Federal authorities in charge of the football tournament told BBC Brasil they would consider using police or military staff should the need arise.
Military police are routinely used at local football matches in Brazil.
But the sport's world governing body, Fifa, stipulates the use of private security for the World Cup.
Private security personnel are supposed to be trained by 21 May, when Fifa takes over control of the stadiums.
The federal authorities told the BBC the training would be done "in time".
"The committee is overseeing this work and we are relaxed about that," said Hilario Medeiros, head of security at the Local Organising Committee.
"If there is not enough private security [during the tournament], we will use public security, armed security. But we are working to make sure we will not have this problem."
The opening game of the World Cup is on 12 June in Brazil's biggest city, Sao Paulo.
Brazil has approximately 220 private security schools, and they will have to train 20,000 stewards in less than 40 days.
The committee has asked for 25,000 private guards to work as stewards during the World Cup.
However, only 5,084 private guards in Brazil have so far received the specific training and documentation needed, according to Brazil's federal police.
Both Fifa and private security companies say there will be enough time to train all the staff and there will be no need for a contingency plan.
Brazilian legislation makes it clear that only professional security guards can act as stewards, and only after completing additional training for major events.
The federal police have said they will not allow stewards to work in World Cup stadiums without such training.
Brazil has so far trained only 20% of the private security guards needed at the 12 stadiums for the World Cup in June and July, the BBC has learned.