUNDLL32.EXE` corpus 28 (sigma 25, splunk 3)
`ParentImage`	ends_with	`\cmd.exe` corpus 13 (sigma 13)

Suspicious Workstation Locking via Rundll32

Event coverage

Stages and Predicates

Stage 1: all of selection_call_img

Stage 2: all of selection_call_parent

Stage 3: all of selection_call_cli

Stage 4: all of selection_function

Indicators

Stage 1: `all of selection_call_img`

Stage 2: `all of selection_call_parent`

Stage 3: `all of selection_call_cli`

Stage 4: `all of selection_function`