They have been trying to access Outlook accounts for the same reason for years, by asking for your password in a deceptive way.
But if this is true it's next-level evil indeed, it seems that Gmail has an open XSS vulnerability, and LinkedIn (and Facebook too?) are using it to outright hack into your account.
Let's call it "a window with a shitty lock" instead of a "back door", if it's an unintentional vulnerability. Then we can just use "back door [left open]" to mean something intentional. Or, you know "key under a rock in the garden" because only certain people know where it is. Actually, I think that's where "Window( with a )S(hitty lock)" 95 first got it's name.