.NET Core Support for macOS High Sierra

# .NET Core Support for macOS High Sierra

## Summary

.NET Core 1.x and 2.0 do not currently support macOS High Sierra (version 10.13, currently in beta) due to changes in macOS. The .NET Core team has been testing macOS High Sierra builds and has filed bugs with Apple so that the two products can work better together.

We intend to have both .NET Core 1.x and 2.0 supporting macOS High Sierra by the time Apple ships the final production release. We are hoping to ship .NET Core 2.0 RTW with High Sierra support.

As .NET Core support for macOS High Sierra is improved, this announcement will be updated with the new status.

## Impact

.NET Core does not work on macOS High Sierra.

## Details

* [dotnet/core #677](https://github.com/dotnet/core/issues/677) -- tracking bug
* [dotnet/corefx #22243](https://github.com/dotnet/corefx/pull/22243) -- workaround for issue 33294006, listed below.

We have reported the following issues to Apple as part of supporting .NET Core on macOS High Sierra.

> Note: The issue links can only be accessed by Apple employees.

* [33275095](https://bugreport.apple.com/web/?problemID=33275095) SSL Failure: macOS 10.13 (17A291j) SecureTransport in server mode can select ciphersuites not applicable to the TLS version.

**.NET Core Note:** When hosting an [SslStream](https://docs.microsoft.com/dotnet/api/system.net.security.sslstream?view=netcore-2.0) limited to TLS 1.0 or TLS 1.1 clients other than Apple SecureTransport may fail to connect.

* [33143630](https://bugreport.apple.com/web/?problemID=33143630) macOS 10.13 (17A291j) SecTrustExecute fails with no error building revocation-enabled chain against a self-issued certificate.

**.NET Core Note:** In this situation an Apple-specific derived type of CryptographicException will be thrown with a message of “No error”.

* [33294006](https://bugreport.apple.com/web/?problemID=33294006) macOS 10.13 (17A291j) libcurl changing to LibreSSL changes thread-safety model from safe to unsafe

**.NET Core Note:** A functioning workaround is in place, but it may regress performance for parallel HTTP requests (to avoid crashes caused by the loss of thread safety).

* [32948209](https://bugreport.apple.com/web/?problemID=32948209) CCCryptorReset returns kCCUnimplemented for ECB on 10.13.

**.NET Core Note:** .NET Core has worked around this regression, so it should not be user-impacting.