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It seems there is an issue in mono's implementation of System.Net.Security.SslStream, or some lower level, class. The app in question uses the WebSockets4Net library to make a web sockets connection. When using secure web socket connections over SSL, an exception is thrown. The WebSockets4Net library uses System.Net.Security.SslStream to do its work.
-> class libs.
Should be fixed with https://github.com/mono/mono/pull/929
Adding the text from the pull request comment here to improve Bugzilla findability
--- From https://github.com/mono/mono/pull/929 ----
When reading and writing data at the same time over an System.Net.Security.SslStream, an exception will be thrown often. Usually the exception is: Mono.Security.Protocol.Tls.TlsException "Bad record MAC"
The issue is that the single TlsCipherSuite instance used for both reading and writing does not synchronize access to the “header” member. If the contents get corrupted the MAC calculation returned by ComputServerRecordMAC or ComputeClientRecordMAC is incorrect.
... Reproduced in OS X, iOS and Windows.