Comment by rfjakob

From man ssh-add:

     -c      Indicates that added identities should be subject to confirmation be‐
             fore being used for authentication.  Confirmation is performed by
             ssh-askpass(1).  Successful confirmation is signaled by a zero exit
             status from ssh-askpass(1), rather than text entered into the re‐
             quester.

I'm a bit surprised that this is not mention in the article, as this seems very useful to make exploits more difficult.