Harden SSL/TLS — A Checklist for Servers, Clients, and CertificatesTransport Layer Security (TLS) secures data in transit across networks. Properly hardening SSL/TLS reduces the attack surface, protects confidentiality and integrity, and prevents protocol downgrade or cryptographic failures. This checklist covers practical steps for servers, clients, and certificates, with examples, rationale, and verification tips.
1. Understand current TLS landscape and goals
- Know which TLS versions are in use and supported by your environment. Disable SSLv2/SSLv3 and TLS 1.0/1.1.
- Aim to support TLS 1.2 and TLS 1.3 only; prioritize TLS 1.3 where possible for its simplified and safer design.
- Goals: prevent downgrade, ensure forward secrecy, use strong ciphers, validate certificates correctly, and reduce metadata leakage.
2. Server-side configuration checklist
2.1 TLS versions and protocol negotiation
- Configure servers to accept only TLS 1.2 and TLS 1.3.
- Prefer TLS 1.3 for new deployments—its handshake reduces round-trips and removes legacy cryptographic constructs.
- Disable renegotiation where supported or ensure secure renegotiation is enforced.
2.2 Cipher suites and key exchange
- For TLS 1.3, rely on the default cipher suites implemented by the server software (they’re already modern).
- For TLS 1.2, explicitly configure cipher suites to:
- Prioritize AEAD ciphers (e.g., AES-GCM, ChaCha20-Poly1305).
- Prefer ECDHE key exchange for forward secrecy.
- Avoid RSA key exchange, static DH, and non-AEAD ciphers (e.g., CBC modes that are vulnerable).
- Example OpenSSL-style TLS 1.2 cipher string (conceptual):
- ECDHE+AESGCM:ECDHE+CHACHA20:!aNULL:!eNULL:!LOW:!MD5:!3DES
2.3 Certificates and key sizes
- Use certificates with 2048-bit RSA at minimum or ECDSA P-256/P-384. Prefer ECDSA for smaller keys and efficient operations.
- Renew certificates before expiration; automate with ACME (Let’s Encrypt) where appropriate.
- Ensure private keys are stored securely (hardware security module [HSM] or OS key stores with strict access controls).
2.4 Certificate chain and trust
- Serve the complete certificate chain (except the root) in the correct order. Missing intermediates cause client trust failures.
- Remove cross-signed or deprecated intermediates that may allow undesired trust paths.
2.5 OCSP and revocation
- Implement OCSP Stapling (must staple the OCSP response to the TLS handshake) to reduce client-side revocation latency and privacy leaks.
- For high-security use, consider OCSP Must-Staple (a certificate extension) to enforce stapling.
2.6 TLS headers and related protections
- Enable HTTP Strict Transport Security (HSTS) with an appropriate max-age and preload if appropriate:
- Start with a short max-age for testing, then increase.
- Use Secure cookies, set SameSite where applicable.
- Set Referrer-Policy, Content-Security-Policy to limit other attack vectors (these are application-level but complement TLS).
2.7 HTTP/2 and ALPN
- Enable ALPN (Application-Layer Protocol Negotiation) to negotiate HTTP/2 and prevent fallback to older protocols. Ensure your TLS configuration supports the negotiated protocol’s requirements.
2.8 Perfect Forward Secrecy (PFS)
- Favor ECDHE for key exchange to ensure PFS. Test that session keys cannot be derived from the server’s long-term private key.
3. Client-side configuration checklist
3.1 TLS libraries and system updates
- Keep TLS libraries (OpenSSL, BoringSSL, NSS, SChannel, Secure Transport) up to date to receive security fixes and new protocol support.
- Disable acceptance of insecure TLS versions in clients. Configure clients to reject TLS 1.0/1.1.
3.2 Certificate validation and strict checks
- Enforce hostname verification and certificate chain validation strictly. Do not allow certificates with mismatched CN/SANs.
- Reject certificates with weak signature algorithms (e.g., SHA-1) or short keys.
