Law authorities have warned they believe criminals are using Android phones to trigger fraudulent tap-and-go payments.
The alert comes in Europol's annual Internet Organised Crime Threat Assessment report.
Experts had previously said that the rollout of smart wallet systems could raise such a threat.
However, the police are unsure exactly how the attacks are being carried out and how common they are.
"The possibility of compromising NFC [near field communication] transactions was explored by academia years ago, and it appears that fraudsters have finally made progress in the area," the report says.