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Yes Theymos and team have done a great job and I'm sure they have really hardened their cyber posture since 2015. Btalk has some pretty serious bounties for finding flaws in the system so they are doing that too. What i do wonder is if there have been any external penetration tests done with remediation and patch work, I'm sure they have been all over it though.
What is the meaning of external penetration tests - is it some kind of trying to find weaknesses in a security system, or what do you mean?
The admin of this forum have been working hard to improve the security system and not only that - a bug bounty is also provided so that someone who finds a weakness can report it as soon as possible instead of exploiting it for personal gain. Someone may have tried to find a weakness in the security system - but they may not have found it, or theymos have fixed it behind the scenes.