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OpenSSL bug exposed up to 255 bytes of server heap and existed since 2011

, a bug has existed in OpenSSL that means innocuous code like: require('tls').connect({port: 443, NPNProtocols: new Uint8Array()}, function(c) {}) or (equivalently, in Python): import ssl, socket assert ssl.HAS_NPN ctx = ssl.create_default_context() ctx.set_npn_protocols([]) ctx.load_verify_locations('root.crt') sock = socket.create_connection(('127.0.0.1', 443)) sock = ctx.wrap_socket(sock, server_hostname='localhost') sock.write('hello') Silently sends up to 255 bytes of the client’s heap to the server. The server must support NPN, and the heap data is encrypted in transit.

You should review your historic usage of SSL_select_next_proto and if you could have ever triggered this bug, I would suggest rolling any secrets available to affected programs. Though valgrind immediately points it out once triggered, and a really basic static pointer bounds analysis should also find it (though likely with a huge false-positive rate in other code.) If SSL_select_next_proto is used straightforwardly in a server’s ALPN callback, there is no impact because other code in openssl validates the client’s extension as being non-empty.

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