The Windows Filtering Platform has blocked a connection
A network connection attempt was blocked by Windows Filtering Platform or host firewall rules.
Event ID 5157 records network connection block events (`Application Name`, `Direction: Inbound / Outbound`, `Source/Dest IP & Port`, `Filter ID`). High volumes of `5157` inbound blocks indicate active port scanning (`T1046`), while outbound blocks indicating unauthorized processes (`powershell.exe`, `curl.exe`, malware droppers) attempting to connect to blocked command-and-control servers or disallowed ports.
Failure Reason / Root Cause:
Network connection attempt dropped by WFP filter rule (`Filter Run-Time ID`).
Raw Event XML snippet
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Microsoft-Windows-Security-Auditing" Guid="{54849625-5478-4994-A5BA-3E3B0328C30D}" />
<EventID>5157</EventID>
<Version>0</Version>
<Level>0</Level>
<Task>12544</Task>
<Opcode>0</Opcode>
<Keywords>0x8020000000000000</Keywords>
<TimeCreated SystemTime="2026-07-17T09:54:44.937Z" />
<EventRecordID>1048576</EventRecordID>
<Correlation />
<Execution ProcessID="612" ThreadID="1024" />
<Channel>Security</Channel>
<Computer>DC01.enterprise.internal</Computer>
<Security />
</System>
<EventData>
<Data Name="SubjectUserSid">S-1-5-18</Data>
<Data Name="SubjectUserName">SYSTEM</Data>
<Data Name="SubjectDomainName">NT AUTHORITY</Data>
<Data Name="SubjectLogonId">0x3e7</Data>
<!-- Event telemetry fields specific to The Windows Filtering Platform has blocked a connection -->
</EventData>
</Event>MITRE ATT&CK mapping
| ID | Technique | Tactic |
|---|---|---|
T1071 | Application Layer Protocol | Command and Control |
T1046 | Network Service Discovery | Discovery |
Recommended actions
- Inspect `Application Name` and `Destination Address` for outbound blocks. If a local application repeatedly attempts connections to external IP addresses on non-standard ports, investigate the endpoint for beaconing malware.