SSL_ERROR_PROTOCOL_VERSION_ALERT: Why the Server Refuses Your TLS Version

SSL_ERROR_PROTOCOL_VERSION_ALERT is a Firefox (NSS) error. The “Secure Connection Failed” page adds NSS's description: “Peer reports incompatible or unsupported protocol version.” It means the server sent the TLS alert protocol_version (number 70) in reply to Firefox's ClientHello: none of the TLS versions Firefox offered is acceptable to it, so it aborted the handshake before any certificate was exchanged.

The direction matters. If the server only speaks an old version and says so, Firefox itself rejects it with SSL_ERROR_UNSUPPORTED_VERSION. This alert is the opposite case: the server, or something in front of it, wants a version the client did not offer, or it cannot parse a modern ClientHello. Chrome reports the same alert as ERR_SSL_VERSION_OR_CIPHER_MISMATCH.

• Firefox's own version range was narrowed. Defaults are security.tls.version.min = 3 (TLS 1.2) and security.tls.version.max = 4 (TLS 1.3). A lowered maximum, often left over from old troubleshooting advice or set by an enterprise policy (SSLVersionMax), makes Firefox offer only TLS 1.2, and a TLS 1.3-only server answers with alert 70.
• TLS 1.3-only server. ssl_protocols TLSv1.3; in nginx, a TLS 1.3-only security policy on a cloud load balancer, or a CDN minimum of 1.3 rejects every client that tops out at TLS 1.2: old browsers, embedded clients and many intercepting proxies.
• Version-intolerant server or middlebox. Some old appliances cannot parse a TLS 1.3-style ClientHello and reply with protocol_version instead of negotiating TLS 1.2.
• TLS interception on your side. Antivirus HTTPS scanning or a corporate proxy re-creates the connection with its own TLS library; if that library is older than the server's policy, the server's alert reaches you through it.

Safari and Chrome do not show the Firefox code, but the server-side causes and fixes are identical.

The alert comes from whichever box terminates TLS for the browser. With a CDN or load balancer in front, check its version policy, not only the origin's.

• Cloudflare: SSL/TLS > Edge Certificates > Minimum TLS Version. At 1.3, clients limited to TLS 1.2 are refused. 1.2 is the sensible floor for public sites.
• AWS, Azure and GCP load balancers: the attached TLS or security policy decides the versions; pick one that allows TLS 1.2 and 1.3.
• nginx: since 1.23.4 the default already includes TLSv1.2 and TLSv1.3, so remove hand-written TLS 1.3-only lines unless every client is known to support 1.3.

# /etc/nginx/conf.d/tls.conf
ssl_protocols TLSv1.2 TLSv1.3;

# Apache 2.4 (mod_ssl)
SSLProtocol -all +TLSv1.2 +TLSv1.3

1. Find which versions the server accepts:

   openssl s_client -connect example.com:443 -servername example.com -tls1_2 -brief </dev/null
   # ... tlsv1 alert protocol version ... SSL alert number 70  -> TLS 1.2 refused
   openssl s_client -connect example.com:443 -servername example.com -tls1_3 -brief </dev/null

2. Cross-check with curl: curl -sv --tls-max 1.2 https://example.com/ -o /dev/null reproduces what a TLS 1.2-only client experiences.
3. Enable TLS 1.2 and 1.3 together on the TLS terminator (config above), reload with nginx -t && systemctl reload nginx or apachectl configtest && systemctl reload apache2.
4. Old appliance in front? If a firewall, WAF or load balancer replies with alert 70 to a modern ClientHello, update its firmware or move TLS termination to a current reverse proxy.
5. Check every hostname and IP, including IPv6: different virtual hosts or nodes behind a balancer can run different policies.

• Open about:config, search security.tls.version, and reset any modified value (Firefox shows changed prefs in bold) to its default.
• Check about:policies: if your organization pins SSLVersionMax, ask IT, as you cannot override it.
• Update Firefox; very old builds do not offer TLS 1.3.
• Temporarily turn off antivirus HTTPS scanning, the VPN or the proxy and retry; try mobile data to rule out the network.

Clearing the cache or cookies changes nothing here, and lowering security.tls.version.min does not help either: the server is asking for a newer version, not an older one. If every browser fails on every network, only the site owner can fix it.

openssl s_client -connect example.com:443 -servername example.com -tls1_2 -brief </dev/null
openssl s_client -connect example.com:443 -servername example.com -tls1_3 -brief </dev/null
# both should print: CONNECTION ESTABLISHED

Then reload the page in Firefox with default TLS prefs. The SSL/TLS checker lists which protocol versions the server accepts, as seen from outside your network.

• SSL_ERROR_NO_CYPHER_OVERLAP — versions match, ciphers do not
• ERR_SSL_PROTOCOL_ERROR — Chrome, reply was not valid TLS
• SSL handshake failed — the general handshake failure
• TLS 1.3 — what changed and why servers enable it