A320 pilot reported a 'dirty socks' fumes event during passenger deplaning. Maintenance determined the fumes were caused by an APU oil leak.

Narrative

After landing in ZZZZ with a faulty weather radar. We turned on the APU clearing the runway with a five minute taxi to the gate. We informed Maintenance Control airborne of our radar issue; and contract maintenance technicians were there when the boarding door was opened. After a couple minutes speaking to contract maintenance and while deplaning we began to smell a sweet odor followed by dirty socks smell; which began to intensify. We immediately shut off the APU bleed to stop contaminating the aircraft. After deplaning the Flight Attendant 3 in the back of the aircraft had the same experience. Maintenance troubled shot and found leaking oil in the seal of the APU duct. They requested us to return to the aircraft and upon reaching the jetbridge we could smell the dirty sock smell immediately at the top of the jetbridge and stronger as we got to the aircraft door. Maintenance also tried to cover the smell with air freshener. We told them this was unacceptable and the odor irritated my nose and throat at this point. We informed Maintenance Control we would not be performing an engine runup as this had led to serious incapacitation previously at [company name].