Skip to content

Latest commit

 

History

History
166 lines (127 loc) · 16.4 KB

sec-design-deep-analysis.md

File metadata and controls

166 lines (127 loc) · 16.4 KB

Deep Analysis of MMKV Security Considerations

1. Objective, Scope, and Methodology

Objective: To conduct a thorough security analysis of the MMKV key-value storage library, focusing on its key components, architecture, data flow, and interactions with the Android operating system. The analysis aims to identify potential security vulnerabilities, assess their impact, and propose specific, actionable mitigation strategies tailored to MMKV's design and intended use within mobile applications. The primary goal is to ensure that applications leveraging MMKV for data persistence do so securely, minimizing risks to data confidentiality, integrity, and availability.

Scope: This analysis covers the MMKV library as described in the provided security design review and the linked GitHub repository (https://github.com/tencent/mmkv). It encompasses:

  • MMKV Core: The primary interface and API.
  • MMKV Logic: Memory mapping, serialization, CRC32 checks.
  • MMKV Coder: Data encoding and decoding.
  • File Storage: Interaction with the Android file system.
  • Inter-process Communication (IPC): MMKV's process locking mechanism.
  • Encryption: MMKV's AES encryption implementation (when enabled).
  • Data Integrity: CRC32 checksum implementation.

The analysis excludes the security of the application using MMKV except where the application's choices directly impact MMKV's security. It also acknowledges the accepted risks related to physical device compromise and user-provided key management.

Methodology:

  1. Code Review (Inferred): Since direct code execution isn't possible, we'll infer the behavior and potential vulnerabilities based on the provided documentation, design diagrams, and common patterns in similar libraries. We'll analyze the described components and their interactions.
  2. Threat Modeling: We'll identify potential threats based on the identified components, data flows, and security controls. We'll consider threats related to data breaches, data corruption, denial of service, and privilege escalation.
  3. Vulnerability Analysis: We'll assess the likelihood and impact of each identified threat, considering existing security controls and accepted risks.
  4. Mitigation Recommendations: We'll propose specific, actionable mitigation strategies to address the identified vulnerabilities. These recommendations will be tailored to MMKV's architecture and intended use.

2. Security Implications of Key Components

Let's break down the security implications of each key component:

  • MMKV Core:

    • Threats: Unauthorized access to MMKV instances, denial of service through excessive resource consumption (e.g., opening too many MMKV instances), potential for race conditions if the process locking mechanism is flawed.
    • Security Controls: Process locking (critical for mitigating race conditions and unauthorized access between processes).
    • Vulnerabilities: Bypassing the process lock (if implemented incorrectly), potential for deadlocks if the locking mechanism is not carefully designed, integer overflows in handling instance IDs or sizes.
    • Mitigation:
      • Strengthen Process Locking: Thoroughly review and test the process locking implementation (using fcntl or similar) to ensure it's robust against race conditions and bypass attempts. Consider using established, well-vetted locking libraries. Implement robust error handling and recovery mechanisms for lock acquisition failures.
      • Resource Limits: Implement limits on the number of MMKV instances that can be created per process and the total size of data stored to prevent resource exhaustion attacks.
      • Input Validation: Validate all inputs to the MMKV Core API, including MMKV IDs, sizes, and other parameters, to prevent integer overflows or other injection vulnerabilities.

  • MMKV Logic:

    • Threats: Data corruption due to errors in memory mapping or serialization/deserialization, attacks exploiting vulnerabilities in the CRC32 implementation (though CRC32 is not a cryptographic hash, weaknesses could lead to undetected corruption).
    • Security Controls: CRC32 checksum for data integrity. Memory mapping (mmap) for performance.
    • Vulnerabilities: Errors in memory management (e.g., buffer overflows, use-after-free) within the mmap handling, potential for data corruption if the serialization/deserialization logic is flawed, CRC32 collision attacks (although the impact is limited to undetected corruption).
    • Mitigation:
      • Robust Memory Management: Carefully review the memory management code related to mmap. Use memory safety tools (e.g., AddressSanitizer, Valgrind – if possible in the development environment) to detect potential memory errors. Consider using safer memory management techniques if feasible.
      • Serialization/Deserialization Security: Thoroughly review and test the serialization/deserialization logic. If custom serialization is used, ensure it's robust against malformed input. Consider using well-vetted serialization libraries (e.g., Protocol Buffers) to minimize the risk of vulnerabilities.
      • Consider Alternatives to CRC32 (Long-Term): While CRC32 is acceptable for basic error detection, explore using a stronger checksum algorithm (e.g., a truncated cryptographic hash like SHA-256) for improved integrity verification, especially if data integrity is critical.

