Pope Francis is no stranger to threats, and the terror group ISIS has issued a chilling new warning against the head of the Catholic Church now that Christmas is fast approaching.

The pro-ISIS Wafa Media Foundation published the new poster showing a car attack, according to Mirror. The poster, which reads "Christmas blood," depicted a masked jihadi driving a BMW straight towards St. Peter's Basilica, where the Pope holds mass. Beside the masked driver is an assault rifle and a rucksack.

ISIS has been using a lot of vehicle attacks recently, and they have already targeted the U.S., United Kingdom, France, and Germany. Experts say it's more difficult for authorities to monitor these kinds of attacks because the perpetrators do not tend to use guns or bombs.

"This kind of attack is very difficult, and probably the most difficult, for law enforcement to protect and prevent," Steve Gomez, former FBI agent, told ABC. "So, we know the threat exists. Any kind of venue, where there are crowds, we have to recognize that the vehicular attack is the preferred method based on ISIS' instructions to its followers worldwide, because it's hard to detect."

Because of law enforcement agencies' "great difficulty" in stopping vehicle attacks, former NYPD police commissioner Ray Kelly said it is essential for them to get "intelligence that indicates that something like this may happen."

NYPD Shield and the FBI's Operation Tripwire have been in close contact with private businesses, such as car rental companies, so that they can get first-hand information about suspicious individuals who might be planning terror attacks. Whenever rental company staff encounter a possible red flag, they should immediately get in touch with the authorities.

Gomez added that people themselves must be more vigilant during special occasions when there are large public gatherings. "The public must know that a threat exists, especially since law enforcement may not have access to every area that may contain a crowd," Gomez said. "They have to recognize that they are in a potentially targeted area and must be alert."