Novel technique allows malicious apps to escape iOS and Android guardrails

Phishers are using a novel technique to trick iOS and Android users into installing malicious apps that bypass safety guardrails built by both Apple and Google to prevent unauthorized apps.

Both mobile operating systems employ mechanisms designed to help users steer clear of apps that steal their personal information, passwords, or other sensitive data. iOS bars the installation of all apps other than those available in its App Store, an approach widely known as the Walled Garden. Android, meanwhile, is set by default to allow only apps available in Google Play. Sideloading—or the installation of apps from other markets—must be manually allowed, something Google warns against.

When native apps aren’t

Phishing campaigns making the rounds over the past nine months are using previously unseen ways to workaround these protections. The objective is to trick targets into installing a malicious app that masquerades as an official one from the targets’ bank. Once installed, the malicious app steals account credentials and sends them to the attacker in real time over Telegram.

“This technique is noteworthy because it installs a phishing application from a third-party website without the user having to allow third-party app installation,” Jakub Osmani, an analyst with security firm ESET, wrote Tuesday. “For iOS users, such an action might break any ‘walled garden’ assumptions about security. On Android, this could result in the silent installation of a special kind of APK, which on further inspection even appears to be installed from the Google Play store.”

The novel method involves enticing targets to install a special type of app known as a Progressive Web App. These apps rely solely on Web standards to render functionalities that have the feel and behavior of a native app, without the restrictions that come with them. The reliance on Web standards means PWAs, as they’re abbreviated, will in theory work on any platform running a standards-compliant browser, making them work equally well on iOS and Android. Once installed, users can add PWAs to their home screen, giving them a striking similarity to native apps.

While PWAs can apply to both iOS and Android, Osmani's post uses PWA to apply to iOS apps and WebAPK to Android apps.

Installed phishing PWA (left) and real banking app (right).

Credit: ESET

Comparison between an installed phishing WebAPK (left) and real banking app (right).

Credit: ESET

The attack begins with a message sent either by text message, automated call, or through a malicious ad on Facebook or Instagram. When targets click on the link in the scam message, they open a page that looks similar to the App Store or Google Play.

ESET’s Osmani continued:

From here victims are asked to install a “new version” of the banking application; an example of this can be seen in Figure 2. Depending on the campaign, clicking on the install/update button launches the installation of a malicious application from the website, directly on the victim’s phone, either in the form of a WebAPK (for Android users only), or as a PWA for iOS and Android users (if the campaign is not WebAPK based). This crucial installation step bypasses traditional browser warnings of “installing unknown apps”: this is the default behavior of Chrome’s WebAPK technology, which is abused by the attackers.

The process is a little different for iOS users, as an animated pop-up instructs victims how to add the phishing PWA to their home screen (see Figure 3). The pop-up copies the look of native iOS prompts. In the end, even iOS users are not warned about adding a potentially harmful app to their phone.

After installation, victims are prompted to submit their Internet banking credentials to access their account via the new mobile banking app. All submitted information is sent to the attackers’ C&C servers.

The technique is made all the more effective because application information associated with the WebAPKs will show they were installed from Google Play and have been assigned no system privileges.

So far, ESET is aware of the technique being used against customers of banks mostly in Czechia and less so in Hungary and Georgia. The attacks used two distinct command-and-control infrastructures, an indication that two different threat groups are using the technique.

“We expect more copycat applications to be created and distributed, since after installation it is difficult to separate the legitimate apps from the phishing ones,” Osmani said.