Program.cs

using System.Collections.Concurrent;
using System.Text.RegularExpressions;

WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
builder.Services.AddCors(options =>
{
    options.AddDefaultPolicy(policy =>
    {
        policy.AllowAnyHeader();
        policy.AllowAnyMethod();
        policy.AllowAnyOrigin();
    });
});

Regex videoPattern = new(@"^/(.*)\.ism/(.*)$");
ConcurrentDictionary<string, (string contentType, byte[] bytes)> cache = new();

const string originalVideoPathPrefix = "https://unnormalslive-koct1.streaming.media.azure.net/.../....ism/";

async Task<(string contentType, byte[] bytes)> SendRequestToOriginal(string postfix)
{
    if (cache.TryGetValue(postfix, out (string, byte[]) result))
        return result;

    using HttpClient client = new();
    HttpResponseMessage response = await client.GetAsync(new Uri(originalVideoPathPrefix + postfix));
    Stream stream = response.Content.ReadAsStream();

    long numBytes = response.Content.Headers.ContentLength ?? 0;
    byte[] bytes = new byte[numBytes];

    long numBytesRead = 0;
    while (numBytesRead < numBytes)
    {
        numBytesRead += await stream.ReadAsync(bytes, (int) numBytesRead, (int) (numBytes - numBytesRead));
    }

    string contentType = response.Content.Headers.ContentType?.ToString() ?? "";
    cache[postfix] = (contentType, bytes);
    return (contentType, bytes);
}

WebApplication app = builder.Build();
app.UseCors();

app.Run(async context =>
{
    Match match = videoPattern.Match(context.Request.Path);
    if (match.Success)
    {
        string postfix = match.Groups[2].Value;
        var (contentType, bytes) = await SendRequestToOriginal(postfix);

        Console.WriteLine("Processed: {0} {1}: {2}, {3} bytes", context.Request.Method, postfix, contentType,
            bytes.Length);

        context.Response.ContentType = contentType;
        context.Response.ContentLength = bytes.Length;

        await context.Response.BodyWriter.WriteAsync(bytes);
    }
    else
    {
        Console.WriteLine("Ignored: {0} {1}", context.Request.Method, context.Request.Path);
    }
});

app.Run();