Stade had won 23 out of 26 Challenge Cup home games at the pool stage and Worcester arrived in Paris as huge underdogs, having lost its past 11 away matches in the competition as well as being without a win in France for a decade.
But the Premiership side upset the Top 14 high-flier with a brilliant Pool 2 performance on the road, Howe crossing for a second time just before the break after Matt Cox scored the opening try.
Ted Hill and Jamie Shillcock also went over in the second half and Jono Lance took his tally with the boot to 13 points, ensuring tries from Kylan Hamdaoui, Laurent Panis and Piet van Zyl were in vain for the 2016/2017 Challenge Cup winner.
La Rochelle showed no mercy on Enisei in Russia, scoring six tries in each half of a 82-21 Pool 4 rout.
Jean Victor Goillot and Pierre Aguillon claimed two five-pointers apiece and Maxime Lafage finished with a 25-point haul in a ruthless mauling.
Top 14 leader Clermont Auvergne laid down a marker with a 41-20 win at Northampton Saints and Dragons was an emphatic 54-17 winner over Timisoara Saracens in the other Pool 1 clash.
Connacht saw off Bordeaux-Begles 22-10 in Pool 3 and Ospreys also struck a blow for the Pro14, George North among the four try scorers in a 27-0 Pool 2 triumph over Pau.
Three-time champion Harlequins thumped Agen 54-22 at The Stoop, while there were also wins for Bristol Bears and Benetton Treviso.