Certificate chain building and verification should be cancellable AuthenticateAs* on OSX and Linux:

• runtime/src/libraries/System.Net.Security/src/System/Net/Security/Pal.OSX/SafeDeleteSslContext.cs

  Line 325 in b44492d

  X509Chain chain = TLSCertificateExtensions.BuildNewChain(

• runtime/src/libraries/Common/src/Interop/Unix/System.Security.Cryptography.Native/Interop.OpenSsl.cs

  Line 213 in 4bdf468

  chain = TLSCertificateExtensions.BuildNewChain(cert, includeClientApplicationPolicy: false);