ERR_SSL_PROTOCOL_ERROR 🟨 JavaScript

ERR_SSL_PROTOCOL_ERROR (This site can't provide a secure connection)

The browser couldn’t negotiate TLS — the server sent an invalid response, often plain HTTP on the HTTPS port.

Seen on: JavaScript

Meaning

Common when a server listens on 443 without TLS configured, when TLS versions/ciphers don’t overlap, or when a proxy interferes.

Common causes

  • HTTPS URL pointing at a plain-HTTP port
  • TLS not configured on the server
  • Outdated TLS versions only
  • Antivirus/proxy interference

⚡ Quick fix

  1. Check that the server has a certificate and TLS on 443
  2. Use http:// for local dev servers that don’t do TLS
  3. Enable TLS 1.2/1.3

Detailed fix by platform

Linux

  1. openssl s_client -connect example.com:443 -servername example.com

How to diagnose

  1. Server — TLS configured on that port?
  2. URL — Using https on a dev server?

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