One such threat is enumeration. Consider the problem of sending a link to a document in an email. Often, you want this to be a shortened link because reasons

One such threat is enumeration. Consider the problem of sending a link to a document in an email. Often, you want this to be a shortened link because reasons (tracking, revocation, etc). If your link shortening scheme uses integers, it's easy for the document to be distributed to unintended recipients. This is why most link shortening products use long random strings, and the ones that don't are shamed into doing so [1]. [1] https://www.wired.com/2016/04/researchers-cracked-microsoft-googles-shortened-urls-spy-people/ https://www.wired.com/2016/04/researchers-cracked-microsoft-...