  cf2_builder_lineTo( CF2_OutlineCallbacks      callbacks,
                      const CF2_CallbackParams  params )
  {
    /* downcast the object pointer */
    CF2_Outline   outline = (CF2_Outline)callbacks;
    CFF_Builder*  builder;


    FT_ASSERT( outline && outline->decoder );
    FT_ASSERT( params->op == CF2_PathOpLineTo );

    builder = &outline->decoder->builder;

    if ( !builder->path_begun )
    {
      /* record the move before the line; also check points and set */
      /* `path_begun'                                               */
      cff_builder_start_point( builder,
                               params->pt0.x,
                               params->pt0.y );
    }

    /* `cff_builder_add_point1' includes a check_points call for one point */
    cff_builder_add_point1( builder,
                            params->pt1.x,
                            params->pt1.y );
  }

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.