Ulster gained revenge over Cardiff after 46-6 and 42-16 defeats at the Arms Park in previous campaigns with a precious away win in Pool 6.
The win means Andy Ward's side will at least go into the sixth and final round of matches at Ravenhill on Friday night with a chance of qualifying for the quarter-finals for the first time since the men from Belfast lifted the Heineken Cup in 1999.
But in order to progress to the final eight, not only must they beat the team that succeeded them as Cup champions, but they must also beat them by four clear tries.
Northampton's narrow 17-14 home win over Biarritz allowed them to improve their position at the top of pool 6 with eight points. Ulster remain in third with six points after their victory in Cardiff, level on points at least with their French rivals.
Cardiff however, remain winless in this season Heineken Cup, and if plans for provincialism come to fruition, the Arms park fans will have seen their side lose their last ever game under the of identity Cardiff at the Arms park.
More than 4,000 loyal fans turned up - presumably because so many other matches in Wales had been called off.
And Cardiff made a match of it with tries through backs Nick Walne and Matt Allen, but the steady boot of Ireland international Humphreys (15 points) and two tries to impressive openside flanker Neil McMillan proved the difference.
Cardiff: Tries - N Walne, M Allen; Con - N Robinson; Pens - N Robinson 3
Ulster: Tries - N McMillan 2, Penalty try; Cons - D Humphreys 3; Pens - D Humphreys 3; DG - B Cunningham. Halftime: Ulster 24-12.