[AD REMOVED]
Summary
What can you do if you discover inside the /etc/ssh_config or inside $HOME/.ssh/config configuration this:
If you are root inside the machine you can probably access any ssh connection made by any agent that you can find in the /tmp directory
Impersonate Bob using one of Bob's ssh-agent:
Why does this work?
When you set the variable SSH_AUTH_SOCK you are accessing the keys of Bob that have been used in Bobs ssh connection. Then, if his private key is still there (normally it will be), you will be able to access any host using it.
As the private key is saved in the memory of the agent uncrypted, I suppose that if you are Bob but you don't know the password of the private key, you can still access the agent and use it.
Another option, is that the user owner of the agent and root may be able to access the memory of the agent and extract the private key.
Long explanation and exploitation
Check the original research here
[AD REMOVED]