long Cluster::Load(long long& pos, long& len) const {
  assert(m_pSegment);
  assert(m_pos >= m_element_start);

 if (m_timecode >= 0) // at least partially loaded
 return 0;

  assert(m_pos == m_element_start);
  assert(m_element_size < 0);

 IMkvReader* const pReader = m_pSegment->m_pReader;

 long long total, avail;

 const int status = pReader->Length(&total, &avail);

 if (status < 0) // error
 return status;

  assert((total < 0) || (avail <= total));
  assert((total < 0) || (m_pos <= total)); // TODO: verify this

  pos = m_pos;

 long long cluster_size = -1;

 {
 if ((pos + 1) > avail) {
      len = 1;
 return E_BUFFER_NOT_FULL;
 }

 long long result = GetUIntLength(pReader, pos, len);

 if (result < 0) // error or underflow
 return static_cast<long>(result);

 if (result > 0) // underflow (weird)
 return E_BUFFER_NOT_FULL;


 if ((pos + len) > avail)
 return E_BUFFER_NOT_FULL;

 const long long id_ = ReadUInt(pReader, pos, len);

 if (id_ < 0) // error
 return static_cast<long>(id_);

 if (id_ != 0x0F43B675) // Cluster ID
 return E_FILE_FORMAT_INVALID;

    pos += len; // consume id


 if ((pos + 1) > avail) {
      len = 1;
 return E_BUFFER_NOT_FULL;
 }

    result = GetUIntLength(pReader, pos, len);

 if (result < 0) // error
 return static_cast<long>(result);

 if (result > 0) // weird
 return E_BUFFER_NOT_FULL;


 if ((pos + len) > avail)
 return E_BUFFER_NOT_FULL;

 const long long size = ReadUInt(pReader, pos, len);

 if (size < 0) // error
 return static_cast<long>(cluster_size);

 if (size == 0)
 return E_FILE_FORMAT_INVALID; // TODO: verify this

    pos += len; // consume length of size of element

 const long long unknown_size = (1LL << (7 * len)) - 1;

 if (size != unknown_size)
      cluster_size = size;
 }

 long long timecode = -1;
 long long new_pos = -1;
 bool bBlock = false;

 long long cluster_stop = (cluster_size < 0) ? -1 : pos + cluster_size;

 for (;;) {
 if ((cluster_stop >= 0) && (pos >= cluster_stop))
 break;


 if ((pos + 1) > avail) {
      len = 1;
 return E_BUFFER_NOT_FULL;
 }

 long long result = GetUIntLength(pReader, pos, len);

 if (result < 0) // error
 return static_cast<long>(result);

 if (result > 0) // weird
 return E_BUFFER_NOT_FULL;

 if ((cluster_stop >= 0) && ((pos + len) > cluster_stop))
 return E_FILE_FORMAT_INVALID;

 if ((pos + len) > avail)
 return E_BUFFER_NOT_FULL;

 const long long id = ReadUInt(pReader, pos, len);

 if (id < 0) // error
 return static_cast<long>(id);

 if (id == 0)
 return E_FILE_FORMAT_INVALID;


 if (id == 0x0F43B675) // Cluster ID
 break;

 if (id == 0x0C53BB6B) // Cues ID
 break;

    pos += len; // consume ID field


 if ((pos + 1) > avail) {
      len = 1;
 return E_BUFFER_NOT_FULL;
 }

    result = GetUIntLength(pReader, pos, len);

 if (result < 0) // error
 return static_cast<long>(result);

 if (result > 0) // weird
 return E_BUFFER_NOT_FULL;

 if ((cluster_stop >= 0) && ((pos + len) > cluster_stop))
 return E_FILE_FORMAT_INVALID;

 if ((pos + len) > avail)
 return E_BUFFER_NOT_FULL;

 const long long size = ReadUInt(pReader, pos, len);

 if (size < 0) // error
 return static_cast<long>(size);

 const long long unknown_size = (1LL << (7 * len)) - 1;

 if (size == unknown_size)
 return E_FILE_FORMAT_INVALID;

    pos += len; // consume size field

 if ((cluster_stop >= 0) && (pos > cluster_stop))
 return E_FILE_FORMAT_INVALID;


 if (size == 0) // weird
 continue;

 if ((cluster_stop >= 0) && ((pos + size) > cluster_stop))
 return E_FILE_FORMAT_INVALID;

 if (id == 0x67) { // TimeCode ID
      len = static_cast<long>(size);

 if ((pos + size) > avail)
 return E_BUFFER_NOT_FULL;

      timecode = UnserializeUInt(pReader, pos, size);

 if (timecode < 0) // error (or underflow)
 return static_cast<long>(timecode);

      new_pos = pos + size;

 if (bBlock)
 break;
 } else if (id == 0x20) { // BlockGroup ID
      bBlock = true;
 break;
 } else if (id == 0x23) { // SimpleBlock ID
      bBlock = true;
 break;
 }

    pos += size; // consume payload
 if (cluster_stop >= 0 && pos > cluster_stop)
 return E_FILE_FORMAT_INVALID;
 }

 if (cluster_stop >= 0 && pos > cluster_stop)
 return E_FILE_FORMAT_INVALID;

 if (timecode < 0) // no timecode found
 return E_FILE_FORMAT_INVALID;

 if (!bBlock)
 return E_FILE_FORMAT_INVALID;

  m_pos = new_pos; // designates position just beyond timecode payload
  m_timecode = timecode; // m_timecode >= 0 means we're partially loaded

 if (cluster_size >= 0)
    m_element_size = cluster_stop - m_element_start;

 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.