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attack-surface.md

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Attack Surface Analysis for borgbackup/borg

Attack Surface: 1. Unauthorized Repository Access (Borg-Specific Aspects)

  • Description: An attacker gains read, write, or delete access to the Borg backup repository by exploiting weaknesses related to Borg's authentication or authorization mechanisms.
  • How Borg Contributes: This is directly related to Borg's core function: managing access to the encrypted repository. Weaknesses in Borg's handling of passwords, keys, or network protocols are the primary concern.
  • Example: An attacker exploits a vulnerability in Borg's borg serve implementation to bypass authentication and gain access to the repository. Or, an attacker discovers a flaw in Borg's key derivation function that allows them to brute-force a weak passphrase more easily.
  • Impact: Complete data breach (confidentiality loss), data loss (availability loss), potential data tampering (integrity loss). Restoration of tampered backups could introduce malware.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strong Passphrases/Key Management: Use strong, unique passphrases. Securely store and manage encryption keys (offline, HSM). Implement key rotation.
    • Secure borg serve Configuration: Always use strong authentication with borg serve. Restrict network access using firewall rules. Avoid exposing borg serve directly to the internet.
    • Use Authenticated Encryption: Always use Borg's authenticated encryption modes (e.g., repokey-blake2, keyfile-blake2). Never use unencrypted modes.
    • Keep Borg Updated: Apply security patches promptly. This is crucial to address any discovered vulnerabilities in Borg's authentication or authorization logic.
    • Avoid BORG_PASSPHRASE: Do not use the BORG_PASSPHRASE environment variable. Use BORG_PASSCOMMAND with a secure command or a keyfile.
    • Secure BORG_PASSCOMMAND: If using BORG_PASSCOMMAND, ensure the command itself is secure and not vulnerable to injection or leakage.

Attack Surface: 2. Repository Tampering/Corruption (Borg-Specific Aspects)

  • Description: An attacker modifies the Borg repository by exploiting vulnerabilities in Borg's repository handling or integrity checks.
  • How Borg Contributes: Borg has built-in integrity checks, but vulnerabilities in these checks or in the repository format handling could allow for targeted corruption.
  • Example: An attacker discovers a flaw in Borg's deduplication logic that allows them to inject malicious data into the repository without triggering integrity checks. Or, a vulnerability in Borg's handling of corrupted archive headers allows an attacker to cause a denial-of-service during restoration.
  • Impact: Data loss (availability loss), data corruption (integrity loss), potential system compromise upon restoration (if malicious data is injected).
  • Risk Severity: High
  • Mitigation Strategies:
    • Regular Integrity Checks: Use borg check --verify-data regularly and automate this process. This relies on Borg's internal checks, so keeping Borg updated is vital.
    • Keep Borg Updated: Apply security patches promptly to address any vulnerabilities in Borg's repository handling or integrity checks.
    • Understand Append-Only Limitations: Append-only mode helps, but it doesn't prevent adding malicious archives. It's a layer of defense, not a complete solution.

Attack Surface: 3. Key Compromise (Direct Borg Handling)

  • Description: The encryption key is compromised due to weaknesses in how Borg handles or stores the key. This excludes general key management best practices.
  • How Borg Contributes: While Borg doesn't store the key persistently unless configured to do so (e.g., with repokey mode), vulnerabilities in how Borg uses the key in memory or during key derivation could lead to compromise.
  • Example: A vulnerability in Borg's key derivation function (KDF) makes it susceptible to a side-channel attack, allowing an attacker to extract the key from memory. Or, a flaw in Borg's handling of the passphrase during input could expose it to other processes.
  • Impact: Complete data breach (confidentiality loss).
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strong Passphrases: Use long, complex passphrases to make brute-force and dictionary attacks more difficult.
    • Keep Borg Updated: Apply security patches to address any vulnerabilities in Borg's key handling or KDF.
    • Avoid BORG_PASSPHRASE: As mentioned before, do not use this environment variable.
    • Secure BORG_PASSCOMMAND: Ensure the command used with BORG_PASSCOMMAND is secure.

Attack Surface: 4. Exploiting Borg Client/Server Vulnerabilities (Direct Code Issues)

  • Description: Exploiting vulnerabilities within the Borg codebase itself (client or borg serve) to gain unauthorized access, execute code, or cause a denial of service.
  • How Borg Contributes: This is entirely dependent on the presence of bugs in Borg's code.
  • Example: A buffer overflow vulnerability in Borg's parsing of a malformed archive allows an attacker to execute arbitrary code on the client machine. Or, a denial-of-service vulnerability in borg serve allows an attacker to crash the server.
  • Impact: Varies depending on the vulnerability; could range from denial of service to complete system compromise (client or server).
  • Risk Severity: High
  • Mitigation Strategies:
    • Keep Borg Updated: This is the primary mitigation. Apply security patches promptly.
    • Code Audits (if feasible): For highly sensitive deployments, consider independent security audits of the Borg codebase.