ssize_t ne2000_receive(NetClientState *nc, const uint8_t *buf, size_t size_)
{
    NE2000State *s = qemu_get_nic_opaque(nc);
    int size = size_;
    uint8_t *p;
    unsigned int total_len, next, avail, len, index, mcast_idx;
    uint8_t buf1[60];
    static const uint8_t broadcast_macaddr[6] =
        { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };

#if defined(DEBUG_NE2000)
    printf("NE2000: received len=%d\n", size);
#endif

    if (s->cmd & E8390_STOP || ne2000_buffer_full(s))
        return -1;

    /* XXX: check this */
    if (s->rxcr & 0x10) {
        /* promiscuous: receive all */
    } else {
        if (!memcmp(buf,  broadcast_macaddr, 6)) {
            /* broadcast address */
            if (!(s->rxcr & 0x04))
                return size;
        } else if (buf[0] & 0x01) {
            /* multicast */
            if (!(s->rxcr & 0x08))
                return size;
            mcast_idx = compute_mcast_idx(buf);
            if (!(s->mult[mcast_idx >> 3] & (1 << (mcast_idx & 7))))
                return size;
        } else if (s->mem[0] == buf[0] &&
                   s->mem[2] == buf[1] &&
                   s->mem[4] == buf[2] &&
                   s->mem[6] == buf[3] &&
                   s->mem[8] == buf[4] &&
                   s->mem[10] == buf[5]) {
            /* match */
        } else {
            return size;
        }
    }


    /* if too small buffer, then expand it */
    if (size < MIN_BUF_SIZE) {
        memcpy(buf1, buf, size);
        memset(buf1 + size, 0, MIN_BUF_SIZE - size);
        buf = buf1;
        size = MIN_BUF_SIZE;
    }

    index = s->curpag << 8;
    if (index >= NE2000_PMEM_END) {
        index = s->start;
    }
    /* 4 bytes for header */
    total_len = size + 4;
    /* address for next packet (4 bytes for CRC) */
    next = index + ((total_len + 4 + 255) & ~0xff);
    if (next >= s->stop)
        next -= (s->stop - s->start);
    /* prepare packet header */
    p = s->mem + index;
    s->rsr = ENRSR_RXOK; /* receive status */
    /* XXX: check this */
    if (buf[0] & 0x01)
        s->rsr |= ENRSR_PHY;
    p[0] = s->rsr;
    p[1] = next >> 8;
    p[2] = total_len;
    p[3] = total_len >> 8;
    index += 4;

    /* write packet data */
    while (size > 0) {
        if (index <= s->stop)
            avail = s->stop - index;
        else
            break;
        len = size;
        if (len > avail)
            len = avail;
        memcpy(s->mem + index, buf, len);
        buf += len;
        index += len;
        if (index == s->stop)
            index = s->start;
        size -= len;
    }
    s->curpag = next >> 8;

    /* now we can signal we have received something */
    s->isr |= ENISR_RX;
    ne2000_update_irq(s);

    return size_;
}

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.