LocalStack ALB forwards malformed chunked request body to ASP.NET Core/Kestrel target

[rogerfar]

LocalStack version: 2026.5.3

When a valid HTTP/1.1 chunked POST request is sent directly to LocalStack’s Application Load Balancer emulation, the downstream ASP.NET Core/Kestrel target receives malformed chunk framing and rejects the request with:

Microsoft.AspNetCore.Server.Kestrel.Core.BadHttpRequestException: Bad chunk size data.

A fixed-length request to the same LocalStack ALB route succeeds.

Minimal target app

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapPost("/echo", async (HttpRequest request) =>
{
    using var reader = new StreamReader(request.Body);
    var body = await reader.ReadToEndAsync();
    return Results.Text(body, "text/plain");
});

app.Run("http://0.0.0.0:8080");

Create a LocalStack Application Load Balancer that forwards to this container on port 8080.

Fixed-length request

curl -v -X POST "http://localhost.localstack.cloud:4566/_aws/elb/ApplicationLoadBalancer/echo" -H "Host: example.localhost.localstack.cloud" -H "Content-Type: application/json" --data "{\"message\":\"hello\"}"

Chunked request

curl.exe -v -X POST "http://localhost.localstack.cloud:4566/_aws/elb/ApplicationLoadBalancer/echo" -H "Host: example.localhost.localstack.cloud" -H "Content-Type: application/json" -H "Transfer-Encoding: chunked" --data-binary "{\"message\":\"hello\"}"

Expected result

Both requests should succeed and the target should receive:

{"message":"hello"}

Actual result

The fixed-length request succeeds.

The chunked request fails. The ASP.NET Core/Kestrel target rejects the forwarded request body with:

Bad chunk size data.

[bentsku]

Hello and thanks for your report!

The fix has been merged and is available in the localstack/localstack-pro:dev image.

Could you please run docker pull localstack/localstack-pro:dev, which is our current dev build, to see if that fixes it for you?
It will then be available in our next release, end of June.

Thanks again!