sec-design-deep-analysis.md

BorgBackup Security Analysis

1. Objective, Scope, and Methodology

Objective: This deep analysis aims to thoroughly examine the security posture of BorgBackup (borg) by dissecting its key components, identifying potential vulnerabilities, and proposing actionable mitigation strategies. The analysis will focus on the core Borg codebase and its interactions with external systems, considering the business and security posture outlined in the provided design review. We will specifically analyze:

• Authenticated Encryption: How Borg implements encryption and authentication, including key derivation, cipher choices, and integrity checks.
• Data Deduplication: The security implications of Borg's deduplication mechanism, including potential side-channel attacks and data leakage.
• Access Control: How Borg manages access to repositories, including append-only mode and client/server permissions.
• Key Management: The security of key handling, storage, and potential risks associated with user-managed keys.
• Repository Structure: How data is organized within a Borg repository and the implications for security.
• Build Process: The security of the build and distribution process, including code signing and dependency management.
• Network Communication (SSH): Security considerations when using SSH for remote repositories.

Scope: This analysis focuses on BorgBackup version 1.2.x (and conceptually, later versions following similar design principles) as described in the provided documentation and inferred from the codebase. Third-party GUI frontends are out of scope, as they are not officially part of the Borg project. We will focus on the core Borg codebase and its interaction with standard system components (e.g., SSH). We will not analyze the security of specific remote repository providers (e.g., BorgBase) beyond the general security considerations of using a remote service.

Methodology:

1. Code Review and Documentation Analysis: We will analyze the provided security design review, available Borg documentation (including the official documentation and source code comments), and relevant parts of the Borg codebase (primarily Python) on GitHub.
2. Architecture Inference: Based on the documentation and code, we will infer the architecture, data flow, and component interactions within Borg.
3. Threat Modeling: We will identify potential threats based on the identified architecture, components, and data flow, considering common attack vectors and vulnerabilities.
4. Vulnerability Analysis: We will analyze the identified threats to determine potential vulnerabilities in Borg's design and implementation.
5. Mitigation Strategy Recommendation: For each identified vulnerability, we will propose specific, actionable mitigation strategies tailored to Borg's architecture and design.

2. Security Implications of Key Components

2.1 Authenticated Encryption:

• Implementation: Borg uses authenticated encryption (AES-CTR with HMAC-SHA256 or ChaCha20-Poly1305) to protect data confidentiality and integrity. Key derivation is performed using scrypt (for key files) or HKDF-SHA256 (for key generation from a passphrase).
• Security Implications:
  • Strong Cryptography: Borg uses well-vetted and widely accepted cryptographic algorithms and modes. The use of authenticated encryption (combining confidentiality and integrity) is crucial.
  • Key Derivation: Scrypt is a strong key derivation function designed to be resistant to brute-force and hardware-based attacks. HKDF-SHA256 is also a secure key derivation function.
  • Cipher Choice: AES-CTR and ChaCha20 are both secure stream ciphers. ChaCha20-Poly1305 is generally preferred for its performance and resistance to certain timing attacks that can theoretically affect AES in some implementations.
  • Vulnerabilities:
    • Weak Passphrases: If a user chooses a weak passphrase, the derived key will also be weak, making the encryption vulnerable to brute-force attacks. This is a user-responsibility issue, but Borg can mitigate it (see below).
    • Side-Channel Attacks: While unlikely, sophisticated side-channel attacks (e.g., timing attacks, power analysis) could potentially be used to extract information about the key or data. This is more of a concern with hardware implementations of cryptography, but software implementations can also be vulnerable.
    • Key Compromise: If the key file or passphrase is compromised, the attacker can decrypt all data in the repository. This is the most significant risk.

2.2 Data Deduplication:

• Implementation: Borg uses content-defined chunking (CDC) with the Buzhash rolling hash algorithm to identify duplicate data blocks.
• Security Implications:
  • Efficiency: Deduplication significantly reduces storage space and bandwidth usage.
  • Vulnerabilities:
    • Collision Attacks: While extremely unlikely with a strong hash function, a malicious actor could theoretically craft a file that produces the same chunk hash as an existing file, potentially leading to data corruption or information disclosure. Borg's use of a cryptographic hash (in addition to the rolling hash) for chunk IDs mitigates this.
    • Information Leakage (Side-Channel): An attacker with access to the repository could potentially infer information about the backed-up data by observing which chunks are present. For example, if a specific chunk is known to be associated with a particular software package, the attacker could infer that the package is present in the backup. This is a minor risk, but it exists.
    • Targeted Data Poisoning: If an attacker can predict or influence the chunking process, they might be able to craft malicious data that, when deduplicated against existing data, results in a corrupted or compromised backup. This is highly unlikely due to the cryptographic hashing of chunk IDs.

