lastpass-cli: a safe and open-source work-around
Rather than embedding a blob in the browser that could have who knows what kinds of unexpected behaviour (like this little gem), there's an alternative: the command-line LastPass client.
Pretty simple to use, and while not as "convenient", it can't spaff your private details into fields without you knowing because you have to manually copy and paste the detail in yourself.
(Yes, there is a risk of something spying on your clipboard … in which case I'd argue you have bigger problems!)
Biting the hand that feeds IT
