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There's nothing white-hat about this. He accessed as much private data as possible, and didn't report the vulnerability to Twitter or to affected users.

this is grey hat at best

What was ruined? And who ruined it?

Access to what had until then been an 0day.

Ibrahim Balic "ruined" it, though public bragging (which is not responsible disclosure).

Ruining access to a 0day sounds like a good thing to me. I don't want people to have access to 0days.

Some people dislike the term "responsible disclosure" and believe it's not a moral imperative:

https://hn.algolia.com/?query=author:tptacek%20responsible%2...

Of course what Ibrahim did wasn't full disclosure either, so he shouldn't be fully congratulated. But bragging about it was better than keeping silent about it in this case.