A remote, unauthenticated TLS client can make a server crash or call through a freed pointer if its sni callback assigns a different context to SSLSocket.context (the documented way to select a certificate per server name) and nothing else keeps the original ssl.SSLContext alive. Typical cases are servers that create an SSLContext per connection or replace it while connections are open; servers that wrap their listening socket with it are not affected.

Mitigation: keep a reference to every SSLContext that sets sni callback for the lifetime of the server. TLS clients are not affected.