Error Code 520: What It Means, Why It Happens, and How to Fix It Quickly

Few things frustrate users more than encountering an unexpected error while trying to access a website or API. Among the most confusing is Error Code 520, a message that looks technical but provides almost no clear explanation. If you operate a web application, use API services, or run automated scraping systems, you’ve probably encountered this error more often than you’d like.

Error 520 is common across modern networks—especially for websites protected by Cloudflare. While it appears vague at first glance, it follows predictable patterns that you can diagnose and resolve with the right approach. In this article, we break down what Error Code 520 means, why it’s triggered, and the proven steps to fix or bypass it.

What Is Error Code 520?

Error Code 520 is a server-side response generated when Cloudflare receives an unexpected or invalid response from the origin server.

In simpler terms:

Cloudflare contacted your web server, but your web server returned something malformed, empty, or broken.

Cloudflare then interprets this as an “unknown error” and returns a 520 response.

Because 520 is not an official HTTP status code, it is unique to Cloudflare’s reverse proxy infrastructure.

When Does Error 520 Typically Occur?

Error 520 usually happens when Cloudflare acts as a gateway between the visitor (or scraper) and the origin server. If anything interrupts the expected request-response handshake, Cloudflare fails the request.

Common Triggers Include:

1. Connection Resets

The origin server closes the connection prematurely.

2. Empty or Corrupted Response Headers

Cloudflare expects valid HTTP headers. If the server responds with:

missing headers
malformed headers
oversized headers

Cloudflare rejects the response.

3. Origin Server Crashes or Hangs

High CPU usage, memory exhaustion, or misconfigured services can stall responses.

4. Firewall or Security Rules Blocking the Request

Security tools such as:

mod_security
CSF
Fail2Ban

may terminate requests before completion.

5. Rate Limiting

If the server blocks rapid or automated requests, Cloudflare receives no valid response.

6. Cloudflare-Specific Issues

In rare cases, Cloudflare itself may experience issues, causing intermittent 520 errors across multiple sites.

How Error Code 520 Appears to Users

Visitors may see:

“Error 520: Web Server Returned an Unknown Error”
“Host Error”
A Cloudflare-branded error page
Intermittent loading failures
Partial page loads followed by error screens

To the end user, the error often feels random and unpredictable.

How Developers Usually Diagnose Error Code 520

Because the issue occurs between Cloudflare and the origin server, debugging requires checking both sides.

1. Review Origin Server Logs

Inspect:

NGINX or Apache logs
PHP-FPM error logs
Application-level logs
System resource metrics

Look for timeouts, crashes, or 500-level errors.

2. Analyze Cloudflare Firewall Events

Cloudflare’s dashboard reveals:

blocked requests
rate-limiting events
WAF triggers
bot protection flags

If Cloudflare blocks the request, the issue may not originate from your server.

3. Disable Firewall Rules Temporarily

Overly aggressive security rules can kill requests before Cloudflare receives a complete response.

4. Check Server Health

Evaluate:

RAM usage
CPU spikes
disk I/O
network constraints

Overloaded servers often drop connections unexpectedly.

5. Test Without Cloudflare

Point your hosts file directly to the origin server. If the error disappears, Cloudflare is acting as the mediator of the issue.

What Causes Error 520 for Automated Bots or Scrapers?

If you run automated tools—such as Python scripts, Playwright, Puppeteer, or API scrapers—you’ll encounter 520 errors more frequently.

This happens because automated traffic often triggers:

WAF protections
bot detection systems
rate limits
connection throttling

In these cases, Error 520 acts as a defensive response, not a server malfunction.

Preventing Error 520 in the Future

The most reliable long-term mitigation strategies include:

enabling Keep-Alive connections
optimizing backend performance
using stable upstream infrastructure
avoiding unnecessarily heavy cookies
enabling caching layers
configuring CDN-friendly server settings

Teams that proactively optimize their backend systems experience far fewer unexplained 520 errors.

Conclusion

Error Code 520 is one of the most confusing connectivity issues because it occurs between Cloudflare and your origin server. While it may seem ambiguous, the underlying causes usually involve malformed responses, server resource limitations, or automated traffic being blocked.

For teams that rely on automated scraping, data pipelines, or API-driven workflows, 520 errors can disrupt entire systems. This is why many developers turn to dedicated scraping infrastructure like MrScraper, which provides proxy rotation, robust anti-bot handling, and stable browser-based scraping—significantly reducing Cloudflare-related failures.