Windows AMSI Related Registry Tampering Via CommandLine

Severity: high
Author: Swachchhanda Shrawan Poudel (Nextron Systems)
Source: upstream

Detects tampering of AMSI (Anti-Malware Scan Interface) related registry values via command line tools such as reg.exe or PowerShell. AMSI provides a generic interface for applications and services to integrate with antimalware products. Adversaries may disable AMSI to evade detection of malicious scripts and code execution.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1562.001` Impair Defenses: Disable or Modify Tools, `T1562.006` Impair Defenses: Indicator Blocking

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `selection_key`

CommandLine|contains: AmsiEnable
CommandLine|contains: '\Software\Microsoft\Windows Script\Settings'

Stage 2: `all of selection_powershell_img`

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

Stage 3: `all of selection_powershell_cmd`

or:
CommandLine|contains: New-ItemProperty
CommandLine|contains: Set-ItemProperty
CommandLine|contains: 'sp '

Stage 4: `all of selection_reg_img`

or:
Image|endswith: '\reg.exe'
OriginalFileName: reg.exe

Stage 5: `all of selection_reg_cmd`

CommandLine|contains: add

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	`AmsiEnable` `New-ItemProperty` corpus 4 (sigma 4) `Set-ItemProperty` corpus 4 (sigma 4) `\Software\Microsoft\Windows Script\Settings` `add` corpus 16 (sigma 16) `sp` corpus 2 (sigma 2)
`Image`	ends_with	`\powershell.exe` corpus 143 (sigma 143) `\pwsh.exe` corpus 140 (sigma 140) `\reg.exe` corpus 46 (sigma 46)
`OriginalFileName`	eq	`PowerShell.EXE` corpus 64 (sigma 60, splunk 4) `pwsh.dll` corpus 72 (sigma 68, splunk 4) `reg.exe` corpus 29 (sigma 29)

Windows AMSI Related Registry Tampering Via CommandLine

MITRE ATT&CK coverage

Event coverage

Stages and Predicates

Stage 1: selection_key

Stage 2: all of selection_powershell_img

Stage 3: all of selection_powershell_cmd