Mobile app fraud continues to rise as advertisers pour billions of ad spend into the platform. Marketing measurement provider DoubleVerify found that from 2017 to 2018, fraud has been rampant in the form of sophisticated invalid traffic impressions and applications.

DoubleVerify identifies and screens common types of mobile app fraud that includes background ad activity, hidden ads, app misrepresentation aka spoofing and measurement manipulation.

All those scans revealed that fraudsters have kept themselves incredibly busy over the last few years. DoubleVerify’s Fraud Lab found that the total number of fraudulent apps has increased by 159 percent from 2017 to 2018, according to figures made available to AList.

Mobile app sophisticated invalid traffic (SIVT) impressions have doubled year-on-year since 2017, DoubleVerify added.

Just over half—57 percent—of fraudulent mobile apps are categorized as “Games” and “Tools & Utilities,” the company found, adding that its fraud tool detected 1.6x more fraudulent apps in 2018 than in the previous year.

“With ad spend increasingly concentrated in mobile – and particularly mobile app, fraudsters are redoubling their efforts to take advantage,” said Roy Rosenfeld, head of DoubleVerify’s Fraud Lab in a prepared statement. “It’s critical that brands understand these risks, in order to allocate spend accordingly and install appropriate safeguards for their digital investments.”

BuzzfeedNews released the findings of a similar mobile app fraud investigation on Thursday, in which several popular Android apps use invasive permissions and dubious code to commit ad fraud. Apps that ranged from children’s reading programs to flashlights and remote controls were found to commit ad fraud while collecting huge amounts of user data.

According to a 2017 report by Singular, 63 percent of marketers don’t use any mobile fraud prevention techniques at all, becoming easy prey for even the most easily preventable attacks.

US marketers will spend an estimated $87 billion on mobile advertising in 2019, according to eMarker, which will account for more than two-thirds of overall US digital ad spend. Outpacing TV for the first time in 2018, mobile ad spend is expected to reach $201 billion in spend globally by 2021.

That being said, it’s no wonder that marketers and fraudsters alike have honed in on mobile. A recent report by Scalarr suggests that marketers will lose nearly $13 billion to mobile app install fraud in 2019. This estimated loss is a significant increase from $7.3 billion in 2018.

In November, the IAB Technology Laboratory released app-ads.txt, a mobile extension of its fraud prevention tool, for beta testing and commentary. Earlier in 2018, The Trustworthy Accountability Group (TAG) added a requirement that all publishers implement the ads.txt standard if they want to become TAG Certified Against Ad Fraud.