int dns_packet_read(DnsPacket *p, size_t sz, const void **ret, size_t *start) {
        assert(p);

        if (p->rindex + sz > p->size)
                return -EMSGSIZE;

        if (ret)
                *ret = (uint8_t*) DNS_PACKET_DATA(p) + p->rindex;

        if (start)
                *start = p->rindex;

        p->rindex += sz;
        return 0;
}

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.