cpp-httplib through 0.5.8 does not filter \r\n in parameters passed into the set_redirect and set_header functions, which creates possibilities for CRLF injection and HTTP response splitting in some specific contexts.
https://github.com/yhirose/cpp-httplib/issues/425
https://gist.github.com/shouc/a9330df817128bc4c4132abf3de09495