Gas stations may be more dangerous than they appear: One in five millennials suspect they've lost money when paying to fill up their tanks because of a fraud called skimming.

Skimming is a crime that occurs when fraudsters attach a device called a skimmer to the card reader and grab data off the magnetic strip on a credit card or debit card. Gas stations are a prime location for this type of scam, Matt Schulz, chief industry analyst at CompareCards, tells CNBC Make It, because they are not required to update their card readers to the safer EMV chip cards until October 2020.

"Gas stations are the lowest-hanging fruit for bad guys when it comes to skimming," Schulz says. In fact, 15 percent of Americans who purchased gas in the last month believe they've been a victim of skimming at the pump, according to a new survey of 1,000 people from CompareCards.

Millennials are more likely to say they've been scammed. Just over 20 percent of millennials suspect that they've been victims of skimming at the pump in the last year. That's compared to about 15 percent of Gen-Xers and just 8 percent of Baby Boomers.

It likely comes down to awareness, Schulz says, adding millennials are very "plugged in" when it comes to news and trends. Millennials are also the group most likely to monitor their finances and recognize when something is wrong. "You're much more likely to watch every single penny when you have fewer pennies," he says.