In recent years, supporting “Always-on SSL/TLS（https）” encryption has become the standard for websites.
However, in reality, many sites are still operating via unencrypted “http”.
We still frequently encounter browser security warnings, not only on personal blogs or small websites, but even on corporate websites.
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SSL/TLS certificates are a critical mechanism for safeguarding website security.
They encrypt the internet connection between the server and the user, preventing data leakage and tampering. At the same time, they verify the legitimacy of the website, proving that it is authentic and safe to use.
| Main Role | Key Benefits |
| Data Encryption | It encrypts data exchanged between the user and the website, such as credit card information, passwords, and personal details. This effectively prevents third parties from eavesdropping on or tampering with the data. |
| Identity Verification | The SSL/TLS Certificate Authority （CA） verifies the website operator to confirm that the site is legitimate and authentic. This helps users easily distinguish your site from malicious phishing sites. |
| Enhanced Trust | The “padlock icon” and “https://” displayed in the browser address bar act as proof that the SSL/TLS certificate is correctly configured. This allows visitors to use the website with peace of mind. |
| Benefits of Enabling SSL/TLS（https） | Risks of Ignoring SSL/TLS （http） | ||
| Protects Data | Secures customer information and transaction data through encryption, ensuring privacy and safety. | Security Risks | High risk of data breaches and leaks. Browsers will display “Not Secure” warnings, alerting users to the lack of encryption. |
| Builds Trust | Establishes a professional and secure corporate image, allowing visitors to use the site with confidence. | Brand Damage & Sales Loss | The site may be completely blocked by browsers. Security warnings damage the brand image, leading to lost business opportunities and a drop in sales. |
| SEO Boost | https is officially recognized as a ranking signal by search engines like Google, helping to improve visibility. | Search Engine Penalties | Risk of suffering from SEO penalties, causing rankings to plummet. In severe cases, the domain may be blacklisted. |
Deploying an SSL/TLS certificate is the foundation of website security and trust.
However, simply installing a certificate does not guarantee that all issues will be resolved.
Even after configuration, browser warnings or connection errors may still occur due to various factors.
| Category | Specific Cause | Details & Notes |
| Certificate Issues | Expired Certificate | One of the most common causes. Certificates currently require annual renewal.
* Validity periods will be shortened in the future: |
| Domain Mismatch | The domain name registered on the certificate does not match the URL the user is currently accessing. | |
| Untrusted Issuer | The site uses a self-signed certificate or a certificate issued by a Certificate Authority（CA） that the browser does not trust. | |
| Server Configuration | Outdated Protocols / Weak Ciphers | The server only supports old versions like TLS 1.0 or 1.1. Modern browsers treat these as security risks and block the connection. |
| Missing Intermediate Certificate | The necessary intermediate certificates are not installed on the server (incomplete installation), breaking the “chain of trust.” | |
| SNI（Server Name Indication）Incompatibility | Issues occurring on older servers or specific environments when hosting multiple “https” domains on a single server IP. | |
| Source Code | Mixed Content | The website contains code with “http” links. If images, JavaScript, or CSS are loaded via “http” on an https page, browsers will block them. |
| Client / Environment | Outdated Browser | The user’s browser version is too old to support modern certificate standards or encryption methods. |
| Firewall / Proxy Interference | Corporate networks performing traffic inspection （MITM） may substitute their own certificates, causing browser errors. |
When a user visits a website that has SSL/TLS issues, their browser will display error messages like the ones shown below.
These are not just simple “errors” . they are vital clues that specifically indicate security risks or connection failures.
Here, we introduce some of the most common error messages you might encounter due to SSL/TLS configuration problems.
| Browser Error Message Example | Representative Error Code | Probable Causes |
| “Not Secure” “Your connection is not private” |
ERR_CERT_INVALID ERR_CERT_AUTHORITY_INVALID SSL_ERROR_BAD_CERT_DOMAIN |
・Certificate has expired.
・Domain mismatch （The certificate is for a different URL）. ・Using a Self-signed certificate. ・The issuer（CA） is not trusted by the browser. |
| “This site can’t provide a secure connection” | ERR_SSL_VERSION_OR_CIPHER_MISMATCH SSL_ERROR_NO_CYPHER_OVERLAP |
・The server’s protocols/ciphers do not match the client browser’s supported versions (e.g., server only supports old TLS).
・Server configuration errors. |
| “Connection is not secure” “Your connection is not private” |
ERR_CERT_COMMON_NAME_INVALID ERR_CERT_DATE_INVALID |
・Certificate has expired.
・Common Name (CN) mismatch (The certificate’s CN does not match the accessed domain). |
| “This site can’t be reached” | ERR_CONNECTION_CLOSED ERR_CONNECTION_REFUSED ERR_CONNECTION_TIMED_OUT ERR_NAME_NOT_RESOLVED |
・Server is down.
・Network or firewall configuration issues. ・DNS resolution failed. ・Server SSL/TLS settings are too old and rejected by the client’s security policy. |
| Red padlock icon or Strikethrough on padlock |
Visual Warning in Browser | ・The site source code contains “Mixed Content” （loading HTTP resources like images/scripts on an HTTPS page）. |
If an SSL/TLS error occurs, immediate action is required to ensure a secure browsing environment for your users.
Below, we outline effective troubleshooting steps and solutions that website administrators can implement themselves.
Simply by following these basic maintenance practices, you can resolve and prevent the majority of SSL/TLS issues.
SSL/TLS is the indispensable foundation of security in the modern internet.
Therefore, it is no longer a question of “whether or not to deploy” a certificate; rather, it is a mandatory requirement serving as the minimum security standard.
For both users and website administrators, understanding this mechanism is the first step toward a safer internet experience.
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