static void drive_machine(conn *c) {
    bool stop = false;
    int sfd, flags = 1;
    socklen_t addrlen;
    struct sockaddr_storage addr;
    int nreqs = settings.reqs_per_event;
    int res;

    assert(c != NULL);

    while (!stop) {

        switch(c->state) {
        case conn_listening:
            addrlen = sizeof(addr);
            if ((sfd = accept(c->sfd, (struct sockaddr *)&addr, &addrlen)) == -1) {
                if (errno == EAGAIN || errno == EWOULDBLOCK) {
                    /* these are transient, so don't log anything */
                    stop = true;
                } else if (errno == EMFILE) {
                    if (settings.verbose > 0)
                        fprintf(stderr, "Too many open connections\n");
                    accept_new_conns(false);
                    stop = true;
                } else {
                    perror("accept()");
                    stop = true;
                }
                break;
            }
            if ((flags = fcntl(sfd, F_GETFL, 0)) < 0 ||
                fcntl(sfd, F_SETFL, flags | O_NONBLOCK) < 0) {
                perror("setting O_NONBLOCK");
                close(sfd);
                break;
            }

            dispatch_conn_new(sfd, conn_new_cmd, EV_READ | EV_PERSIST,
                                     DATA_BUFFER_SIZE, tcp_transport);
            stop = true;
            break;

        case conn_waiting:
            if (!update_event(c, EV_READ | EV_PERSIST)) {
                if (settings.verbose > 0)
                    fprintf(stderr, "Couldn't update event\n");
                conn_set_state(c, conn_closing);
                break;
            }

            conn_set_state(c, conn_read);
            stop = true;
            break;

        case conn_read:
            res = IS_UDP(c->transport) ? try_read_udp(c) : try_read_network(c);

            switch (res) {
            case READ_NO_DATA_RECEIVED:
                conn_set_state(c, conn_waiting);
                break;
            case READ_DATA_RECEIVED:
                conn_set_state(c, conn_parse_cmd);
                break;
            case READ_ERROR:
                conn_set_state(c, conn_closing);
                break;
            case READ_MEMORY_ERROR: /* Failed to allocate more memory */
                /* State already set by try_read_network */
                break;
            }
            break;

        case conn_parse_cmd :
            if (try_read_command(c) == 0) {
                /* wee need more data! */
                conn_set_state(c, conn_waiting);
            }

            break;

        case conn_new_cmd:
            /* Only process nreqs at a time to avoid starving other
               connections */

            --nreqs;
            if (nreqs >= 0) {
                reset_cmd_handler(c);
            } else {
                pthread_mutex_lock(&c->thread->stats.mutex);
                c->thread->stats.conn_yields++;
                pthread_mutex_unlock(&c->thread->stats.mutex);
                if (c->rbytes > 0) {
                    /* We have already read in data into the input buffer,
                       so libevent will most likely not signal read events
                       on the socket (unless more data is available. As a
                       hack we should just put in a request to write data,
                       because that should be possible ;-)
                    */
                    if (!update_event(c, EV_WRITE | EV_PERSIST)) {
                        if (settings.verbose > 0)
                            fprintf(stderr, "Couldn't update event\n");
                        conn_set_state(c, conn_closing);
                    }
                }
                stop = true;
            }
            break;

        case conn_nread:
            if (c->rlbytes == 0) {
                complete_nread(c);
                break;
            }
            /* first check if we have leftovers in the conn_read buffer */
            if (c->rbytes > 0) {
                int tocopy = c->rbytes > c->rlbytes ? c->rlbytes : c->rbytes;
                if (c->ritem != c->rcurr) {
                    memmove(c->ritem, c->rcurr, tocopy);
                }
                c->ritem += tocopy;
                c->rlbytes -= tocopy;
                c->rcurr += tocopy;
                c->rbytes -= tocopy;
                if (c->rlbytes == 0) {
                    break;
                }
            }

            /*  now try reading from the socket */
            res = read(c->sfd, c->ritem, c->rlbytes);
            if (res > 0) {
                pthread_mutex_lock(&c->thread->stats.mutex);
                c->thread->stats.bytes_read += res;
                pthread_mutex_unlock(&c->thread->stats.mutex);
                if (c->rcurr == c->ritem) {
                    c->rcurr += res;
                }
                c->ritem += res;
                c->rlbytes -= res;
                break;
            }
            if (res == 0) { /* end of stream */
                conn_set_state(c, conn_closing);
                break;
            }
            if (res == -1 && (errno == EAGAIN || errno == EWOULDBLOCK)) {
                if (!update_event(c, EV_READ | EV_PERSIST)) {
                    if (settings.verbose > 0)
                        fprintf(stderr, "Couldn't update event\n");
                    conn_set_state(c, conn_closing);
                    break;
                }
                stop = true;
                break;
            }
            /* otherwise we have a real error, on which we close the connection */
            if (settings.verbose > 0) {
                fprintf(stderr, "Failed to read, and not due to blocking:\n"
                        "errno: %d %s \n"
                        "rcurr=%lx ritem=%lx rbuf=%lx rlbytes=%d rsize=%d\n",
                        errno, strerror(errno),
                        (long)c->rcurr, (long)c->ritem, (long)c->rbuf,
                        (int)c->rlbytes, (int)c->rsize);
            }
            conn_set_state(c, conn_closing);
            break;

