[snowwolf]

>> According to your own interpretation of the law

Look, I am not a laywer, and I am happy for someone to correct me here, but this is the wording of the law:

"A personal data breach means a breach of security leading to the accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to, personal data."

Is there anything in that sentence that means a successful credential stuffing attack would not fit the criteria?

[randerson]

a) the data wasn't breached through the credential stuffing attack, it was breached at an earlier time on another site. b) the credential stuffing attack itself is authorized access (because from the site's perspective, the user provided the correct username and password), not unauthorized access.

[snowwolf]

>> a) the data wasn't breached through the credential stuffing attack, it was breached at an earlier time on another site.

Remember GDPR is specifically concerned with "A personal data breach" The original breach that led to the password being leaked was likely also a personal data breach (unless the only thing the hackers managed to access was the username/password database - and even then email address can constitute personal data in some cases), but there is definitely a personal data breach as a result of the credential stuffing attack (in the Deliveroo case, more than likely full home address, possibly other addresses too like work, possibly name, some level of credit card data, order history, etc.).

>> b) the credential stuffing attack itself is authorized access (because from the site's perspective, the user provided the correct username and password), not unauthorized access.

It's certainly authenticated access, but I think you'll struggle to convince a lawyer that it was authorised.

[mannykannot]

I am not disagreeing with your position that such an access is not authorized by the person whose confidentially is compromised, but the phrases from the UK ICO that you quote in making your argument do not say that the mitigations you propose would provide an adequate defense for the website provider, either. Taken in isolation and at face value (which is what you do to make your case), those phrases lead inevitably to the conclusion that password-only authentication cannot possibly suffice as ICO-compliant authorization for access to any personal data whatsoever.