Thanks for this comment. It seems that ARINC 429 has been replaced by ARINC 644 in most new aircraft.

From reading the Wikipedia article, they are indeed logically one-way (although the underlying protocol involves two-way communication). It has no security at all.

However, it seems that communication between any avionics systems and anything user-accessible goes through a Network Extension Device (NED). These are required to either be physically (not only logically) unidirectional _or_ have built-in security.

So it might be physically impermeable or it might be a buggy 10-year old firewall. Doesn't exactly inspire confidence given the subject of the article.