"Wizards have banks?"
"Just the one. Gringotts. Run by goblins."
Harry dropped the bit of sausage he was holding.
"Yeah -- so yeh'd be mad ter try an' rob it, I'll tell yeh that. Never mess with goblins, Harry. Gringotts is the safest place in the world fer anything yeh want ter keep safe -- 'cept maybe Hogwarts. As a matter o' fact, I gotta visit Gringotts anyway. Fer Dumbledore. Hogwarts business." Hagrid drew himself up proudly. "He usually gets me ter do important stuff fer him. Fetchin' you gettin' things from Gringotts -- knows he can trust me, see.
"Got everythin'? Come on, then."
Harry Potter and the Philosopher's Stone Pg 48 (emphasis mine)
“I need to break into a Gringotts vault.”
Harry had not meant to say it so badly...
“Harry—” said Hermione, but she was cut off by Griphook.
“Break into a Gringotts vault?” repeated the goblin, wincing a little as he shifted his position upon the bed. “It is impossible.”
“No, it isn’t,” Ron contradicted him. “It’s been done.”
“Yeah,” said Harry. “The same day I first met you, Griphook. My birthday, seven years ago.”
Harry Potter and the Deathly Hallows Pg 410 (emphasis mine)
“Never mind that stuff ... Is it true? Did you break into Gringotts? Did you escape on a dragon? It’s everywhere, everyone’s talking about it, Terry Boot got beaten up by Carrow for yelling about it in the Great Hall at dinner!”
“Yeah, it’s true,” said Harry.
Harry Potter and the Deathly Hallows Pg 480 (emphasis mine)
If Harry, Ron, and Hermione were able to completely tamper the security of a super massively big bank holding billions of galleons, and other precious things, how could it be considered secure?
There could be many many such people, such attackers!