Storing files locally is a common task for mobile applications. Files that are stored unencrypted can be read out and modified by an attacker with physical access to the device. Access to sensitive data can be harmful for the user of the application, for example when the device gets stolen.

## Ask Yourself Whether

- The file contains sensitive data that could cause harm when leaked.

There is a risk if you answered yes to any of those questions.

## Recommended Secure Coding Practices

It’s recommended to password-encrypt local files that contain sensitive information. The class EncryptedFile can be used to easily encrypt files.

## Sensitive Code Example

    Files.write(path, content); // Sensitive

    FileOutputStream out = new FileOutputStream(file); // Sensitive

    FileWriter fw = new FileWriter("outfilename", false); // Sensitive

## Compliant Solution

    String masterKeyAlias = MasterKeys.getOrCreate(MasterKeys.AES256_GCM_SPEC);

    File file = new File(context.getFilesDir(), "secret_data");
    EncryptedFile encryptedFile = EncryptedFile.Builder(
        file,
        context,
        masterKeyAlias,
        EncryptedFile.FileEncryptionScheme.AES256_GCM_HKDF_4KB
    ).build();

    // write to the encrypted file
    FileOutputStream encryptedOutputStream = encryptedFile.openFileOutput();

## See

- OWASP - Top 10 2021 Category A4 - Insecure Design
- OWASP - Mobile AppSec Verification Standard - Data Storage and Privacy Requirements
- OWASP - Mobile Top 10 2016 Category M2 - Insecure Data Storage
- OWASP - Top 10 2017 Category A3 - Sensitive Data Exposure
- OWASP - Top 10 2017 Category A6 - Security Misconfiguration
- CWE - CWE-311 - Missing Encryption of Sensitive Data