LogForging.cs

using System;
using System.Diagnostics;
using System.IO;
using System.Net;
using System.Web;
using Microsoft.Extensions.Logging;

class ILogger
{
    public void Warn(string message) { }
}

public class LogForgingHandler : IHttpHandler
{

    public void ProcessRequest(HttpContext ctx)
    {
        String username = ctx.Request.QueryString["username"]; // $ Source
        ILogger logger = new ILogger();
        // BAD: Logged as-is
        logger.Warn(username + " logged in"); // $ Alert
        // GOOD: New-lines removed
        logger.Warn(username.Replace(Environment.NewLine, "") + " logged in");
        // GOOD: New-lines removed
        logger.Warn(username.Replace(Environment.NewLine, "", StringComparison.InvariantCultureIgnoreCase) + " logged in");
        // GOOD: New-lines replaced
        logger.Warn(username.ReplaceLineEndings("") + " logged in");
        // GOOD: Html encoded
        logger.Warn(WebUtility.HtmlEncode(username) + " logged in");
        // BAD: Logged as-is to TraceSource
        new TraceSource("Test").TraceInformation(username + " logged in"); // $ Alert

        Microsoft.Extensions.Logging.ILogger logger2 = null;
        // BAD: Logged as-is
        logger2.LogError(username); // $ Alert

        // GOOD: uses safe extension method that sanitizes internally
        logger.WarnSafe(username + " logged in");
        // BAD: uses unsafe extension method that does not sanitize
        logger.WarnUnsafe(username + " logged in");
    }

    public bool IsReusable
    {
        get
        {
            return true;
        }
    }
}

static class LoggerExtensions
{
    public static void WarnSafe(this ILogger logger, string message)
    {
        logger.Warn(message.Replace(Environment.NewLine, ""));
    }

    public static void WarnUnsafe(this ILogger logger, string message)
    {
        logger.Warn(message); // $ Alert
    }
}