TLS/SSL Security: What to Check and Why

TLS is the encryption layer that protects every HTTPS connection. But not all TLS is equal — old versions have known breaks, weak cipher suites can be downgraded or cracked, and an expired certificate kills user trust instantly. Here's what matters and how to verify it.

TLS vs SSL: What's the Difference?

SSL (Secure Sockets Layer) is the predecessor to TLS (Transport Layer Security). SSL 2.0 and 3.0 are both fully broken and have been deprecated for over a decade. TLS 1.0 and TLS 1.1 followed and are now also deprecated. When people say "SSL" colloquially today, they almost always mean TLS 1.2 or TLS 1.3. Your server shouldn't support any version called "SSL" at all.

TLS Versions: What's Safe

Nginx — disable old TLS

ssl_protocols TLSv1.2 TLSv1.3;

Apache — disable old TLS

SSLProtocol -all +TLSv1.2 +TLSv1.3

Cipher Suites

A cipher suite is a combination of algorithms used to encrypt the TLS connection: key exchange, authentication, bulk encryption, and message authentication. Not all cipher suites are equal — some use known-weak algorithms that can be broken with enough compute.

What to avoid

• NULL ciphers — no encryption at all. Authentication only.
• EXPORT ciphers — deliberately weakened 40-bit encryption, legacy of US export controls. Exploited by FREAK and LOGJAM attacks.
• RC4 — stream cipher with statistical biases that make it breakable. Disabled in all modern browsers.
• DES / 3DES — 56-bit and 112-bit keys. Vulnerable to SWEET32 birthday attacks in 3DES with long sessions.
• Anonymous key exchange (aNULL) — no server authentication, trivially MitM'd.

Recommended cipher string for Nginx (TLS 1.2 + 1.3)

ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256;
ssl_prefer_server_ciphers off;

TLS 1.3 has its own fixed cipher list and doesn't use the ssl_ciphers directive — the weak options are removed from the spec entirely, which is one of TLS 1.3's major improvements.

Certificate Expiry

An expired TLS certificate causes browsers to show a hard security warning to all visitors — and most users will leave rather than click through. Certificate expiry is entirely preventable with monitoring and automated renewal.

• Let's Encrypt certificates expire every 90 days. Use certbot's cron job or a hosting provider that auto-renews to avoid surprises.
• DV, OV, EV certificates from commercial CAs now max out at 398 days (about 13 months). Plan renewals well before expiry.
• Set up monitoring: tools like UptimeRobot, Checkly, and WebAudit's own scanner alert you when expiry is within 30 days.
• WebAudit deducts 5 points when expiry is within 30 days and 15 points for an already-expired certificate.

Self-Signed Certificates

A self-signed certificate is one where the certificate issuer is the same as the subject — the server signed its own certificate rather than having it signed by a trusted Certificate Authority (CA). Browsers don't trust self-signed certificates because there's no third party vouching that you are who you claim to be.

Self-signed certificates are fine for local development and internal tools where you can distribute and trust the certificate manually. For any public-facing site, use a certificate from a trusted CA. Let's Encrypt provides free DV certificates for public domains — there's no excuse for self-signed certificates on production web servers.

The Connection to HSTS

TLS and HSTS are complementary. TLS encrypts the connection; HSTS ensures the browser always uses TLS. A strong TLS configuration without HSTS still leaves the first connection vulnerable to SSL stripping. See the HSTS guide for how to lock down both sides of the equation.

Quick Checklist

• Disable SSL 2.0, SSL 3.0, TLS 1.0, TLS 1.1
• Enable TLS 1.2 and TLS 1.3
• Remove NULL, EXPORT, RC4, DES, 3DES, aNULL cipher suites
• Use ECDHE key exchange (supports Perfect Forward Secrecy)
• Certificate from a trusted CA, not self-signed
• Certificate expiry > 30 days; set up automated renewal
• Deploy HSTS after confirming TLS is correct on all subdomains