Klarna users are being signed in to random accounts

Their German counterpart, Sofortüberweisung, didn't properly blacklist test credentials given out by banks e.g. to developers in the beginning, so people could simply use those and pay for goods and services with fake accounts.

For me there are so many red flags with all these services, as they basically "steal" your credentials to log into your online banking. And while they claim that they only use the credentials to make transfers they could as well look at all my other account data. I really wonder how such a scheme can be legal and how banks can allow this, as they normally tell people to never give their credentials to anyone. The situation of course recently improved with the mandated 2FA for logins and transfers, but still there are so many attack vectors in this model that it boggles my mind how it can still exist.

Have been using SÜ for years until I learned that they not just facilitate the transfer but abuse their role to dump bank transfer data worth several months. I don't use that service anymore.

That sounds pretty bad! I always thought the login flow was super sketchy, but wasn't aware of this part — has this been covered/analyzed somewhere or is it evident from their terms or something?