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Not only can you make a good guess at how many customers/etc exist, you can guess individual ones.

World’s easiest hack. You’re looking at /customers/3836/bills? What happens if you change that to 4000? They’re a big company. I bet that exists.

Did they put proper security checks EVERYWHERE? Easy to test.

But if you’re at /customers/{big-long-hex-string}/bill the chances of you guessing another valid ID are basically zero.

Yeah it’s security through obscurity. But it’s really good obscurity.