HTTP 425 Too Early

HTTP 425 is Too Early: the server is unwilling to risk processing a request that might be replayed (RFC 8470 section 5.2). It exists for TLS 1.3 early data, also called 0-RTT. When a client resumes an earlier session, it can send its first request before the handshake finishes and save a round trip, but anyone who recorded that early data can send it again, and the server can’t always tell the copy from the original.

How the pieces fit

• The client that sends a request in early data must be ready for a 425. RFC 8470 says a user agent should retry automatically, and never in early data again; Firefox does.
• A TLS-terminating proxy that forwards a request before its own handshake completes must add Early-Data: 1, so the origin knows the request is replayable.
• The origin must answer 425 to a request marked Early-Data: 1 that it can’t safely process. It shouldn’t send 425 otherwise, since a client that didn’t use early data may not know what to do with it.
• Proxies in between must pass a 425 back when the request they forwarded carried Early-Data.

POST /api/transfers HTTP/1.1
Host: bank.example
Early-Data: 1
Content-Type: application/json

HTTP/1.1 425 Too Early

The client sends the request again once the handshake is done, and the proxy forwards it without Early-Data.

Which requests to let through

Replays hurt only requests with side effects. A common rule is to accept GET, HEAD and OPTIONS in early data and answer 425 to everything else. That only holds if your GETs really are safe: a GET /unsubscribe?id=... that changes state shouldn’t run from early data either.

nginx

nginx’s documentation pairs ssl_early_data with forwarding the $ssl_early_data variable, which is 1 while the handshake is incomplete:

server {
    listen 443 ssl;
    ssl_protocols TLSv1.3;
    ssl_early_data on;

    location / {
        proxy_pass http://app;
        proxy_set_header Early-Data $ssl_early_data;
    }
}

The app then refuses what it won’t risk:

app.use((req, res, next) => {                    // Express
  if (req.get('Early-Data') === '1' && !['GET', 'HEAD', 'OPTIONS'].includes(req.method)) {
    return res.status(425).end();
  }
  next();
});

Gotchas

• HTTP/3 too. QUIC has 0-RTT, and RFC 9114 requires RFC 8470’s replay protections for HTTP/3.
• Not a rate limit or an outage. That’s 429 or 503; a 425 is about one request’s timing.
• Caching. RFC 8470 says a 425 isn’t cacheable by default, and its body doesn’t represent any resource.

Search 425 429 in the tool below to compare it with Too Many Requests.