But, to be a practical attack, the client software would first have to be modified to conceal the suspiciously big number of bogus failed logins from the end user (if I understood clearly)
From the original article https://mega-awry.io/ (emphasis mine):
> Since the server code is not published, we cannot implement a Proof-of-Concept (PoC) in which the adversary actually controls MEGA. Instead, we implemented a MitM attack by installing a bogus TLS root certificate on the victim.
> ...
> The log in attempt fails, which is only a limitation of the MitM setting. A malicious cloud provider can perform this attack without the user noticing.
Thanks for clarifying it. Yes, the writing about it is surprisingly unclear. Even on the original paper and the dedicated website for this bug, there's a frustrating lack of clarity about what they mean by "login" and how much the user is in the loop of this attack, which could be an important gating factor making it unlikely to ever have been a practical exploit