"What they don't want to have happen is have a bunch of charge-offs related to that," he said.

By all indications, that could actually happen. Bitcoin's winning streak ended in December, and prices fell sharply and quickly (today Bitcoin is worth about $6,663).

Nonetheless, many people are now on a quest to become "Bitcoin billionaires" by investing in Bitcoin and other cryptocurrencies. According to a survey out just last month by financial consulting company deVere Group, six out of 10 people with no exposure to cryptocurrencies said they would consider including it in their portfolios, and 70% of the respondents who already held cryptocurrencies said they planned to increase their exposure in the next 12 months.

"An increasing general awareness of cryptocurrencies and how they work, plus a growing sense that cryptocurrency regulation is now inevitable, are perhaps the main reasons why such a high percentage of people are now open to looking at the possibilities of crypto for their portfolios," deVere Group founder and CEO Nigel Green said. "The survey also highlights that the majority of those who do currently hold some cryptocurrency as part of their investment portfolio believe that despite ongoing volatility, the potential rewards will outweigh the potential risks."

That may leave credit unions with some tough decisions to make, but how credit unions as a whole are handling cryptocurrency risk is hard to say, perhaps because the member activity around it is still relatively low – or perhaps because it's all so new for the credit unions. Several credit unions were reluctant to talk to CU Times about their cryptocurrency policies or the risks that members' cryptocurrency purchases could pose. But many are taking steps to inform members about cryptocurrency.

The Tucson, Ariz.-based Vantage West Credit Union, which has $1.9 billion in assets and about 150,000 members, for example, has posted content on its website that is intended to help members understand what cryptocurrency is and how it works. The credit union said it isn't dealing in cryptocurrency beyond that.

"We will continue to evaluate the evolution of the technology as well. There may come a day that cryptocurrency has wide enough adoption to be taken seriously as part of our member services mix, but we're not there yet," the credit union wrote on its site.

Karen Hack, who is the director of marketing at the Sylvania, Ohio-based Directions Credit Union, which has $715 million in assets and about 86,000 members, said her credit union tries to post timely financial information on its blog.

"However, we have yet to have any members ask for Bitcoin or show any interest in this form of currency right now, and our credit union does not have any policy in place for this right now," she told CU Times.

Credit unions and other card issuers could implement a simple solution to mitigate the risk: Deny transactions involved with cryptocurrency. But it's not that easy, Falk said.

"It's always dicey about blocking transactions. Now the normal ones, like online gambling, and the [U.S. Treasury's Office of Foreign Assets Control] OFAC countries that you can't do business in, those are pretty standard across the industry," he explained. "When you start cherry-picking out, though, and saying, 'Okay, you can't do this, you can do this, you can't do that,' consumers, especially the younger consumers, get a little bit concerned."

Another problem is that some card issuers have started classifying cryptocurrency transactions as cash advances, which can be very profitable.

"So you just have to make a business-risk decision on what you're going to do," he said. "I think that some of the big issuers that we've seen block these transactions, I think over time … we're going to see those loosen back up. I think right now it's a reaction, and I think that based on their card portfolios, they're in a better position than I am to know whether or not their portfolios are seeing a lot more cryptocurrency transactions and the risk is growing. And so therefore, they want to blunt that right now until things kind of get more regulated."

Credit unions have to look at their individual card portfolio risk and take a balanced approach. And if they do decide to block transactions, they need to be sure they have a good communications plan with members and educate them about cryptocurrency, Falk warned.

"This is where it gets into the morality of the whole thing: Is it the card issuer's responsibility to police where their cardholders are using their cash advance money?" he asked. "Let's say the person goes to a casino and they eat their entire $20,000 credit line through ATM withdrawals in the casino. Do you ever see a financial institution insert themselves between that person and a gambling addiction, for example? No. You hear about it on the back end: 'Hey, this person defaulted because he had a gambling addiction and he can't pay his credit card off because he went bankrupt because of it.' But that's a bigger societal, moral-hazard issue."

Credit unions wouldn't be the first to restrict transactions, though. JPMorgan Chase & Co., Bank of America Corp. and Citigroup are all restricting cryptocurrency purchases on their credit cards, according to a Bloomberg report. Also, Google and Facebook both recently rolled out new policies that will ban cryptocurrency-related ads.

"We want people to continue to discover and learn about new products and services through Facebook ads without fear of scams or deception. That said, there are many companies who are advertising binary options, ICOs and cryptocurrencies that are not currently operating in good faith," Facebook said in an announcement.

"The thing to remember is that much of this is unregulated, and with that lack of regulation, basically there's a lack of consumer protections," Falk said.

In any case, cryptocurrency's newfound popularity reminds him of a familiar story of risk and hype that credit unions have heard before.

"I have a credit line and I go out and I buy $20,000 worth of Bitcoin today and tomorrow it's worth $5,000, I'm upside down," he said. "But hey, let's face it, the mortgage crisis was all about that, right?"