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Android provides several options to save persistent application data, one of which is External Storage (/sdcard, /mnt/sdcard). "External storage" examples include a micro- or standard-sized SD card internal to the device, Android device storage mounted to a PC, and the Android/obb directory.

Files saved to the external storage prior to Android 4.1 are world-readable. Prior to Android 1, files saved to external storage are world-writable. From Android 1 to Android 4.3, only the WRITE_EXTERNAL_STORAGE permission is required for an app to write to any external storage file stored by any app. Starting with Android 4.4, groups and modes of files are created based on a directory structure, which allows an app permission to manage/read/write files within a directory structure based on its package name. Starting with Android 4.4, users (including apps as users) are isolated from primary external storage spaces of other apps controlled by the Android device.

Consequent to the lack of restrictions described above, files written to external storage can be modified or read . 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(for the Android versions which allow read/write) and by anyone with access to the files if stored on an off-device external storage device such as a PC (or if the in-device external storage media is removed and mounted elsewhere).

The Android API Guides [Android n.d.] 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 unless encrypted first, 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 {
  fileFile 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) {
      // handle error
    }
  }
}

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 #1 (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) {
      // handle error
    }
  }
}

Related Vulnerabilities

• JVN#92038939
  TITLE: mixi for Android information management vulnerability
  URL: http://jvn.jp/en/jp/JVN92038939/index.html
• JVN#05102851
  TITLE: Yome Collection for Android issue in management of IMEI URL: http://jvn.jp/en/jp/JVN05102851/index.html

Related Guidelines

Bibliography