The most common TLS and SSL configuration errors found across the web — ranked by frequency, with prevalence data and actionable fixes.
Legacy TLS versions with known vulnerabilities (POODLE, BEAST) remain enabled on server. Modern browsers now block these but internal tools may still use them.
Disable TLS 1.0 and 1.1 in your server config. For Nginx: `ssl_protocols TLSv1.2 TLSv1.3;` — for Apache: `SSLProtocol -all +TLSv1.2 +TLSv1.3`.
Certificate has expired or will expire within 7 days. Browsers display hard-stop warnings preventing users from proceeding. Causes immediate revenue loss.
Enable auto-renewal via Let's Encrypt Certbot, AWS ACM, or your CA's ACME integration. Set 30/14/7-day expiry alerts with CertNotify.
Intermediate certificate missing from server response. Desktop browsers often cache intermediates, but mobile and API clients frequently fail — causing intermittent errors.
Concatenate your domain cert + intermediate cert(s) in the correct order in your PEM file. Test with: `openssl s_client -connect domain.com:443 -showcerts`.
Server supports export-grade, RC4, 3DES, or NULL cipher suites which are cryptographically broken. Attackers can downgrade connections to these weak ciphers.
Use Mozilla SSL Config Generator to get a recommended cipher list for your server. Remove any RC4, DES, EXPORT, and NULL ciphers explicitly.
HTTP Strict Transport Security is not set. Without it, first-visit HTTP requests are vulnerable to SSL stripping attacks, where attackers intercept plaintext connections.
Add: `Strict-Transport-Security: max-age=31536000; includeSubDomains; preload` to your server response headers. Then submit to the HSTS preload list.
Self-signed certificate is not trusted by browsers or operating systems. All users see certificate warnings. Legitimate services should never use self-signed certs.
Replace with a certificate from a trusted CA. Let's Encrypt provides free, auto-renewing certificates via Certbot or ACME protocol.
Certificate is valid but does not cover the requested hostname. The SAN (Subject Alternative Name) list does not include the accessed subdomain or domain variant.
Reissue certificate to include all required domains in the SAN list. Wildcard certs (*.domain.com) cover all direct subdomains but not sub-subdomains.
Server does not support cipher suites that provide Perfect Forward Secrecy (ECDHE, DHE). Recorded traffic could be decrypted if the server private key is later compromised.
Prioritise ECDHE cipher suites in your server configuration. Ensure DHE key size is at least 2048 bits if DHE is used.
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