2.3 Access Control:

• Implementation: Borg supports repository-level access control using append-only mode and client/server permissions (when using borg serve).
• Security Implications:
  • Append-Only Mode: This prevents an attacker with write access from modifying or deleting existing backups, limiting the damage they can cause. This is a strong protection against ransomware.
  • Client/Server Permissions: borg serve allows restricting clients to specific operations (e.g., read-only, append-only).
  • Vulnerabilities:
    • Misconfiguration: If append-only mode is not enabled or client/server permissions are not properly configured, an attacker with write access could compromise the entire repository.
    • Limited Granularity: Access control is primarily at the repository level. There's no built-in support for file-level or user-level permissions within a repository.
    • SSH Configuration: When using SSH, the security of the repository relies heavily on the security of the SSH configuration and the user accounts on the remote server.

2.4 Key Management:

• Implementation: Borg uses key files to store encryption keys. Users are responsible for securely storing these key files. Keys can be generated from a passphrase or randomly generated.
• Security Implications:
  • User Responsibility: The security of the backups depends entirely on the user's ability to protect the key file.
  • Vulnerabilities:
    • Key Loss: If the key file is lost, the backups are unrecoverable.
    • Key Compromise: If the key file is compromised (e.g., stolen, accessed by malware), the attacker can decrypt all data in the repository.
    • Lack of Key Rotation: Borg doesn't have built-in mechanisms for automatic key rotation, although it can be done manually.
    • No Key Escrow: There's no built-in mechanism for key escrow or recovery in case of key loss.

2.5 Repository Structure:

• Implementation: A Borg repository consists of a directory containing configuration files, data chunks, and index files.
• Security Implications:
  • Data Integrity: Borg uses cryptographic hashes to verify the integrity of data chunks and index files.
  • Vulnerabilities:
    • Direct File Access: If an attacker gains direct access to the repository files (e.g., through a compromised server), they could potentially tamper with the data or metadata, even if they don't have the encryption key. Append-only mode mitigates this to some extent.
    • Metadata Leakage: The repository structure and metadata (e.g., file names, sizes, timestamps) are not encrypted by default (although they can be with the --encryption=repokey-blake2 or --encryption=keyfile-blake2 options). This could leak information about the backed-up data.

2.6 Build Process:

• Implementation: Borg's build process uses standard Python packaging tools and GitHub Actions for CI.
• Security Implications:
  • Code Review: Code reviews help prevent malicious code from being introduced into the codebase.
  • Automated Testing: Automated tests help ensure that the code functions as expected and that security vulnerabilities are not introduced.
  • Linters: Linters help enforce code style and identify potential errors.
  • Dependency Management: Dependencies are managed using setup.cfg and requirements.txt.
  • Vulnerabilities:
    • Compromised Dependencies: If a dependency is compromised, it could introduce malicious code into Borg.
    • Compromised Build System: If the GitHub Actions environment or PyPI account is compromised, an attacker could distribute a malicious version of Borg.
    • Unsigned Releases: While developers are encouraged to sign commits, Borg releases themselves are not typically cryptographically signed. This makes it harder to verify the integrity of downloaded packages.

2.7 Network Communication (SSH):

• Implementation: Borg can use SSH to access remote repositories.
• Security Implications:
  • Secure Channel: SSH provides a secure, encrypted channel for communication between the client and the server.
  • Authentication: SSH uses key-based authentication, which is more secure than password-based authentication.
  • Vulnerabilities:
    • Weak SSH Configuration: If the SSH server is misconfigured (e.g., weak ciphers, password authentication enabled), it could be vulnerable to attack.
    • Compromised SSH Server: If the SSH server is compromised, the attacker could gain access to the Borg repository.
    • Man-in-the-Middle Attacks: If the SSH client's known_hosts file is not properly managed, it could be vulnerable to man-in-the-middle attacks.

