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.)
https://github.com/babelouest/rhonabwy/commit/f9fd9a1c77e48b514ebb3baf0360f87eef3d846e