SteadyNode

Cloudflare · HTTP 520

Cloudflare Error 520: Unknown Origin Error

Trace Cloudflare error 520 to an origin that closes a connection or returns an empty or malformed HTTP response. Compare edge headers with origin logs and test the origin directly.

Origin / Application

Decode a CDN or gateway error

Paste a captured HTTP response or enter a status and headers. Only the status and response headers are sent; common credential headers are filtered first.

Supports raw headers, full HTTP responses, JSON with status/statusCode and headers, cURL -v/-i response output, and HAR exports. HAR selects a server error first. cURL commands are never executed; request headers, response bodies, and common credential headers are excluded.

Diagnostic report

Error code / name

CF-520

Fault domain

Origin / Application

Why it happened

Cloudflare connected to the origin, but the origin closed the connection or returned a response that Cloudflare could not parse. Oversized headers, application crashes, and intermediary resets can all produce an unexpected response.

Actionable fixes

  1. Match the CF-Ray value and timestamp to reverse-proxy and application logs.
  2. Check origin logs for worker crashes, connection resets, and malformed status lines.
  3. Compare response headers and body size from the origin with a direct request from an allowed network.

Why this error happened

Cloudflare connected to the origin, but the origin closed the connection or returned a response that Cloudflare could not parse. Oversized headers, application crashes, and intermediary resets can all produce an unexpected response.

Actionable fixes

  1. Match the CF-Ray value and timestamp to reverse-proxy and application logs.
  2. Check origin logs for worker crashes, connection resets, and malformed status lines.
  3. Compare response headers and body size from the origin with a direct request from an allowed network.

Network failure point

Client -> Cloudflare Edge -> TCP/TLS OK -> Origin sends empty or invalid HTTP response -> X

Verify origin directly via CLI

curl -sS -v -I --connect-timeout 5 --max-time 20 --resolve example.com:443:ORIGIN_IP https://example.com/health

Configuration fix

nginx.confnginx
location / {
    proxy_http_version 1.1;
    proxy_set_header Host $host;
    proxy_pass http://app;
}

Catch this failure before it becomes an outage

SteadyNode's multi-region HTTP checks record status and response headers, while Upstream Translator highlights unexpected origin responses before they become prolonged outages.

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Frequently asked questions

What does Cloudflare error 520 mean?

Cloudflare received an empty, malformed, or otherwise unexpected response from the origin.

Can a firewall cause a 520?

Yes. A firewall or proxy can reset an established connection; correlate the CF-Ray value with origin and network logs before changing rules.

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