CURLcode Curl_connected_proxy(struct connectdata *conn, int sockindex)
{
  CURLcode result = CURLE_OK;

  if(conn->bits.socksproxy) {
#ifndef CURL_DISABLE_PROXY
    /* for the secondary socket (FTP), use the "connect to host"
     * but ignore the "connect to port" (use the secondary port)
     */
    const char * const host = conn->bits.httpproxy ?
                              conn->http_proxy.host.name :
                              conn->bits.conn_to_host ?
                              conn->conn_to_host.name :
                              sockindex == SECONDARYSOCKET ?
                              conn->secondaryhostname : conn->host.name;
    const int port = conn->bits.httpproxy ? (int)conn->http_proxy.port :
                     sockindex == SECONDARYSOCKET ? conn->secondary_port :
                     conn->bits.conn_to_port ? conn->conn_to_port :
                     conn->remote_port;
    conn->bits.socksproxy_connecting = TRUE;
    switch(conn->socks_proxy.proxytype) {
    case CURLPROXY_SOCKS5:
    case CURLPROXY_SOCKS5_HOSTNAME:
      result = Curl_SOCKS5(conn->socks_proxy.user, conn->socks_proxy.passwd,
                         host, port, sockindex, conn);
      break;

    case CURLPROXY_SOCKS4:
    case CURLPROXY_SOCKS4A:
      result = Curl_SOCKS4(conn->socks_proxy.user, host, port, sockindex,
                           conn);
      break;

    default:
      failf(conn->data, "unknown proxytype option given");
      result = CURLE_COULDNT_CONNECT;
    } /* switch proxytype */
    conn->bits.socksproxy_connecting = FALSE;
#else
  (void)sockindex;
#endif /* CURL_DISABLE_PROXY */
  }

  return result;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.