Google Play was recently found to be hosting more than 400 apps that turned infected phones into listening posts that could siphon sensitive data out of the protected networks they connected to, security researchers said Thursday.

One malicious app infected with the so-called DressCode malware had been downloaded from 100,000 to 500,000 times before it was removed from the Google-hosted marketplace, Trend Micro researchers said in a post. Known as Mod GTA 5 for Minecraft PE, it was disguised as a benign game, but included in the code was a component that established a persistent connection with an attacker controlled server. The server then had the ability to bypass so-called network address translation protections that shield individual devices inside a network. Trend Micro has found 3,000 such apps in all, 400 of which were available through Play.

"This malware allows threat actors to infiltrate a user's network environment," Thursday's report stated. "If an infected device connects to an enterprise network, the attacker can either bypass the NAT device to attack the internal server or download sensitive data using the infected device as a springboard."

The report continued:

The malware installs a SOCKS proxy on the device, building a general purpose tunnel that can control and give commands to the device. It can be used to turn devices into bots and build a botnet, which is essentially a network of slave devices that can be used for a variety of schemes like distributed denial-of-service (DDoS) attacks—which have become an increasingly severe problem for organizations worldwide—or spam email campaigns. The botnet can use the proxied IP addresses also generated by the malware to create fake traffic, disguise ad clicks, and generate revenue for the attackers.

A Google spokesman said in an e-mail: "We're aware of the issue and we're taking the necessary actions."

Trend Micro's report comes three weeks after researchers from separate security firm Checkpoint said they detected 40 DressCode-infected apps in Google Play

Trend said that only a small portion of each malicious app contained the malicious functions, a feature that makes detection difficult. In 2012, Google introduced a cloud-based security scanner called Bouncer that scours Play for malicious apps. Since then, thousands of malicious apps have been detected by researchers. This raises a question: if outside parties can find them, why can't Google find them first?

Post updated to add comment from Google.