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Threat Model Analysis for letsencrypt/boulder

Threat: Private Key Compromise via Software Vulnerability

  • Threat: Private Key Compromise via Software Vulnerability

    • Description: An attacker exploits a vulnerability (e.g., buffer overflow, remote code execution) in Boulder's code (specifically in the code interacting with the HSM or handling key material) to gain access to the CA's private key(s). This could involve crafting malicious ACME requests or exploiting vulnerabilities in exposed Boulder services.
    • Impact: Complete CA compromise. The attacker can issue fraudulent certificates for any domain, impersonate the CA, and potentially decrypt past communications (if forward secrecy is not used). This destroys trust in the CA and all certificates it has issued.
    • Affected Component: boulder-ca (specifically, functions interacting with the HSM API, such as crypto/pkcs11.go or similar, and any code handling private key material directly), potentially boulder-ra if key material is passed insecurely between components.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Rigorous Code Review: Conduct thorough code reviews, focusing on security-critical components.
      • Static Analysis: Use static analysis tools to identify potential vulnerabilities.
      • Fuzz Testing: Perform fuzz testing on the ACME interface and internal APIs.
      • Dependency Management: Keep all dependencies up-to-date and audit them for vulnerabilities.
      • Memory Safe Languages: Boulder is written in Go, which offers some memory safety, but careful coding is still essential.
      • Principle of Least Privilege: Run Boulder components with the minimum necessary privileges.
      • HSM Security: Ensure the HSM is configured securely and its firmware is up-to-date.
      • Intrusion Detection/Prevention: Deploy intrusion detection and prevention systems to monitor for suspicious activity.

Threat: DNS-01 Challenge Hijacking via Boulder Validation Logic Flaws

  • Threat: DNS-01 Challenge Hijacking via Boulder Validation Logic Flaws

    • Description: An attacker exploits vulnerabilities within Boulder's implementation of the DNS-01 challenge validation process. This is distinct from compromising external DNS servers. The vulnerability could allow the attacker to bypass checks or manipulate the validation process to falsely prove control of a domain.
    • Impact: The attacker can obtain a valid certificate for a domain they do not own, allowing them to impersonate the legitimate website, intercept traffic, or conduct phishing attacks.
    • Affected Component: boulder-va (specifically, the DNS-01 challenge validation logic in va/dns.go or similar).
    • Risk Severity: High
    • Mitigation Strategies:
      • Rigorous Code Review: Thoroughly review the DNS-01 validation code for logic errors and potential bypasses.
      • Input Validation: Strictly validate all inputs related to the DNS-01 challenge, including domain names and TXT record values.
      • Fuzz Testing: Fuzz test the DNS-01 validation logic with various malformed inputs.
      • Unit and Integration Tests: Write comprehensive unit and integration tests to cover all aspects of the DNS-01 validation process.
      • Regular Security Audits: Conduct regular security audits of the validation code.

Threat: Denial of Service via Challenge Flood

  • Threat: Denial of Service via Challenge Flood

    • Description: An attacker initiates a large number of ACME challenges (e.g., HTTP-01, DNS-01) without completing them. This consumes resources on the CA server, preventing legitimate users from obtaining certificates. This specifically targets Boulder's ability to handle a large volume of requests.
    • Impact: Denial of service. Legitimate users are unable to obtain certificates.
    • Affected Component: boulder-va (the validation authority, handling all challenge types), potentially boulder-ra (if challenges are queued or processed there).
    • Risk Severity: High
    • Mitigation Strategies:
      • Challenge Timeouts: Implement short timeouts for incomplete challenges.
      • Resource Limits: Limit the number of pending challenges per account or IP address within Boulder's configuration.
      • Rate Limiting (on Challenges): Implement rate limits specifically for challenge initiation within Boulder.
      • Asynchronous Processing: Use asynchronous processing for challenge validation to avoid blocking the main server threads. Ensure Boulder's architecture supports this efficiently.

Threat: Time Manipulation via NTP Compromise (affecting Boulder's logic)

  • Threat: Time Manipulation via NTP Compromise (affecting Boulder's logic)

    • Description: An attacker compromises the CA's NTP server or manipulates network traffic to alter the CA's system time. This directly impacts Boulder's internal time-dependent logic, causing it to issue certificates with incorrect validity periods or accept expired challenges.
    • Impact: Certificates may be issued with incorrect validity periods, leading to trust issues or allowing attackers to bypass time-based security checks. Expired challenges might be accepted, leading to fraudulent certificate issuance.
    • Affected Component: boulder-ca (specifically, functions related to certificate issuance and validity period calculation), boulder-va (challenge validation). Affects any component relying on system time.
    • Risk Severity: High
    • Mitigation Strategies:
      • Multiple NTP Sources: Configure Boulder to use multiple, trusted NTP sources.
      • NTP Authentication: Use authenticated NTP (e.g., NTS) if supported by Boulder and the NTP servers.
      • Sanity Checks: Implement sanity checks within Boulder's code on certificate validity periods and challenge timestamps. Reject obviously incorrect values.

Threat: Database Corruption via SQL Injection (in Boulder's code)

  • Threat: Database Corruption via SQL Injection (in Boulder's code)

    • Description: An attacker exploits a SQL injection vulnerability within Boulder's database interaction code to modify or delete data in the CA database. This could lead to data corruption, denial of service, or potentially unauthorized certificate issuance (if the attacker can manipulate authorization records).
    • Impact: Data loss, denial of service, potential unauthorized certificate issuance, compromise of CA integrity.
    • Affected Component: Any Boulder component interacting with the database (e.g., boulder-ra, boulder-ca, boulder-va), specifically code using SQL queries (likely in files under storage/ or similar).
    • Risk Severity: High
    • Mitigation Strategies:
      • Parameterized Queries: Use parameterized queries or prepared statements for all database interactions within Boulder's code.
      • Input Validation: Strictly validate and sanitize all user-supplied input before using it in SQL queries within Boulder.
      • ORM (Object-Relational Mapper): If Boulder uses an ORM, ensure it's a well-vetted one and is used correctly. If not, consider its use.
      • Regular Code Audits: Regularly audit the database interaction code for potential SQL injection vulnerabilities.

Threat: Configuration File Exposure (leading to Boulder compromise)

  • Threat: Configuration File Exposure (leading to Boulder compromise)

    • Description: An attacker gains access to Boulder's configuration files, potentially revealing sensitive information such as database credentials, or other configuration settings that could be used to directly attack Boulder.
    • Impact: Exposure of sensitive information, potentially leading to database compromise, or other attacks specifically targeting Boulder.
    • Affected Component: All Boulder components, as they rely on the configuration.
    • Risk Severity: High
    • Mitigation Strategies:
      • Secure File Permissions: Set strict file permissions on configuration files, allowing access only to the necessary users and processes that Boulder runs as.
      • Avoid Storing Secrets in Plaintext: Do not store sensitive information (e.g., passwords) in plaintext in Boulder's configuration files. Use environment variables or a secrets management system.
      • Regular Audits: Regularly audit Boulder's configuration files for sensitive information and insecure settings.