3. Mitigation Strategies

Based on the identified vulnerabilities, here are specific, actionable mitigation strategies for BorgBackup:

3.1 Authenticated Encryption:

• Mitigation:
  • Passphrase Strength Enforcement: Implement a mechanism to enforce strong passphrases. This could include:
    • Minimum length requirements.
    • Complexity requirements (e.g., requiring uppercase, lowercase, numbers, and symbols).
    • Checking against a list of common passwords.
    • Providing a visual strength meter.
    • Educating users about the importance of strong passphrases.
  • Side-Channel Attack Mitigation: While difficult to fully mitigate in software, consider:
    • Using constant-time comparison functions where appropriate.
    • Staying up-to-date with the latest cryptographic libraries, which often include mitigations for known side-channel attacks.
  • Key Compromise Mitigation: (See Key Management below)

3.2 Data Deduplication:

• Mitigation:
  • Collision Attack Mitigation: Borg already mitigates this by using cryptographic hashes for chunk IDs. No further action is needed.
  • Information Leakage Mitigation: This is a minor risk. Mitigation is difficult without significantly impacting performance. Consider documenting this risk clearly.
  • Targeted Data Poisoning Mitigation: Borg already mitigates this with cryptographic hashing. No further action needed.

3.3 Access Control:

• Mitigation:
  • Improved Documentation: Provide clearer documentation and examples on how to properly configure append-only mode and client/server permissions.
  • Default to Secure Settings: Consider making append-only mode the default setting for new repositories.
  • SSH Security Guidance: Provide detailed guidance on how to securely configure SSH for use with Borg, including:
    • Using key-based authentication only.
    • Disabling root login.
    • Using strong ciphers and MACs.
    • Regularly updating SSH software.
    • Using a dedicated user account for Borg backups.
    • Using ForceCommand in authorized_keys to restrict the commands that can be executed via SSH.
  • Granular Permissions (Future Enhancement): Explore the possibility of adding more granular permissions in the future (e.g., file-level or user-level permissions within a repository). This is a complex feature, but it would enhance security.

3.4 Key Management:

• Mitigation:
  • Key Rotation Guidance: Provide clear documentation and tools to help users manually rotate their encryption keys.
  • Key Escrow/Recovery (Future Enhancement): Explore options for integrating with key management services (KMS) or hardware security modules (HSMs) to provide key escrow and recovery capabilities. This is a complex feature, but it would significantly improve the security of key management.
  • Multi-Factor Authentication (Future Enhancement): Consider adding support for multi-factor authentication (MFA) for accessing repositories, especially when using borg serve.
  • Key File Security Guidance: Emphasize best practices for securing key files, such as:
    • Storing them on a separate, encrypted device.
    • Using a strong passphrase to protect the key file itself (if applicable).
    • Making regular backups of the key file.
    • Avoiding storing the key file in the same location as the backup repository.

3.5 Repository Structure:

• Mitigation:
  • Encrypted Metadata (Default): Consider making metadata encryption (using --encryption=repokey-blake2 or --encryption=keyfile-blake2) the default setting for new repositories.
  • File System Permissions: Provide guidance on setting appropriate file system permissions on the repository directory to prevent unauthorized access.
  • Integrity Monitoring: Consider implementing a mechanism to monitor the integrity of the repository files and alert the user if any tampering is detected. This could be a separate tool or integrated into Borg.

3.6 Build Process:

• Mitigation:
  • Dependency Auditing: Regularly audit dependencies for security vulnerabilities using tools like pip-audit or Dependabot.
  • Signed Releases: Implement a process for cryptographically signing Borg releases (e.g., using GPG). This allows users to verify the integrity of downloaded packages.
  • Build System Security: Ensure that the GitHub Actions environment is secure and that access to the PyPI account is protected with strong passwords and two-factor authentication.
  • Software Bill of Materials (SBOM): Consider generating an SBOM for each release to provide transparency about the components used in Borg.

3.7 Network Communication (SSH):

• Mitigation:
  • SSH Configuration Guidance: (See Access Control above)
  • Known Hosts Management: Provide guidance on how to properly manage the SSH client's known_hosts file to prevent man-in-the-middle attacks. This could include using SSH certificates or a centralized key management system.
  • Network Segmentation: If possible, isolate the Borg server on a separate network segment to limit the impact of a potential compromise.

4. Conclusion

BorgBackup is a well-designed and secure backup solution that incorporates strong cryptographic practices and access control mechanisms. However, like any software, it has potential vulnerabilities. The most significant risks are related to user-managed key management and the potential for compromise of the repository through weak SSH configurations or direct file access. By implementing the mitigation strategies outlined above, the Borg project can further enhance its security posture and reduce the risk of data loss or compromise. The recommendations related to key management (escrow, MFA, rotation) and build process (signed releases, SBOM) are particularly important for long-term security.