Code change:
clarify that user codes are unguessable, - update user code examples,
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This is very minor, but there's a contradiction between the usercode provided in the example in section 3.3.3. and the recommendations that are mentioned in the text above it. The text says: And However, the example provided below it goes against these two recommendations, as it contains a special character (a hyphen) rather than just letters and numbers, and it's longer than eight characters in length: I'd suggest either tweaking the wording, or tweaking the example so that the usercode provided in the example is aligned to the recommendations mentioned in the text.
The specification does not require user codes to be unguessable. Section 3.3.3 (Display of a Short User Code) states that user codes have to be unique and should be short-lived, but this does not imply that codes should be unguessable. It seems that Section 13.27 (Exhaustion of Random Value Space) does not apply to user codes, but to values that are clearly random values "such as nonces, tokens, and randomized URIs". If attackers can guess user codes, the same attack described in URL is possible.
Good catch! The intent is for these to be unguessable, so we can add that explicitly.

Related discussion:
text: When the end user is directed to enter a short code through the response-interact-usercode mode, the client instance communicates the user code to the end user and directs the end user to enter that code <del> at an associated URI.  This mode is designed to be used when the client instance is not able to communicate or facilitate launching an arbitrary URI.  The associated URI could be statically configured with the client instance or in the client software's documentation. As a consequence, these URIs SHOULD be short.  The user code URI MUST be reachable from the end user's browser, though the URI is usually be opened on a separate device from the client instance itself.  The URI MUST be accessible from an HTTP GET request and MUST be protected by HTTPS or equivalent means. </del> <ins> at an associated URI.  The client instance MAY format the user code in such a way as to facilitate memorability and transfer of the code, so long as this formatting does not alter the value as accepted at the user code URI.  For example, a client instance receiving the user code "A1BC3DFF" could choose to display this to the user as "A1BC 3DFF", breaking up the long string into two shorter strings.  In this example, the space in between the two parts would be removed upon its entry into the user code URI. This mode is designed to be used when the client instance is not able to communicate or facilitate launching an arbitrary URI.  The associated URI could be statically configured with the client instance or in the client software's documentation.  As a consequence, these URIs SHOULD be short.  The user code URI MUST be reachable from the end user's browser, though the URI is usually be opened on a separate device from the client instance itself.  The URI MUST be accessible from an HTTP GET request and MUST be protected by HTTPS or equivalent means. </ins> In many cases, the URI indicates a web page hosted at the AS, allowing the AS to authenticate the end user as the RO and