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attack-surface.md

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Attack Surface Analysis for letsencrypt/boulder

Attack Surface: 1. Challenge Manipulation (DNS-01, HTTP-01, TLS-ALPN-01)

  • Description: Attackers exploit weaknesses in Boulder's implementation of the ACME challenge-response process to obtain certificates for domains they don't control. This is distinct from attacks on external DNS or web servers.
    • How Boulder Contributes: Boulder's core function is to validate these challenges. Vulnerabilities in its validation logic, handling of external requests (even if the requests themselves are secure), or timing are the key concerns. This includes how Boulder parses responses, handles errors, and enforces timing constraints.
    • Example: An attacker exploits a race condition within Boulder's code during HTTP-01 validation to bypass checks. Or, a flaw in Boulder's DNS response parsing allows a carefully crafted malicious response to be accepted.
    • Impact: Unauthorized issuance of certificates, leading to potential phishing, man-in-the-middle attacks, and loss of trust.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Robust Input Validation: Strictly validate all data received from external sources within Boulder's code. This includes rigorous checks on DNS responses, HTTP headers and bodies, and TLS parameters.
      • Secure Coding Practices: Focus on preventing race conditions, buffer overflows, and other common coding errors in the challenge validation logic.
      • Regular Code Review: Thoroughly review the code responsible for challenge validation, with a specific focus on security-critical sections.
      • Fuzz Testing: Use fuzz testing to identify vulnerabilities in Boulder's handling of malformed or unexpected challenge responses.
      • Penetration Testing: Regularly conduct penetration tests specifically targeting the challenge-response mechanism as implemented by Boulder.

Attack Surface: 2. ACME Protocol Implementation Flaws (JWS, Order Processing)

  • Description: Bugs in Boulder's implementation of the ACME protocol itself (RFC 8555) allow attackers to bypass security checks or cause unexpected behavior. This is about Boulder's code, not the protocol itself.
    • How Boulder Contributes: Boulder is the ACME server implementation. Any deviations from the RFC, incorrect state handling, or flaws in cryptographic operations are direct vulnerabilities.
    • Example: An attacker crafts a malformed JWS (JSON Web Signature) that bypasses Boulder's signature validation due to a bug in Boulder's JWS handling code. Or, a flaw in Boulder's order processing logic allows an attacker to finalize an order without completing the required challenges because of a state machine error in Boulder.
    • Impact: Unauthorized certificate issuance, account hijacking, denial-of-service.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Strict Adherence to RFC 8555: Ensure Boulder's code meticulously follows the ACME protocol specification.
      • Comprehensive Unit and Integration Tests: Test every aspect of Boulder's ACME protocol implementation, including edge cases and error handling.
      • Fuzz Testing: Use fuzz testing to identify vulnerabilities in Boulder's handling of malformed or unexpected ACME requests.
      • Code Review: Regularly review the code responsible for ACME protocol handling, focusing on potential security flaws and conformance to the RFC.
      • Formal Verification (Ideal): If feasible, consider using formal verification techniques to prove the correctness of critical parts of the ACME protocol implementation.

Attack Surface: 3. Vulnerable Dependencies (Boulder's Direct Dependencies)

  • Description: Vulnerabilities in libraries directly used by Boulder's code (Go packages, etc.) can be exploited. This focuses on dependencies within Boulder's control, not external services.
    • How Boulder Contributes: Boulder's security is directly tied to the security of the libraries it imports and uses. A vulnerability in a Go package used for cryptography, networking, or data parsing is a direct threat to Boulder.
    • Example: A vulnerability is discovered in a Go library used by Boulder for TLS handling. An attacker exploits this vulnerability to compromise Boulder's communication because Boulder directly uses the vulnerable library.
    • Impact: Varies depending on the vulnerability, but could range from denial-of-service to complete compromise of the CA.
    • Risk Severity: High (can be Critical depending on the dependency and vulnerability)
    • Mitigation Strategies:
      • Dependency Management: Use go mod to track and update dependencies. Pin dependencies to specific versions where appropriate.
      • Software Composition Analysis (SCA): Use an SCA tool to automatically identify known vulnerabilities in Boulder's direct dependencies.
      • Regular Updates: Regularly update all of Boulder's Go dependencies to the latest secure versions. This is a continuous process.
      • Vulnerability Scanning: Use vulnerability scanners that can analyze Boulder's compiled binary and its dependencies.
      • Vendor Security Advisories: Monitor security advisories for all of Boulder's dependencies.

Attack Surface: 4. Internal API Exposure (Boulder's Own APIs)

  • Description: Unprotected or vulnerable internal APIs within Boulder itself allow attackers to control the CA.
    • How Boulder Contributes: Boulder has internal APIs for its own management and operation. If these APIs are exposed without proper security (authentication, authorization, input validation), they become a direct attack vector within Boulder.
    • Example: An attacker discovers an unauthenticated internal API endpoint within Boulder's code that allows them to issue certificates without going through the ACME process.
    • Impact: Complete compromise of the CA, unauthorized certificate issuance.
    • Risk Severity: High
    • Mitigation Strategies:
      • Authentication and Authorization: Implement strong authentication and authorization for all of Boulder's internal API endpoints.
      • Network Segmentation (Limited Impact): While network segmentation can help, it's not a primary defense for internal APIs. The focus should be on securing the APIs themselves.
      • API Security Testing: Regularly test the security of Boulder's internal APIs, including penetration testing and fuzz testing. This testing should be part of Boulder's development lifecycle.
      • Input Validation: Rigorously validate all inputs to Boulder's internal API endpoints, even if they are intended for internal use.
      • Code Review: Pay close attention to the security of internal API code during code reviews.