Suspicious XOR Encoded PowerShell Command

Severity: medium
Author: Sami Ruohonen, Harish Segar, Tim Shelton, Teymur Kheirkhabarov, Vasiliy Burov, oscd.community, Nasreddine Bencherchali
Source: upstream

Detects presence of a potentially xor encoded powershell command

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059.001` Command and Scripting Interpreter: PowerShell
Defense Evasion	`T1027` Obfuscated Files or Information, `T1140` Deobfuscate/Decode Files or Information

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `all of selection_img`

or:
Description: 'Windows PowerShell'
Image|endswith: '\powershell.exe'
Image|endswith: '\pwsh.exe'
OriginalFileName: PowerShell.EXE
OriginalFileName: pwsh.dll
Product: 'PowerShell Core 6'

Stage 2: `all of selection_cli_xor`

CommandLine|contains: bxor

Stage 3: `all of selection_cli_other`

or:
CommandLine|contains: '-join '
CommandLine|contains: '-join'''
CommandLine|contains: '-join"'
CommandLine|contains: '-join`'
CommandLine|contains: '::Join'
CommandLine|contains: ForEach
CommandLine|contains: '[char]'
CommandLine|contains: 'for '
CommandLine|contains: 'for('

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	`-join` `-join"` `-join'` -join` `::Join` `ForEach` `[char]` `bxor` `for` corpus 2 (sigma 2) `for(`
`Description`	eq	`Windows PowerShell` corpus 2 (sigma 2)
`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)
`Product`	eq	`PowerShell Core 6` corpus 2 (sigma 2)