  • MMKV Coder:

    • Threats: Attacks exploiting vulnerabilities in the encoding/decoding logic, potentially leading to data corruption or code execution (if the decoded data is used in a vulnerable way).
    • Security Controls: None directly within MMKV Coder; relies on the security of the data types being encoded.
    • Vulnerabilities: Buffer overflows or other memory corruption vulnerabilities in the encoding/decoding process, especially if custom encoding is used. Type confusion vulnerabilities if the decoding process doesn't properly validate the type of data being decoded.
    • Mitigation:
      • Secure Encoding/Decoding: If custom encoding/decoding is used, ensure it's thoroughly reviewed and tested for security vulnerabilities. Use memory safety tools to detect potential errors. Consider using standard, well-vetted encoding formats (e.g., Protocol Buffers, JSON with a schema) to reduce the risk of vulnerabilities.
      • Type Validation: Implement strict type validation during decoding to prevent type confusion vulnerabilities. Ensure that the decoded data is of the expected type before it's used.

  • File Storage:

    • Threats: Unauthorized access to MMKV data files, data tampering, data deletion, attacks exploiting vulnerabilities in the Android file system.
    • Security Controls: Relies on the operating system's file system security (permissions, sandboxing).
    • Vulnerabilities: Incorrect file permissions allowing other applications to access MMKV data files, vulnerabilities in the Android file system allowing unauthorized access.
    • Mitigation:
      • Strict File Permissions: Ensure that MMKV data files are created with the most restrictive permissions possible (e.g., MODE_PRIVATE in Android). Regularly review and audit the file permissions to ensure they haven't been accidentally changed.
      • File System Security: Rely on the Android operating system's security features to protect the file system. Keep the Android system updated to the latest security patches.
      • Data Encryption at Rest (Reinforcement): Even with file permissions, encrypting the data within the MMKV files adds another layer of defense.

  • Inter-process Communication (IPC):

    • Threats: Race conditions, data inconsistencies, denial of service, unauthorized access to data from other processes.
    • Security Controls: Process locking mechanism (using fcntl or similar).
    • Vulnerabilities: Flaws in the process locking mechanism allowing multiple processes to access the same data concurrently, leading to data corruption or inconsistencies. Deadlocks if the locking mechanism is not carefully designed.
    • Mitigation:
      • Robust Locking (as mentioned above): Thoroughly review and test the process locking implementation. Use established, well-vetted locking libraries. Implement robust error handling and recovery mechanisms.
      • Locking Granularity: Consider the granularity of the locking mechanism. Fine-grained locking (e.g., locking individual key-value pairs) can improve concurrency but increases complexity. Coarse-grained locking (e.g., locking the entire MMKV instance) is simpler but can reduce concurrency. Choose the appropriate granularity based on the application's needs.

  • Encryption (AES):

    • Threats: Weak key derivation, key compromise, attacks exploiting vulnerabilities in the AES implementation (unlikely, but possible).
    • Security Controls: AES encryption (configurable).
    • Vulnerabilities: Use of weak encryption keys, insecure key storage, side-channel attacks on the AES implementation. The biggest vulnerability is the reliance on user-provided keys without secure key derivation.
    • Mitigation:
      • Secure Key Derivation (KDF): Implement a strong KDF (e.g., PBKDF2, scrypt, Argon2) to derive encryption keys from user passwords or other secrets. This is crucial to prevent weak keys. Do not allow users to directly provide the encryption key.
      • Secure Key Storage (Android Keystore): Use the Android Keystore system to store encryption keys securely. This protects the keys from unauthorized access, even if the application is compromised.
      • AES Implementation: Use the Android system's built-in AES implementation (through javax.crypto.* or the Conscrypt provider) rather than a custom implementation. This ensures that the implementation is well-vetted and regularly updated.
      • Key Rotation: Implement a mechanism for key rotation to limit the impact of a key compromise.

  • Data Integrity (CRC32):

    • Threats: Undetected data corruption.
    • Security Controls: CRC32 checksum.
    • Vulnerabilities: CRC32 is not cryptographically secure and collisions can be crafted.
    • Mitigation:
      • Consider Stronger Checksum (Long-Term): As mentioned before, consider a stronger checksum algorithm (e.g., truncated SHA-256) for improved integrity verification.

3. Architecture, Components, and Data Flow (Inferred)

Based on the provided information, we can infer the following:

  1. Architecture: MMKV is a library embedded within an Android application. It uses memory mapping (mmap) to provide fast access to persistent data stored in a file. It employs a process locking mechanism to ensure data consistency across multiple processes.