        case conn_swallow:
            /* we are reading sbytes and throwing them away */
            if (c->sbytes == 0) {
                conn_set_state(c, conn_new_cmd);
                break;
            }

            /* first check if we have leftovers in the conn_read buffer */
            if (c->rbytes > 0) {
                int tocopy = c->rbytes > c->sbytes ? c->sbytes : c->rbytes;
                c->sbytes -= tocopy;
                c->rcurr += tocopy;
                c->rbytes -= tocopy;
                break;
            }

            /*  now try reading from the socket */
            res = read(c->sfd, c->rbuf, c->rsize > c->sbytes ? c->sbytes : c->rsize);
            if (res > 0) {
                pthread_mutex_lock(&c->thread->stats.mutex);
                c->thread->stats.bytes_read += res;
                pthread_mutex_unlock(&c->thread->stats.mutex);
                c->sbytes -= res;
                break;
            }
            if (res == 0) { /* end of stream */
                conn_set_state(c, conn_closing);
                break;
            }
            if (res == -1 && (errno == EAGAIN || errno == EWOULDBLOCK)) {
                if (!update_event(c, EV_READ | EV_PERSIST)) {
                    if (settings.verbose > 0)
                        fprintf(stderr, "Couldn't update event\n");
                    conn_set_state(c, conn_closing);
                    break;
                }
                stop = true;
                break;
            }
            /* otherwise we have a real error, on which we close the connection */
            if (settings.verbose > 0)
                fprintf(stderr, "Failed to read, and not due to blocking\n");
            conn_set_state(c, conn_closing);
            break;

        case conn_write:
            /*
             * We want to write out a simple response. If we haven't already,
             * assemble it into a msgbuf list (this will be a single-entry
             * list for TCP or a two-entry list for UDP).
             */
            if (c->iovused == 0 || (IS_UDP(c->transport) && c->iovused == 1)) {
                if (add_iov(c, c->wcurr, c->wbytes) != 0) {
                    if (settings.verbose > 0)
                        fprintf(stderr, "Couldn't build response\n");
                    conn_set_state(c, conn_closing);
                    break;
                }
            }

            /* fall through... */

        case conn_mwrite:
          if (IS_UDP(c->transport) && c->msgcurr == 0 && build_udp_headers(c) != 0) {
            if (settings.verbose > 0)
              fprintf(stderr, "Failed to build UDP headers\n");
            conn_set_state(c, conn_closing);
            break;
          }
            switch (transmit(c)) {
            case TRANSMIT_COMPLETE:
                if (c->state == conn_mwrite) {
                    while (c->ileft > 0) {
                        item *it = *(c->icurr);
                        assert((it->it_flags & ITEM_SLABBED) == 0);
                        item_remove(it);
                        c->icurr++;
                        c->ileft--;
                    }
                    while (c->suffixleft > 0) {
                        char *suffix = *(c->suffixcurr);
                        cache_free(c->thread->suffix_cache, suffix);
                        c->suffixcurr++;
                        c->suffixleft--;
                    }
                    /* XXX:  I don't know why this wasn't the general case */
                    if(c->protocol == binary_prot) {
                        conn_set_state(c, c->write_and_go);
                    } else {
                        conn_set_state(c, conn_new_cmd);
                    }
                } else if (c->state == conn_write) {
                    if (c->write_and_free) {
                        free(c->write_and_free);
                        c->write_and_free = 0;
                    }
                    conn_set_state(c, c->write_and_go);
                } else {
                    if (settings.verbose > 0)
                        fprintf(stderr, "Unexpected state %d\n", c->state);
                    conn_set_state(c, conn_closing);
                }
                break;

            case TRANSMIT_INCOMPLETE:
            case TRANSMIT_HARD_ERROR:
                break;                   /* Continue in state machine. */

            case TRANSMIT_SOFT_ERROR:
                stop = true;
                break;
            }
            break;

        case conn_closing:
            if (IS_UDP(c->transport))
                conn_cleanup(c);
            else
                conn_close(c);
            stop = true;
            break;

        case conn_max_state:
            assert(false);
            break;
        }
    }

    return;
}

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.