Fortinet has filed a petition with the U.S. Patent and Trademark workplace to re-examine a security patent held by Trend Micro.

The Trend Micro patent in question, 5,623,600, covers virus detection and removal processes. The patent has led to some skirmishes within the security business, notably a battle between Trend Micro and Barracuda Networks. Fortinet says its petition identifies previous art that creates Trend Micro’s patent invalid.

Fortinet has tussled with Trend Micro before. Trend Micro sued Fortinet in 2004 and therefore the 2 firms settled a patent dispute in 2006. The legal peace did not last long. Fortinet sued Trend Micro again—following a 2008 and 2009 duel—in January as a result of Fortinet thought of the patent it licensed in 2006 to be invalid. Fortinet additionally alleged that Trend Micro threatened its customers.