In Rhonabwy through 1.1.13, HMAC signature verification uses a strcmp function that is vulnerable to side-channel attacks, because it stops the comparison when the first difference is spotted in the two signatures. (The fix uses gnutls_memcmp, which has constant-time execution.)
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.
A device's real time power consumption may be monitored during security token evaluation and the information gleaned may be used to determine the value of the reference token.