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MASWE-0002: Sensitive Data Stored Unencrypted Outside of Private Storage

Overview

Category: MASVS-STORAGE
Vulnerability: Sensitive Data Stored Unencrypted in Shared / External Storage Locations
CWEs: CWE-732 (Incorrect Permission Assignment), CWE-312, CWE-922, CWE-321, CWE-326, CWE-320
MASTG Tests: MASTG-TEST-0002, MASTG-TEST-0006
MITRE ATT&CK Mobile: T1409 (Access Stored Data), T1533 (Data from External Storage)
CVSS v4.0 Score: 7.5 HIGH (CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N)
NIST Standards: NIST SP 800-163 Rev. 1 (§4.1), NIST SP 800-53 (AC-3, SC-28)
Industry Compliance: PCI-DSS v4.0 (Req 3.4), Google Play MASA (§1.2)

MASWE-0002 covers vulnerabilities where sensitive information (bearer tokens, session cookies, passwords, credit card PANs) is written to external or shared storage (getExternalFilesDir(), SD cards, Downloads). Because external storage is accessible via ADB, removed physical media, and on Android < 10 by any application requesting READ_EXTERNAL_STORAGE, cleartext files and pseudo-encrypted files with hardcoded or co-located keys represent a severe risk of unauthorized disclosure.

🛑 Vulnerability Vectors (The "Attacker" Perspective)

Our :features:maswe0002 module demonstrates 5 critical external storage anti-patterns:

  1. External Storage Plaintext Leak: Writing raw credentials and authentication tokens into /sdcard/Android/data/.../files/maswe0002_plaintext.json.
  2. Hardcoded Encryption Key: Encrypting external storage files using a static, hardcoded AES key embedded in Kotlin bytecode. Easily extracted via decompilation.
  3. Encryption Key Stored on Filesystem: Storing the symmetric AES key directly in /sdcard/.../secret.key alongside the encrypted payload.
  4. Weak Cipher Mode (AES/ECB): Encrypting sensitive data using Electronic Codebook (ECB) mode (AES/ECB/PKCS5Padding). Identical plaintext blocks produce identical ciphertext blocks, allowing pattern reconstruction.
  5. Reused Encryption Key Across Devices: Using predictable seeds (e.g. constant strings or device attributes) that allow ciphertext from one device to be cloned and decrypted on any other device.

📊 Attack Flow Sequence

sequenceDiagram
    autonumber
    actor Attacker as 😈 Attacker / Malware
    participant Storage as 📁 External Storage (/sdcard/Android/data/...)
    participant App as 📱 Vulnerable App (:app-vulnerable)
    participant Backend as ☁️ Enterprise Backend

    App->>Storage: 1. Write tokens to maswe0002_plaintext.json
    App->>Storage: 2. Write secret.key alongside encrypted data
    Attacker->>Storage: 3. Read plaintext JSON or dump secret.key
    Storage-->>Attacker: 4. Cleartext JWT and crypto key recovered
    Attacker->>Backend: 5. Replay JWT token to impersonate victim
    Backend-->>Attacker: 6. Complete account takeover

🛡️ Mitigations & Secure Implementation (The "Secure" Perspective)

Our :features:maswe0002 secure mirror demonstrates comprehensive defenses:

  1. Redirect to Private Internal Sandbox: Store sensitive data exclusively in context.filesDir. Internal storage is protected by kernel-level Linux UID permissions and SELinux policies.
  2. Android Keystore Hardware-Backed Keys: Keys are generated within the hardware TEE / StrongBox Keymaster. Private keys can never be exported or decompiled.
  3. Key Encapsulation (Zero Filesystem Storage): Cryptographic keys are referenced solely by Keystore alias. No key files are ever saved to disk.
  4. Authenticated AEAD Encryption (AES-256-GCM): Encrypt external data using EncryptedFile with AES256_GCM_HKDF_4KB authenticated encryption.
  5. Device-Bound Key Isolation: Hardware Keystore keys cannot be migrated, cloned, or replayed across devices.

🔬 Automated Verification Suite (features/maswe0002/poc/)

Script Type Purpose
frida_hook.js Dynamic Instrumentation Hooks getExternalFilesDir, SecretKeySpec, and Cipher.getInstance to intercept keys and unencrypted leaks
semgrep_rule.yml Static Analysis (SAST) Detects external file calls, ECB modes, and hardcoded key declarations in CI/CD
adb_verify.sh ADB Device Automation Inspects /sdcard/Android/data/... for leaked JSON files and secret.key artifacts