static void append_stats(const char *key, const uint16_t klen,
                  const char *val, const uint32_t vlen,
                  const void *cookie)
{
    /* value without a key is invalid */
    if (klen == 0 && vlen > 0) {
        return ;
    }

    conn *c = (conn*)cookie;

    if (c->protocol == binary_prot) {
        size_t needed = vlen + klen + sizeof(protocol_binary_response_header);
        if (!grow_stats_buf(c, needed)) {
            return ;
        }
        append_bin_stats(key, klen, val, vlen, c);
    } else {
        size_t needed = vlen + klen + 10; // 10 == "STAT = \r\n"
        if (!grow_stats_buf(c, needed)) {
            return ;
        }
        append_ascii_stats(key, klen, val, vlen, c);
    }

    assert(c->stats.offset <= c->stats.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.