In openssl versions before 0.10.70, ssl::select_next_proto can return a slice pointing into the server argument's buffer but with a lifetime bound to the client argument. In situations where the server buffer's lifetime is shorter than the client buffer's, this can cause a use after free. This could cause the server to crash or to return arbitrary memory contents to the client.
openssl 0.10.70 fixes the signature of ssl::select_next_proto to properly constrain the output buffer's lifetime to that of both input buffers.
In standard usage of ssl::select_next_proto in the callback passed to SslContextBuilder::set_alpn_select_callback, code is only affected if the server buffer is constructed within the callback. For example:
Not vulnerable - the server buffer has a 'static lifetime:
builder.set_alpn_select_callback(|_, client_protos| {
ssl::select_next_proto(b"\x02h2", client_protos).ok_or_else(AlpnError::NOACK)
});
Not vulnerable - the server buffer outlives the handshake:
let server_protos = b"\x02h2".to_vec();
builder.set_alpn_select_callback(|_, client_protos| {
ssl::select_next_proto(&server_protos, client_protos).ok_or_else(AlpnError::NOACK)
});
Vulnerable - the server buffer is freed when the callback returns:
builder.set_alpn_select_callback(|_, client_protos| {
let server_protos = b"\x02h2".to_vec();
ssl::select_next_proto(&server_protos, client_protos).ok_or_else(AlpnError::NOACK)
});
