What Is the 520 Status Code?

The 520 status code is classified as "Unknown Error" by Cloudflare. It means the proxy server connected to your origin server, but the origin web server returned an empty response, a malformed response, or an otherwise invalid http response that Cloudflare couldn't interpret. HTTP status code 520 is a proprietary error code used primarily by Cloudflare-it is not a standard http status code defined in any IETF specification.

Unlike a 404 (resource not found) or 500 (internal server error), the 520 error code signals a breakdown in proper communication between Cloudflare's reverse proxy and the origin. The error message users see-"Web server returned an unknown error"-is Cloudflare's catch all response for server replies it cannot parse.

Common symptoms include intermittent error pages, partial page loads, failed file uploads, and API requests returning an unknown error. For teams using web scrapers, crawlers, or platforms like Olostep, 520 errors indicate a failure in server communication with Cloudflare that warrants origin-side investigation.

How a 520 Error Happens in Practice

Picture this: a client sends a GET request to your domain. Cloudflare, acting as a reverse proxy, accepts the connection and forwards it to your origin server. The origin accepts the TCP connection but then crashes-perhaps an unhandled exception or an out-of-memory kill-and sends back nothing. Cloudflare receives that empty response and surfaces a 520 status code to the client's browser.

This pattern also appears on POST endpoints, large file uploads, and long-running API calls. A TCP timeout occurs if the origin server takes longer than 300 seconds to respond. Cloudflare checks for a valid HTTP status line and response headers; when the web server is returning an unexpected response-binary garbage, truncated headers, or no headers at all-it responds with "returning an unknown error." On high-traffic sites or automation-heavy workloads where thousands of concurrent requests hit the same server, these failures become especially visible.

What Is the 520 HTTP Status Code?

HTTP status code 520 indicates an unknown error from the server. Cloudflare returns a 520 error when it cannot interpret server responses under normal HTTP semantics. It is not part of the official IETF specification but appears in server logs, monitoring dashboards, and browser error pages as an http status value.

Key distinctions from related Cloudflare status codes:

Code	Meaning
520	TCP connection established, but server response is empty, malformed, or unexpected
521	Origin server refused the TCP connection entirely
522	TCP connection timed out before handshake completed
524	Origin accepted connection but took too long to respond
Web scraping frameworks and APM tools should treat 520 as a non-standard http status code and still classify it as a server error.

Typical Causes of a 520 Error Message

A 520 error usually stems from origin server bugs, network issues, or misconfiguration between Cloudflare and your origin. While the error shows up at the proxy, the root cause is almost always the origin web server or hosting provider configuration. Transient traffic spikes-from bots, AI agents, or scraping jobs-can expose latent potential issues that manifest as 520 status codes.

Empty Responses and Malformed HTTP from the Origin

520 errors can occur due to empty responses from the origin server. Concretely, Cloudflare opens a TCP connection but receives no HTTP status line, incomplete headers, or an abruptly closed connection. Common triggers include unhandled exceptions in PHP, Node.js, or Python apps, segmentation faults, and OOM kills. Server crashes can cause a 520 error due to application failures or timeouts. 520 errors often occur due to empty or malformed server responses-check your origin server's error logs around the exact timestamp of each incident.

Oversized or Invalid HTTP Headers

Cloudflare enforces strict limits on response headers. Cloudflare's limits for header sizes are usually 16 KB individually or 32 KB collectively. Excessive cookies, verbose debugging headers, or long authorization tokens can push past these thresholds and trigger a 520. Remove excessive headers to stay within Cloudflare's 16KB limit. Inspect response headers using cURL or browser developer tools to catch unusually large or duplicate headers before they cause problems.

Firewall, WAF, and Security Software Conflicts

Security plugins or firewalls can accidentally block Cloudflare IP addresses. Misconfigured firewalls can block Cloudflare IPs, causing 520 errors-this is one of the most common causes. Your firewall rules, mod_security rulesets, or third-party WAFs may drop connections or inject non-HTTP data into responses from requests originating from Cloudflare's network. Verify that your firewall settings allow all official Cloudflare IP ranges. Rate limiting that blocks too many requests from CDN ip addresses is another frequent culprit, especially when filtering malicious traffic aggressively.

DNS Records and Networking Issues

Incorrect or outdated dns records in the Cloudflare dashboard can point to the wrong origin IP, leading to invalid responses that surface as 520. Ensure your dns records point to the correct IP address by cross-checking A, AAAA, and CNAME entries against your hosting provider. Try setting the affected dns record temporarily to "DNS only" (grey cloud) to test the direct path. Stable DNS and network configuration are crucial for automation-heavy workloads like web crawling and data extraction pipelines.

Origin Server Overload and Resource Limits

Server overload occurs when the origin accepts connections but cannot generate valid responses due to CPU saturation, memory exhaustion, or exhausted worker pools. Typical sources include batch ETL jobs, aggressive bots, and full-table database scans. Monitor system metrics and correlate resource spikes with 520 events. Misconfigured servers can trigger 520 errors affecting SEO and server performance alike. Long-running endpoints like report generation or large export downloads are particularly vulnerable.

TLS, HTTP/2, and Browser-Specific Edge Cases

Protocol mismatches-especially involving HTTP/2 to origin-can cause Cloudflare to receive invalid frames. Disable HTTP/2 if your origin server doesn't support it properly. Documented issues exist where specific browser or VPN users trigger 520 due to TLS extension mishandling. Check SSL/TLS certificates for validity and chain correctness. For API-heavy workloads, prefer stable server settings like HTTP/1.1 to origin until your server configuration is proven robust under load.

