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Windows: reduce false positives on Suspicious PowerShell (AMSI Bypass) rule #2311

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Windows: reduce false positives on Suspicious PowerShell (AMSI Bypass) rule #2311

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6 changes: 3 additions & 3 deletions rules/windows/suspicious_powershell_obfuscation.yml
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
# Rule version v1.2.0 (validated 2026-06-24)
# Rule version v1.3.0 (validated 2026-07-01)
dataTypes:
- wineventlog
name: 'Windows: Suspicious PowerShell (Encoded / Download Cradle / AMSI Bypass)'
Expand All @@ -9,11 +9,11 @@ impact:
category: Execution
technique: 'T1059.001 - Command and Scripting Interpreter: PowerShell'
adversary: origin
description: 'Detects high-risk PowerShell script-block content: download cradles, encoded/hidden execution, reflective loading and AMSI bypass markers. Matches the 4104 script-block text (not the -EncodedCommand flag, to avoid benign false positives).'
description: 'Detects high-risk PowerShell script-block content: download cradles, encoded/hidden execution, reflective loading and AMSI bypass markers. Matches the 4104 script-block text (not the -EncodedCommand flag, to avoid benign false positives). v1.3.0: excludes the benign injected PSBreakpoint/AMSI "sentinel" instrumentation harness (markers: sentinelbreakpoints, \windows\sentinel\, Po_wer_Spl_oit_Indicators) that otherwise false-positives on the literal "AmsiInitFailed" token it emits on every PowerShell session. The exclusion is per-script-block, so a real payload executed through the harness is a separate 4104 event and still fires.'
references:
- https://attack.mitre.org/techniques/T1059/001/
where: |
equals("log.eventCode", "4104") && (regexMatch("log.eventDataScriptBlockText", "(?i)(amsiutils|amsiinitfailed|amsiscanbuffer|virtualalloc|writeprocessmemory|getdelegateforfunctionpointer|invoke-mimikatz|invoke-shellcode|invoke-dllinjection|createremotethread)") || (regexMatch("log.eventDataScriptBlockText", "(?i)(downloadstring|downloadfile|downloaddata|invoke-webrequest|net.webclient|start-bitstransfer)") && regexMatch("log.eventDataScriptBlockText", "(?i)(iex|invoke-expression|-enc |-encodedcommand|-w hidden|-windowstyle hidden|frombase64string)")))
equals("log.eventCode", "4104") && !regexMatch("log.eventDataScriptBlockText", "(?i)(sentinelbreakpoints|windows.sentinel.[0-9]|po_wer_spl_oit_indicators)") && (regexMatch("log.eventDataScriptBlockText", "(?i)(amsiutils|amsiinitfailed|amsiscanbuffer|virtualalloc|writeprocessmemory|getdelegateforfunctionpointer|invoke-mimikatz|invoke-shellcode|invoke-dllinjection|createremotethread)") || (regexMatch("log.eventDataScriptBlockText", "(?i)(downloadstring|downloadfile|downloaddata|invoke-webrequest|net.webclient|start-bitstransfer)") && regexMatch("log.eventDataScriptBlockText", "(?i)(iex|invoke-expression|-enc |-encodedcommand|-w hidden|-windowstyle hidden|frombase64string)")))
groupBy:
- dataSource
deduplicateBy: []
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