In modern cloud environments, threats don’t wait for meetings, approvals, or planning sessions.
Sometimes an attack hits so fast that your only advantage is instinct, experience, and the ability to act immediately.

Last month, I experienced exactly that — a coordinated impersonation attempt from multiple bad actors in Europe using public cloud hosting (GCP) as their relay. They created their own connectors and attempted to impersonate internal executives and accounting contacts.

The attack bypassed standard controls because:

• They used legitimate cloud IP ranges
• They generated perfect SPF/DKIM passes
• Their mail flow looked “clean” until you read the headers
• They used crafted envelope senders + forged display names

The only way to stop them instantly — before users were tricked — was to drop two transport rules at highest priority using PowerShell.
These acted as “circuit breakers” until perimeter firewall rules could be deployed.

Below is the exact PowerShell approach, redacted and rewritten for general use.

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🚨 Reflex Script #1 — Emergency “Kill Switch” Rule

Purpose: If attackers are impersonating an internal address like [email protected], this rule blocks any external sender who uses that address in the envelope from or header from.

# Connect to Exchange Online
Connect-ExchangeOnline

# Create emergency kill-switch rule
New-TransportRule -Name "KILL SWITCH: Block external spoofing of noreply" `
-FromScope External `
-HeaderContainsMessageHeader "From" `
-HeaderContainsWords "noreply@" `
-SetSCL 9 `
-StopRuleProcessing $true `
-Priority 0

What this rule does instantly:

• Stops external senders pretending to be noreply@
• Sets SCL=9 so the message is quarantined or rejected (depending on policy)
• Stops evaluation of all other rules — making it hit within milliseconds

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🚨 Reflex Script #2 — Block ALL External Senders Using a Protected Address

Attackers often rotate payloads or try other internal addresses.
This second rule blocks all attempts — even if they change tactics.

New-TransportRule -Name "BLOCK ALL External From Protected Address" `
-FromScope External `
-SenderAddressMatchesPatterns "noreply@", "billing@", "alerts@" `
-SetSCL 9 `
-StopRuleProcessing $true `
-Priority 1

You can modify the patterns depending on the address being abused.

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🛡️ Why This Worked Instantly

These scripts bypass the UI delay and:

• Apply before EOP content filters
• Hit prior to Safe Links/Safe Attachments
• Trigger even if messages pass SPF/DKIM/DMARC
• Intercept mail before it reaches the user’s mailbox
• Provide time to analyze, trace, and escalate

This is why reflex PowerShell is critical for senior-level engineers — the GUI is too slow during live attacks.

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🔐 Permanent Fix: Layer 3 / Layer 7 Firewall Enforcement

Once the immediate threat was stopped with PowerShell, the permanent fix required:

Layer 3 (IP-based blocklists)

Blocking:

• Abused GCP IP ranges
• Known threat actor networks
• Anonymous compute nodes

Layer 7 (Application-layer filtering)

Policies included:

• Block SMTP traffic from unknown hosts
• Block unauthorized connector-based submissions
• Strict URL filtering for phishing redirectors
• Geo-blocking regions with no business presence

Once these firewall measures were active, the PowerShell Kill Switch rules were safely disabled to avoid unnecessary mail flow impact.

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💡 Lessons Learned

1. Bad actors are fast — you must be faster.
2. Transport rules + PowerShell are your instant “circuit breakers.”
3. SPF/DKIM/DMARC are not enough when attackers leverage cloud infrastructure.
4. Layer 3 and Layer 7 controls create the “permanent seal.”
5. Instant response + longer-term architecture = real protection.

This is the type of real-world, battle-tested example that hiring panels want to hear.
Not theory — lived experience.

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