[New Rules] PowerShell Suspicious Payload Encoded and Compressed (#1580)

* Create defense_evasion_posh_compressed.toml

* Update defense_evasion_posh_compressed.toml

* Add GzipStream, cover common variations withou using wildcard

* Update defense_evasion_posh_compressed.toml

* Apply suggestions from code review

Co-authored-by: Justin Ibarra <brokensound77@users.noreply.github.com>

* Add false_positives

Co-authored-by: Justin Ibarra <brokensound77@users.noreply.github.com>

rules/windows/defense_evasion_posh_compressed.toml

@@ -0,0 +1,69 @@
+[metadata]
+creation_date = "2021/10/19"
+maturity = "production"
+updated_date = "2021/10/19"
+
+[rule]
+author = ["Elastic"]
+description = """
+Identifies the use of .Net functionality for decompression and base64 decoding combined in PowerShell scripts, which Malware and security tools
+heavily use to deobfuscate payloads and load them directly in memory to bypass defenses.
+"""
+false_positives = ["Legitimate PowerShell Scripts which makes use of compression and encoding"]
+from = "now-9m"
+index = ["winlogbeat-*", "logs-windows.*"]
+language = "kuery"
+license = "Elastic License v2"
+name = "PowerShell Suspicious Payload Encoded and Compressed"
+risk_score = 47
+rule_id = "81fe9dc6-a2d7-4192-a2d8-eed98afc766a"
+severity = "medium"
+tags = ["Elastic", "Host", "Windows", "Threat Detection", "Defense Evasion"]
+timestamp_override = "event.ingested"
+type = "query"
+
+query = '''
+event.category:process and 
+  powershell.file.script_block_text : (
+    (System.IO.Compression.DeflateStream or System.IO.Compression.GzipStream or IO.Compression.DeflateStream or IO.Compression.GzipStream) and
+    FromBase64String
+  )
+'''
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+[[rule.threat.technique]]
+id = "T1140"
+name = "Deobfuscate/Decode Files or Information"
+reference = "https://attack.mitre.org/techniques/T1140/"
+
+[[rule.threat.technique]]
+id = "T1027"
+name = "Obfuscated Files or Information"
+reference = "https://attack.mitre.org/techniques/T1027/"
+
+[rule.threat.tactic]
+id = "TA0005"
+name = "Defense Evasion"
+reference = "https://attack.mitre.org/tactics/TA0005/"
+
+
+[[rule.threat]]
+framework = "MITRE ATT&CK"
+
+[[rule.threat.technique]]
+id = "T1059"
+reference = "https://attack.mitre.org/techniques/T1059/"
+name = "Command and Scripting Interpreter"
+
+  [[rule.threat.technique.subtechnique]]
+  id = "T1059.001"
+  reference = "https://attack.mitre.org/techniques/T1059/001/"
+  name = "PowerShell"
+
+[rule.threat.tactic]
+id = "TA0002"
+reference = "https://attack.mitre.org/tactics/TA0002/"
+name = "Execution"
+