PowerShell Base64 Download Cradle via FromBase64String and Invoke-Expression (via ps_script)

ATT&CK techniques

Execution → Defense Evasion

Recon
Resource Dev
Initial Access
Execution
- T1059.001PowerShell
Persistence
Priv Esc
Defense Evasion
- T1140Deobfuscate/Decode Files or Information
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule detects PowerShell that decodes a base64 blob with FromBase64String and immediately runs it through Invoke-Expression, the staged download-and-execute cradle used by the LNK phishing chains analyzed by STRT to fetch and launch a second-stage script. Adversaries leverage this pattern to reconstruct and execute obfuscated code in memory, so this behavior warrants review during phishing investigation.

Reporting behind it

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PowerShell Base64 Download Cradle via FromBase64String and Invoke-Expression (via ps_script)

Pivot detection · T1059.001 · 9 related

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.

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