3.3 Certificate pinning and trust on first use (TOFU)
- Consider certificate pinning (HTTP Public Key Pinning is deprecated — use pinning with care in applications) or Trust On First Use for specific high-value services. Implement pin validation updates and recovery mechanisms to avoid lockout.
3.4 OCSP and revocation checks
- Validate revocation status (OCSP/CRL) where feasible. Prefer mechanisms that minimize privacy exposure (OCSP stapling on server is best).
3.5 Client-side telemetry and privacy
- Avoid leaking unnecessary metadata during TLS handshakes (SNI can leak hostname — use Encrypted ClientHello (ECH) when supported). Keep client implementations updated to support ECH as it matures.
4. Certificates and PKI best practices
4.1 Use reputable CAs and short-lived certs
- Use trusted Certificate Authorities and consider short-lived certificates (e.g., 90 days) with automated renewal to limit key exposure.
4.2 Key management
- Generate keys with secure RNGs and protect private keys with HSMs or tightly controlled OS permissions. Use separate keys per service when practical.
- Rotate keys periodically and after any suspected compromise.
4.3 Certificate transparency and logging
- Ensure certificates are logged in Certificate Transparency (CT) logs; many browsers require CT for publicly trusted certificates.
4.4 Signature algorithms
- Use SHA-256 or stronger for certificate signatures; avoid SHA-1 and MD5.
5. Testing, verification, and monitoring
5.1 Automated scanning and continuous testing
- Use scanners (ssllabs.com-like tests, curl/wget with modern TLS options, or internal scanners) to verify configuration and grade security.
- Include TLS tests in CI/CD pipelines for services to catch regressions.
5.2 Monitoring and alerting
- Monitor certificate expiry and renewal status; alert well before expiry.
- Monitor for unusual TLS handshake failures, protocol downgrades, or large numbers of renegotiations.
5.3 Penetration testing and fuzzing
- Periodically perform penetration testing and fuzzing of TLS endpoints to detect unexpected behavior or logic flaws.
6. Example configurations
6.1 Nginx (concise example)
- TLS 1.⁄1.3 enabled, strong ciphers, HSTS, OCSP stapling. (Add your server-specific values for cert paths and keys.)
6.2 Apache
- Enable mod_ssl with modern ciphers, SSLProtocol set to TLSv1.2 TLSv1.3, configure SSLCipherSuite for ECDHE+AEAD, enable SSLStapling.
6.3 OpenSSL server (conceptual)
- Use recent OpenSSL version and configure cipher lists to prefer ECDHE and AEAD ciphers.
7. Common pitfalls and how to avoid them
- Serving incomplete certificate chains — always test with multiple clients.
- Relying on default settings of old software — default may be insecure; verify.
- Enabling obsolete ciphers for compatibility without compensating controls — prefer negotiation policies or version-specific endpoints.
- Forgetting to enable OCSP stapling — causes slow revocation checks or privacy leaks.
8. Quick checklist (actionable items)
- Disable SSLv2/SSLv3/TLS 1.0/1.1.
- Support TLS 1.2 and TLS 1.3 only; prefer TLS 1.3.
- Use ECDHE for key exchange; prefer AEAD ciphers (AES-GCM, ChaCha20-Poly1305).
- Use RSA ≥2048 or ECDSA P-⁄384; prefer ECDSA where appropriate.
- Serve full certificate chain; enable OCSP stapling and consider Must-Staple.
- Enable HSTS and ALPN; configure secure cookies and relevant headers.
- Keep TLS libraries updated and monitor certificates.
- Automate certificate renewal (ACME) and store private keys securely.
9. Resources for further reading
- RFCs: TLS 1.2 (RFC 5246), TLS 1.3 (RFC 8446).
- Practical guides and server-specific docs (OpenSSL, Nginx, Apache).
- Online scanners and testing tools for independent verification.
Harden TLS proactively and validate changes across a representative set of clients. Small misconfigurations can break compatibility or leave weak fallback paths—test thoroughly and automate where possible.
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