My client side code generates UUIDs and sends them to the server.
For example, '6ea140caa83b485f9c98ebaacfb536ce' would be a valid uuid4 to send back.
Is there any way to detect or prevent a user sending back a valid but "user generated" uuid4 like 'babebabebabe4abebabebabebabebabe'?
For example, one way to prevent a certain class of these would be looking at the number of occurrences of 0's and 1's in the binary representation of the number. This could work for a string like '00000000000040000000000000000000' but not for all strings.
It depends a little ...
there is no way to be entirely sure, but depending on the UUID version/subtype you are using there MIGHT be a way to detect at least some irregular values:
https://www.rfc-editor.org/rfc/rfc4122#section-4.1 defines the original version 1 of UUIDs, and a layout for the uuid fields ...
you could for example check if the version and variant fields are valid...
if your UUID generation actually uses Version 1 you could, in addition to the first test of version and variant, test if the timestamp is in a valid range ... for example, it might be unlikely that the UUID in question was generated in the year 1600 ... or in the future
so tests like there could be applied to check if the value actually makes sense, or is complete gibberish ... it can not protect you against someone thinking: ok ... lets analyze this and provide a manually choosen value that satisfies all conditions