Potential Encoded PowerShell Patterns In CommandLine

Severity: low
Author: Teymur Kheirkhabarov (idea), Vasiliy Burov (rule), oscd.community, Tim Shelton
Source: upstream

Detects specific combinations of encoding methods in PowerShell via the commandline

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059.001` Command and Scripting Interpreter: PowerShell
Defense Evasion	`T1027` Obfuscated Files or Information

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `selection_img`

or:
Image|endswith: '\powershell.exe'
Image|endswith: '\pwsh.exe'
OriginalFileName: PowerShell.EXE
OriginalFileName: pwsh.dll

Stage 2: `all of selection_to_1`

or:
CommandLine|contains: ToByte
CommandLine|contains: ToDecimal
CommandLine|contains: ToInt
CommandLine|contains: ToSByte
CommandLine|contains: ToSingle
CommandLine|contains: ToUint

Stage 3: `all of selection_to_2`

or:
CommandLine|contains: String
CommandLine|contains: ToChar
CommandLine|contains: ToString

Stage 4: `1 of selection_gen_1`

CommandLine|contains: char
CommandLine|contains: join

Stage 5: `1 of selection_gen_2`

CommandLine|contains: join
CommandLine|contains: split

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	`String` `ToByte` `ToChar` `ToDecimal` `ToInt` `ToSByte` `ToSingle` `ToString` `ToUint` `char` `join` `split`
`Image`	ends_with	`\powershell.exe` corpus 143 (sigma 143) `\pwsh.exe` corpus 140 (sigma 140)
`OriginalFileName`	eq	`PowerShell.EXE` corpus 64 (sigma 60, splunk 4) `pwsh.dll` corpus 72 (sigma 68, splunk 4)

Potential Encoded PowerShell Patterns In CommandLine

MITRE ATT&CK coverage

Event coverage

Stages and Predicates

Stage 1: selection_img

Stage 2: all of selection_to_1

Stage 3: all of selection_to_2