🛡️ SIEM Logs Explained: How to Build Secure and Auditable .NET Apps

By Tareq Marji -

 

In today’s security-conscious world, it’s no longer enough to just log errors and crashes. You need to log security-related events too — for compliance, auditing, and incident response.

That’s where SIEM logs come in.

🧠 What is SIEM?

SIEM (Security Information and Event Management) is a security solution that helps organizations collect, store, analyze, and alert on security-related logs and events.

Popular SIEM platforms include:

  • 🛡️ Splunk
  • 📊 IBM QRadar
  • ☁️ Microsoft Sentinel
  • 🧭 Elastic SIEM
  • 🔍 ArcSight

📦 What Are SIEM Logs?

SIEM logs are not a specific format. They’re structured logs that contain security-relevant events, such as:

  • User logins / logouts
  • Failed login attempts
  • Access to sensitive data
  • Privilege escalation
  • System configuration changes

These logs are sent from apps and servers to a central SIEM system, where they are:

  1. Parsed
  2. Correlated
  3. Stored
  4. Alerted upon

🧱 Anatomy of a Good SIEM Log

A proper SIEM log should include:

Field

Example

Timestamp

2025-05-26T08:45:12Z

Event Type

LoginSuccess, LoginFailure, DataAccess

User

jdoe@example.com

IP Address

192.168.1.15

Application

MyApp.WebAPI

Status

Success or Failure

Context

Custom data (e.g., endpoint, entity ID)

⚙️ How to Send SIEM Logs in .NET

✅ Step 1: Use a Logging Library (Serilog Recommended)

dotnet add package Serilog

dotnet add package Serilog.Sinks.File

You can also use:

  • Serilog.Sinks.Syslog (for direct syslog output)
  • Serilog.Sinks.Elasticsearch (for Elastic SIEM)

✅ Step 2: Configure Logger

csharp

Log.Logger = new LoggerConfiguration()

    .Enrich.WithMachineName()

    .Enrich.WithThreadId()

    .WriteTo.File("logs/security.json", rollingInterval: RollingInterval.Day)

    .CreateLogger();

✅ Step 3: Log Security Events

csharp

Log.Information("User login attempt: {@LoginInfo}", new {

    Timestamp = DateTime.UtcNow,

    EventType = "LoginAttempt",

    User = "admin@example.com",

    IPAddress = "192.168.0.10",

    Status = "Success",

    Application = "MyApp.WebAPI"

});

🔁 You can serialize this as JSON so it can be easily ingested by SIEM tools.

📡 Sending Logs to a SIEM System

Here are some ways you can forward logs to SIEM platforms:

Destination

Method

Splunk

HTTP Event Collector (HEC)

Elastic SIEM

Send to Elasticsearch

Syslog server

Use Serilog + Syslog sink

Azure Sentinel

Azure Monitor / Log Analytics agent

Example (Syslog via Serilog):

csharp

dotnet add package Serilog.Sinks.SyslogMessages

dotnet add package Serilog.Sinks.Network

csharp

CopyEdit

.WriteTo.UdpSyslog("siem-server.example.com", port: 514)

🧪 What Events Should You Log?

Here’s a basic SIEM event checklist:

✅ Login success / failure
✅ Token generation / refresh
✅ User registration / password changes
✅ Access to protected resources
✅ Permission/role changes
✅ Exceptions related to authentication or security

🛠 Best Practices

  • 🧱 Use structured logging (e.g., JSON)
  • 🔒 Avoid sensitive data (no passwords, tokens)
  • ⏳ Include timestamps in UTC
  • 📌 Use consistent naming for event types
  • 🧪 Test with your SIEM system to ensure proper ingestion

📌 Summary

✅ SIEM logs are structured, security-focused logs
✅ .NET apps can emit these logs using Serilog, NLog, etc.
✅ Logs should contain detailed, structured security events
✅ Logs can be sent via files, HTTP, Syslog, or cloud agents
✅ These logs help with security monitoring, alerting, and audits

 

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