Cardiff halted Birmingham's winning run as Yousef Salech secured a 1-0 victory to continue the Bluebirds' climb up the Championship.
Salech was named among the home substitutes initially, but defender Krystian Bielik suffered an injury in the warm-up and had to drop out on an afternoon of relentless rain in the Welsh capital.
The late change saw Cardiff reshuffle their resources and Salech plunder a first-half winner, his sixth goal of the season coming 10 minutes after James Beadle had saved his spot-kick.
Cardiff have now recorded three straight home wins as they continue to show real signs of adapting to life in the second tier following promotion from League One.
Birmingham had arrived on the back of three successive wins without conceding, and remain in the play-off places after suffering their first defeat since September 9.
Cardiff manager Brian Barry-Murphy was serving a touchline ban after receiving his third yellow card of the season.
Both sets of players wore black armbands to mark the 60th anniversary of the Aberfan disaster, in which 116 children were among the 144 who died in the Welsh mining village, and a minute's silence before kick-off was impeccably observed.
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Salech featured for the first time in five weeks after a shoulder injury, but could not pull down a Joel Bagan pass that would have taken him through on goal.
Luis Vazquez's wait for a first Birmingham goal went on as the Argentinian striker was unable to connect with Jay Stansfield's cross.
Cardiff were awarded a spot-kick after 21 minutes when Tomoki Iwata dragged down Perry Ng at a corner. Salech sent the penalty low to Beadle's right, but the effort lacked power and the Birmingham goalkeeper pushed the ball to safety.
It was the second successive spot-kick Cardiff had missed following Chris Willock's failure to convert against Watford.
But Salech quickly made amends by holding off Phil Neumann from Ollie Tanner's cross and producing a tidy turn and finish past Beadle.
Some Birmingham players held their arms aloft believing Neumann had been sent to the floor by a foul, but referee Tim Robinson clearly saw it as a case of Salech being stronger.
Beadle came to Birmingham's rescue at the start of the second half, denying Cian Ashford with his legs after Tanner and Salech had opened up the visitors.
Salech fired just past the base of a post as Cardiff increased intensity in the search for a second goal and Beadle fielded Ashford's attempt from distance.
Birmingham had not tested Nathan Trott until Stansfield met Alex Cochrane's cross, but it was a routine stop to say the least and Cardiff were untroubled until the closing stages.
Marvin Ducksch headed Max Bird's cross wide and Birmingham's best chance came two minutes from time when Trott superbly kept out Demarai Gray from point-blank range.
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The managers
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