  2. Components: As described above (MMKV Core, MMKV Logic, MMKV Coder, File Storage).

  3. Data Flow:

    • Write Operation:

      1. The application calls the MMKV Core API to write data.
      2. MMKV Core acquires a process lock.
      3. MMKV Coder encodes the data.
      4. MMKV Logic serializes the data and writes it to the memory-mapped region.
      5. MMKV Logic calculates the CRC32 checksum.
      6. The data is flushed to the underlying file (managed by File Storage).
      7. MMKV Core releases the process lock.

    • Read Operation:

      1. The application calls the MMKV Core API to read data.
      2. MMKV Core acquires a process lock.
      3. MMKV Logic reads the data from the memory-mapped region.
      4. MMKV Logic verifies the CRC32 checksum.
      5. MMKV Coder decodes the data.
      6. MMKV Core returns the data to the application.
      7. MMKV Core releases the process lock.

    • Encryption (if enabled): Encryption and decryption would occur within the MMKV Logic component, after encoding and before decoding, respectively.

4. Specific Security Considerations

Given the nature of MMKV as a key-value storage library for Android applications, the following security considerations are particularly important:

  • Data Sensitivity: The application using MMKV must carefully consider the sensitivity of the data being stored. Sensitive data (e.g., user credentials, API keys, personal information) must be encrypted using MMKV's encryption feature, with strong key derivation and secure key storage.
  • Inter-process Communication: If the application uses MMKV to share data between multiple processes, the process locking mechanism is critical. Any flaws in this mechanism could lead to data corruption or race conditions.
  • File System Security: MMKV relies on the Android file system's security. The application should ensure that MMKV data files are created with the most restrictive permissions possible.
  • Key Management: Secure key management is paramount when encryption is used. The application must use a strong KDF and the Android Keystore system to protect encryption keys.
  • Denial of Service: While MMKV is designed for performance, an attacker could potentially cause a denial-of-service by creating a large number of MMKV instances or storing excessively large values, exhausting system resources.

5. Actionable Mitigation Strategies (Tailored to MMKV)

These are the most critical and actionable mitigations, prioritized based on impact and feasibility:

  1. Highest Priority - Secure Key Management (for Applications Using Encryption):

    • Implement a strong KDF (PBKDF2, scrypt, Argon2) to derive encryption keys from user-provided passwords or other secrets. Provide clear guidance and examples in the MMKV documentation on how to use these KDFs correctly.
    • Strongly recommend and document the use of the Android Keystore system for storing encryption keys. Provide code examples demonstrating how to integrate MMKV with the Android Keystore.
    • Never allow users to directly provide the encryption key.

  2. High Priority - Strengthen Process Locking:

    • Thoroughly review and test the process locking implementation (using fcntl or similar) to ensure it's robust against race conditions and bypass attempts. Consider using a well-vetted, platform-specific locking library.
    • Implement comprehensive unit and integration tests to verify the correctness of the locking mechanism under various concurrent access scenarios.
    • Implement robust error handling and recovery mechanisms for lock acquisition failures.

  3. High Priority - Input Validation:

    • Validate all inputs to the MMKV Core API, including MMKV IDs, sizes, and other parameters, to prevent integer overflows or other injection vulnerabilities.

  4. Medium Priority - Robust Memory Management:

    • Carefully review the memory management code related to mmap. Use memory safety tools (AddressSanitizer, Valgrind) during development to detect potential memory errors.

  5. Medium Priority - Serialization/Deserialization Security:

    • Thoroughly review and test the serialization/deserialization logic. If custom serialization is used, ensure it's robust against malformed input. Consider using well-vetted serialization libraries (e.g., Protocol Buffers).

  6. Medium Priority - Secure Encoding/Decoding:

    • If custom encoding/decoding is used, ensure it's thoroughly reviewed and tested. Use memory safety tools. Consider standard, well-vetted encoding formats.

  7. Medium Priority - Strict File Permissions:

    • Ensure that MMKV data files are created with MODE_PRIVATE in Android. Document this clearly.

  8. Long-Term - Consider Alternatives to CRC32:

    • Explore using a stronger checksum algorithm (e.g., a truncated cryptographic hash like SHA-256) for improved integrity verification.

  9. Ongoing - Regular Security Audits:

    • Conduct regular security audits of the MMKV codebase to identify and address potential vulnerabilities.
    • Stay informed about security best practices and emerging threats related to Android development and data storage.

By implementing these mitigation strategies, the security posture of MMKV and the applications that use it can be significantly improved, protecting sensitive data and ensuring the reliability of the storage solution.