Additive Manufacturing (AM) adoption is increasing in home and industrial settings, but information security for this technology is still immature. Thus far, three security threat categories have been identified: technical data theft, sabotage, and illegal part manufacturing. In this paper, we expand to a new threat category: misuse of digital design files as a subliminal communication channel. We identify and explore attacks by which arbitrary information can be embedded steganographically in the most common digital design file format, the STL, without distorting the printed object. Because the technique will not change the manufactured object’s geometry, it is likely to remain unnoticed and can be exploited for data transfer. Further, even with knowledge of our methods, defenders cannot distinguish between actual data transfer and random manipulation of the files. This is the first info-hiding attack on this system, conducted despite the fact that random changes may spoil the physical artifact and result in detection.