2018 has been a challenging year for the peer-to-peer payment service Venmo, owned and operated by PayPal Holdings.

While the year saw continued growth in transaction volume, and the launch of new services such as instant cashout (January) and a debit card (June), the year has also seen a significant uptick in fraud, from 0.25% to 0.4%, according to a report in the Wall Street Journal over the weekend.

Fraud can happen on Venmo in a number of ways. Hackers can take over existing accounts and remove their money, or create synthetic accounts with stolen card credentials and send money to confederates. PayPal generally reimburses Venmo users victimized by fraud, according to the report.

Venmo fraud related to chargebacks and float appeared soon after the service was launched, as detailed in this 2015 report.

Shirley Inscoe, a senior analyst with the consultancy Aite Group, told Bank Innovation that recent data breaches and authentication shortfalls are two reasons why P2P payments tend to see more fraud than the rest of the industry:

There can be several reasons for Venmo’s fraud experience; for example, with all the data breaches, it is easy for fraudsters to impersonate a consumer and use the person’s account information to sign up for Venmo. Financial institutions are also grappling with account takeover fraud, and a fraudster who successfully gains access to an account can drain the funds quickly using Venmo transfers. False registrations (or application fraud) and account takeovers are the two more significant challenges financial institutions face in the current market, and will continue to be until technology improvements are implemented to more reliably authenticate consumers.

Venmo had recently reported a cheery 78% growth in transaction volume for the third quarter, to $17 billion. “Our monetization efforts appear to be reaching a tipping point,” PayPal CEO Dan Schulman told CNBC.

But shortly after PayPal reported earnings, news broke that Zelle, the bank-backed P2P offering, was outpacing Venmo’s growth. (Much of Zelle’s growth is reported to be intra-bank payments, but no matter.) Square Cash, another rival, is also consistently ranked higher than Venmo in downloads on both Android and iOS app stores.

And now comes the news that fraud is on the upswing on the platform, and legitimate accounts have been shuttered in an effort to combat, causing an outburst of anger among users. Venmo has also limited or eliminated access to services such as instant cashout, or making payments via the website rather than mobile app. The company noted in an internal memo that the website accounted for just 2% of transaction volume, but 15% of fraud.

Venmo’s commerce service Pay with Venmo is also growing 185% month-over-month, but from a small base. Pay with Venmo for Uber is growing 300%, Schulman said in October.