Cloudflare error pages · error message

Cloudflare: Error 520: Web server returns an unknown error

RetryableHTTP 520Origin server errors (520-524)

Last verified

The exact message
Error 520: Web server is returning an unknown error

What it means

520 is a Cloudflare-specific status. Cloudflare connected to the site's origin server, but the origin returned something Cloudflare could not use: an empty or malformed response, a response without a status line, or a connection that was dropped. It is a catch-all for unexpected origin behavior.

For visitors and automation, 520 is not about you. Nothing in your request is blocked; the site's own server misbehaved. Retrying later is the only client-side action.

For the site owner, Cloudflare lists causes including origin crashes or misconfigurations, response headers over 128 KB (often from excessive cookies), empty or malformed responses, incorrect HTTP/2 configuration at the origin, and Authenticated Origin Pulls enabled on Cloudflare while the origin is not configured for it.

What it looks like

HTTP/1.1 520
Server: cloudflare

Error 520 ... Web server is returning an unknown error ... Ray ID: 8a1b2c3d4e5f6789

Why it happens

  • The origin application crashed or reset the connection (common with some PHP applications).
  • Response headers exceed 128 KB, often because of large or numerous cookies.
  • The origin sent an empty or malformed response with no status code.
  • The origin advertises HTTP/2 but does not support it properly.
  • Authenticated Origin Pulls is enabled but the origin rejects Cloudflare's client certificate.

How to fix it

  1. Visitors and automation: retry later with backoff; report persistent errors to the site with the Ray ID.
  2. Site owners: check origin error logs for crashes at the timestamps of the errors.
  3. Measure response header size and reduce cookies if it approaches 128 KB.
  4. Disable HTTP/2 to Origin under Speed, Settings, Protocol Optimization to test whether HTTP/2 is the cause.
  5. As a temporary diagnostic, set the DNS record to DNS-only; if the problem persists, collect the Ray ID, /cdn-cgi/trace output and HAR files for Cloudflare support.

How to stop it recurring

Site owners: monitor origin crash rates, keep cookies small, test HTTP/2 support before enabling it, and alert on 5xx rates in Cloudflare's analytics. Integrators: treat 520 as transient with capped retries.

Official documentation: developers.cloudflare.com/support/troubleshooting/http-status-codes/cloudflare-5xx-errors/error-520. See all Cloudflare error messages.

Related errors

Definitions

Quick answers

What does "Error 520: Web server returns an unknown error" mean in Cloudflare?

520 is a Cloudflare-specific status. Cloudflare connected to the site's origin server, but the origin returned something Cloudflare could not use: an empty or malformed response, a response without a status line, or a connection that was dropped. It is a catch-all for unexpected origin behavior. For visitors and automation, 520 is not about you. Nothing in your request is blocked; the site's own server misbehaved.

How do I fix "Error 520: Web server returns an unknown error"?

1. Visitors and automation: retry later with backoff; report persistent errors to the site with the Ray ID. 2. Site owners: check origin error logs for crashes at the timestamps of the errors. 3. Measure response header size and reduce cookies if it approaches 128 KB. 4. Disable HTTP/2 to Origin under Speed, Settings, Protocol Optimization to test whether HTTP/2 is the cause. 5. As a temporary diagnostic, set the DNS record to DNS-only; if the problem persists, collect the Ray ID, /cdn-cgi/trace output and HAR files for Cloudflare support.

Retries and timeouts without writing the loop

Autonoly's HTTP Request node and browser steps have per-node retries, timeouts and continue-on-fail, so a transient 429 or 503 can back off instead of failing the run. It has no proxies or IP rotation: if a site blocks automated traffic, that block applies to Autonoly too.

Also see Error handling in workflows.