mod_http_oauth2: Bind refresh tokens to client Prevent one OAuth client from using the refresh tokens issued to another client as required by RFC 6819 section 5.2.2.2 See also draft-ietf-oauth-security-topics-22 section 2.2.2 Thanks to OAuch for pointing out this issue

<tr>
  <th>What is {x} {op} {y}?</th><td>
    <input name="captcha_challenge" type="hidden" value="{challenge}">
    <input name="captcha_reply" pattern="[0-9]+" required type="number">
  </td></tr>