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Android provides several options to save persistent application data, one of which is External Storage (/sdcard, /mnt/sdcard).

Files saved to the external storage are world-readable. Consequently, they can be modified by other apps installed on the device or by the user (by enabling USB mass storage and manipulating files from a PC).

The Android API Guides [Android Guides 2013] Storage Options states:

Caution: External storage can become unavailable if the user mounts the external storage on a computer or removes the media, and there’s no security enforced upon files you save to the external storage. All applications can read and write files placed on the external storage and the use can remove them.

Developers should not store sensitive data to external storage devices because files stored externally have no guarantee of availability, integrity, and confidentiality.

Noncompliant Code Example

The following code creates a file on the external storage and saves sensitive information to the file:

private String filename = "myfile"

private String string = "sensitive data such as credit card number"
FileOutputStream fos = null;

try {
  file file = new File(getExternalFilesDir(TARGET_TYPE), filename);
  fos = new FileOutputStream(file, false);
  fos.write(string.getBytes());
} catch (FileNotFoundException e) {
  // handle FileNotFoundException
} catch (IOException e) {
  // handle IOException
} finally {
  if (fos != null) {
    try {
  	fos.close();
    } catch (IOException e) {
    }
  }
}

Proof of Concept

Typically, an application stores files in the directory as follows:

/sdcard/Android/data/com.company.app/files/save/appdata/save_appdata

Compliant Solution (Save a File on Internal Storage)

The following code uses the openFileOutput() method to create "myfile" in an application data directory with permission set to MODE_PRIVATE so that other apps cannot access the file:

private String filename = "myfile"
private String string = "sensitive data such as credit card number"
FileOutputStream fos = null;

try {
   fos = openFileOutput(filename, Context.MODE_PRIVATE);
   fos.write(string.getBytes());
   fos.close();
} catch (FileNotFoundException e) {
  // handle FileNotFoundException
} catch (IOException e) {
  // handle IOException
} finally {
  if (fos != null) {
    try {
      fos.close();
    } catch (IOException e) {
    }
  }
}

Related Vulnerabilities

• JVN#92038939 mixi for Android information management vulnerability
• JVN#05102851 Yome Collection for Android issue in management of IMEI

Related Guidelines

Bibliography