Wlrmdr.EXE Uncommon Argument Or Child Process

Severity: medium
Author: frack113, manasmbellani
Source: upstream

Detects the execution of "Wlrmdr.exe" with the "-u" command line flag which allows anything passed to it to be an argument of the ShellExecute API, which would allow an attacker to execute arbitrary binaries. This detection also focuses on any uncommon child processes spawned from "Wlrmdr.exe" as a supplement for those that posses "ParentImage" telemetry.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1218` System Binary Proxy Execution

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `selection_parent`

ParentImage|endswith: '\wlrmdr.exe'

Stage 2: `all of selection_child_img`

or:
Image|endswith: '\wlrmdr.exe'
OriginalFileName: WLRMNDR.EXE

Stage 3: `all of selection_child_cli_flags_s`

CommandLine|contains: '-s '

Stage 4: `all of selection_child_cli_flags_f`

CommandLine|contains: '-f '

Stage 5: `all of selection_child_cli_flags_t`

CommandLine|contains: '-t '

Stage 6: `all of selection_child_cli_flags_m`

CommandLine|contains: '-m '

Stage 7: `all of selection_child_cli_flags_a`

CommandLine|contains: '-a '

Stage 8: `all of selection_child_cli_flags_u`

CommandLine|contains: '-u '

Stage 9: `not 1 of filter_main_*`

or:
ParentImage: ''
ParentImage: -
ParentImage: 'C:\Windows\System32\winlogon.exe'
ParentImage: null

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	`-a` corpus 2 (sigma 2) `-f` `-m` corpus 2 (sigma 2) `-s` corpus 3 (sigma 3) `-t` `-u` corpus 3 (sigma 3)
`Image`	ends_with	`\wlrmdr.exe`
`OriginalFileName`	eq	`WLRMNDR.EXE`
`ParentImage`	ends_with	`\wlrmdr.exe`
`ParentImage`	eq	`-` corpus 5 (sigma 5) `C:\Windows\System32\winlogon.exe`

Wlrmdr.EXE Uncommon Argument Or Child Process

MITRE ATT&CK coverage

Event coverage

Stages and Predicates

Stage 1: selection_parent

Stage 2: all of selection_child_img

Stage 3: all of selection_child_cli_flags_s

Stage 4: all of selection_child_cli_flags_f

Stage 5: all of selection_child_cli_flags_t

Stage 6: all of selection_child_cli_flags_m

Stage 7: all of selection_child_cli_flags_a

Stage 8: all of selection_child_cli_flags_u

Stage 9: not 1 of filter_main_*