If this were a game of chess, checkmate is getting arbitrary code to run in the context of the user. If you have an existing ssh process running as the same user then you can attach it with a debugger and inject code into it.
A debugger injecting code into a process under the same context isn't a vulnerability. It's supposed to be able to do that. And OpenSSH is supposed to allow you to multiplex sessions. It's a feature.
The vulnerability is the attacker being able to run arbitrary code as the user.
That's a feature, not a bug. SSH is a power tool; if your users can not be trusted with power tools, it is your responsibility to provide them with something brightly coloured, drool-proof and locked down.
