Nearly half of Android devices are vulnerable to an attack that could replace a legitimate app with malicious software that can collect sensitive data from a phone.

Google, Samsung, and Amazon have released patches for their devices, but 49.5 percent of Android users are still vulnerable, according to Palo Alto Networks, which discovered the problem. Google said it has not detected attempts to exploit the flaw.

A malicious application installed using the vulnerability, called “Android Installer Hijacking,” would have full access to a device, including data such as usernames and passwords, wrote Zhi Xu, a senior staff engineer with Palo Alto.

The company wrote two exploits that take advantage of the flaw, which involves how APKs (Android application packages) are installed.

The vulnerability only affects applications that are installed from a third-party app store. Security experts generally recommend using caution when downloading apps from those sources.

Apps downloaded from third parties place their APK installation files in a device’s unprotected local storage, such as an SD card, Xu wrote. From there, a system application called PackageInstaller finishes the installation. The flaw allows an APK file to be modified or replaced during installation without anyone knowing.

An attack would work like this: A user downloads what appears to be a legitimate application. The application asks for certain permissions on the device. During that process, Palo Alto found it was possible to swap or modify the APK file in the background because the PackageInstaller fails to verify it, Xu wrote.

After clicking the install button, “the PackageInstaller can actually install a different app with an entirely different set of permissions,” he wrote.

Android devices do not need to be rooted for the attack to work, although rooting does make devices more vulnerable.

When the flaw was discovered, in January 2014, close to 90 percent of all Android devices were affected. That has since dropped to 49.5 percent, but many devices have not been patched.

Palo Alto’s exploits were successful against Android versions 2.3, 4.0.3 to 4.0.4, 4.1.x, and 4.2.x. The 4.4 version of Android fixes the issue. Some Android 4.3 devices may still be affected, however, since some manufacturers have not patched yet, Xu wrote.

Google has published a patch here, and Amazon recommends downloading the latest version of the Amazon AppStore, which will update its Fire devices, Xu wrote.

Palo Alto has also developed an Android app that will detect if a device is still vulnerable.