Forster connected to a Darren Currie free-kick in the 64th minute in a match short on entertainment at Portman Road.
Ipswich should have opened the scoring just before the break, but Sam Parkin scuffed his shot with just Cardiff goalkeeper Neil Alexander to beat.
Parkin hit the bar with a header late on and goalkeeper Lewis Price pulled off a superb reaction save to deny Cardiff substitute Alan Lee.
Cardiff manager Dave Jones:
"I think I can take more positives than negatives from the match. One mistake proved costly. It was a silly free kick to give away.
"We got into good positions in the first-half and should have got something out of them, but we didn't.
"I have brought in nine new players and seven of them were on duty. I have told them they need to gel quickly but I cannot fault them."
Ipswich: Price, Wilnis, De Vos, Naylor, Richards, Currie, Magilton (Juan 90), Garvan, Westlake, Forster (Peters 77), Parkin.
Subs Not Used: Supple, Horlock, McDonald.
Booked: De Vos.
Goals: Forster 64.
Cardiff: Alexander, Darlington, Purse, Cox, Barker, Whitley, Ardley, Parry (Lee 74), Boland (Mulryne 86), Cooper, Jerome.
Subs Not Used: Warner, Weston, Loovens.
Ref: K Friend (Leicestershire).