Is Google’s Android Play Store full of adware and malware or isn’t it? According to HP’s latest Cyber Risk Report, it all depends which firm’s mobile antivirus scanner you run.

The IT giant’s researcher’s first tried to get some handle on the problem by comparing a sample of 500,000 apps from the Play Store against a two million-strong database of known malware and adware samples, coming up with some startling numbers.

Looking at straight malware, HP uncovered a list of recognised Trojans that had been downloaded by users anything from 1.1 million to 11 million times globally. When it came to less serious but still potentially troublesome adware apps, these had been downloaded anything up to tens of millions of times.

The issue of malware sneaking on to Google’s Play is well documented and the firm has gradually tightened controls to make this harder, or at least to ensure that when it does happen they are pulled quickly. However, the firm also uncovered a wider issue of how different antivirus firms categorise apps whose main offence is to serve ads and gather personal data after being installed by willing Android users.

Running 7,000 known ad-serving apps through the databases used by mobile security programs from a range of software vendors turned up some intriguing results. Firms such as ESET, Fortinet, DrWeb, and Sophos recorded between 4,507 and 5,121 as suspect adware, while others such as Symantec took issue with barely half a dozen of the same sample.

Others such as Kaspersky, Trend Micro, BitDefender, McAfee and AVG, sat somewhere in the middle, calling out anything from a few hundred to the low thousands.

Who is right and does it even matter?

“The industry has not yet come to consensus,” noted HP’s researchers, suggesting that some of the mobile security scanners might have databases oriented towards Windows malware.

However, the fact Android had more of these adware detections than Apple’s iOS suggested deeper differences in the way the app stores were meant to work.

“One way of looking at it could be that Google is friendlier to developers, and that it does not reject apps for aesthetic reasons. Another way of looking at it could be that Google’s revenue is more ad-related than Apple’s hardware-centric revenue, and therefore Apple can enforce a more consumer-friendly app store policy,” said HP.

“The industry needs to work together to come up with consistent definitions of what constitutes malicious or unwanted behaviors and a sensible app store policy and guidelines, accommodating to app users, app makers, and third-party ad providers while preventing abuses.”

One advantage Android and mobile users do have over the Windows world is the power to de-recommend apps that abuse the permissions asked, or serve too many ads.

The fact that Play Store apps often serve adware has been pointed out by security companies before, with Lookout Mobile Security taking a particularly tough stance on the issue that involved naming ad networks it thought had crossed a red line. Other firms just think that too many popular apps now come with this downside, a direct consequence of the free model in which small developers have to make some money to justify their effort and adopt ad systems they don't always understand.

On a separate but related note, in the same report HP’s Fortify division analysed 180 mobile applications developed for business use, finding that a roughly equal number showed poor implementations of encryption, mostly weak algorithms or storing data with no encryption at all.