In the other match in the group leaders Biarritz, who are three points clear of second-placed Saracens, won 38-24 against Treviso in Italy.
Ulster coach Mark McCall:
"My understanding is you can go upstairs for the try scorer but not the person who gave the pass. In that respect the officials got it wrong, but we don't feel cheated because it wasn't a try.
"It was very disappointing to concede that penalty under the sticks at the end. It will be difficult for us to qualify now. Ideally we needed to win and if not we needed a bonus point."
Saracens boss Steve Diamond:
"I was very happy to beat Ulster because wins are hard to come by in this group. If we get a bonus point against Treviso then we have a chance in Biarritz.
"Biarritz is a difficult environment to play in. We beat them at home but it's different going down there - I did that with Sale and they put 40 points on us."
Saracens: Scarbrough; Bailey, Castaignede, Johnston, Vaikona;
Jackson, Bracken; Yates, Byrne, Visagie; Raiwalui, Chesney;
Vyvyan, B Russell, Skirving.
Replacements: B Russell for Scarbrough (79), Bartholomeusz for Castaignede (65), Dickens for Bracken (56), Seymour for Raiwalui (76), Broster for B Russell (24), Randell for Skirving (58).
Not Used: Cairns.
Sin bin: Yates (22).
Ulster: Cunningham; Bowe, Maggs, Trimble, Steinmetz; Humphreys,
Campbell; B Young, R Best, S Best; Harrison, McCullough; N Best, McMillan, Wilson.
Replacements: Topping for Cunningham (66), Boss for Campbell (52), J Fitzpatrick for B Young (52), Ferris for N Best (33), Caldwell for Ferris (63).
Not Used: Brady, Larkin.
Att: 8,571
Referee: Joel Jutge (France)