How to Fix an HTTP 520 Error Step by Step

Standard troubleshooting includes checking origin server logs and verifying firewall settings. Start from origin health, then DNS and networking, then Cloudflare-specific toggles. Teams running scrapers or AI agents should also add resilience-implementing retry logic and backoff-on the client side.

1. Check Your Origin Server and Application Logs First

Check your origin server's error logs for issues. SSH into your host, confirm that web server processes (Nginx, Apache, PHP-FPM) are running, and review recent logs for fatal exceptions or OOM kills matching 520 timestamps. Use curl directly against the origin IP to verify it returns a clean server response.

2. Validate DNS Records and Hosting Provider Settings

Confirm Cloudflare DNS entries match your hosting provider's control panel. Set the affected dns record to "DNS only" and re-test from a browser to isolate whether Cloudflare is part of the failure path. Mismatches can route traffic to staging or obsolete servers returning an invalid response.

3. Review Firewalls, WAF Rules, and Cloudflare IP Allowlists

Whitelist Cloudflare IP ranges in your firewall settings across OS-level firewalls (iptables, ufw) and any WAFs. Inspect security tool logs for blocked requests matching Cloudflare's user agents. Temporarily disable aggressive rulesets to see if the frequency of 520 errors drops. Balance security with access for legitimate CDN traffic and web data pipelines.

4. Tune Cloudflare Features and Protocol Settings

In the Cloudflare dashboard, inspect HTTP/2 to origin, TLS mode, cloudflare cache rules, and Rocket Loader. Disable HTTP/2 to origin if 520 errors cluster around specific client types. Review Page Rules or Transform Rules that add headers. Use CF-RAY IDs from error pages to correlate problematic requests with particular zones or paths. Test changes on a staging domain first.

5. Collaborate with Your Hosting Provider and Cloudflare Support

Persistent 520 errors warrant escalation. Provide your hosting provider with timestamps, origin logs, and traffic levels. Give Cloudflare support the affected domain, exact URL paths, CF-RAY header values, and results of /cdn-cgi/trace. Well-documented incident reports help build runbooks for future events.

520 Errors, Automation, and Web Data Workflows (Olostep's Perspective)

Platforms like Olostep's Web Data API sit upstream of cloudflare protected sites and must gracefully handle 520 status codes as transient infrastructure failures. Robust scraping logic should distinguish between 4xx client errors, 5xx server errors, and special cases like 520 to decide when to retry or mark a URL as unhealthy.

Designing Resilient Clients for 520 and Other Transient Errors

Engineering teams building scrapers or AI research agents should implement retry logic with exponential backoff for 520 errors. Cap retries per URL and log all occurrences with correlation IDs. Treat 520 differently from permanent failures like 404-valuable pages should not be removed from crawl queues. Olostep's Web Data API incorporates these patterns by default.

How Olostep Helps You Work Around Origin Instability

Olostep manages connection pools, timeouts, and region selection to reduce the impact of intermittent 520 errors. It can provide structured JSON or Markdown output even when target websites intermittently return empty or malformed HTML, by consolidating multiple attempts. For customers operating at scale-e-commerce monitoring, healthcare content enrichment, AI visibility-Olostep surfaces repeated 520 patterns to help target remediation at specific domains. Usage-based pricing makes it straightforward for startups to experiment without major upfront infrastructure investment.

Preventing Future 520 Status Codes

Prevention involves stable server configuration, proactive monitoring, sane security rules, and capacity planning. This is especially important for businesses whose APIs or AI pipelines must deliver near-real-time data without gaps. 520 errors can negatively impact website SEO and user experience. Search engines expect standard HTTP status codes for indexing, and 520 errors can prevent search engines from indexing pages or lead to delayed indexing of website content. 520 errors can diminish user experience and SEO performance over time.

Harden and Monitor the Origin Server

Keep your web server and application runtimes up to date. Implement real-time monitoring for 5xx errors via tools like Prometheus or Datadog. Set alerts for CPU, memory, and connection thresholds. Run regular load tests simulating realistic traffic from browsers, APIs, and web scraping agents to ensure proper communication under pressure.

Maintain Clean DNS and Networking Practices

Periodically audit dns records to remove deprecated hosts and legacy ip addresses. Document which domains point to which origin servers. Implement health checks and failover mechanisms so that if one origin becomes unstable, traffic can reroute to a healthy backup. Clean DNS practices are critical when running both human-facing websites and machine-driven workloads off the same network infrastructure.

Align Security and Performance Goals

Strong security must be balanced against serving valid HTTP responses to legitimate clients and CDNs. Collaborate between security, DevOps, and product teams to classify good automated traffic-monitoring tools, partner APIs, Olostep-based scrapers-and avoid blocking it. Review firewall and WAF rulesets periodically. Use Cloudflare features like Bot Management judiciously, starting with monitor-only modes. A well-tuned security posture reduces both attack surface and random 520 errors from overzealous response body modification.

Key Takeaways on the 520 Error Code

The 520 error code is a Cloudflare-specific signal that the origin server returns an unknown, empty, or invalid HTTP response-almost always an origin-side issue.
Consistent 520 status codes warrant investigation into application crashes, resource limits, DNS and firewall configuration, and Cloudflare protocol settings.
Cloudflare returns a 520 error for unknown server responses, making the troubleshooting process a matter of working backward from the proxy to the origin.
Robust client-side handling-especially for AI agents, web scrapers, and data pipelines-should gracefully manage transient 520 errors with retries and backoff.
Olostep offers a practical way for teams to offload the complexity of dealing with flaky origins and non-standard http status codes while obtaining reliable, structured web data.

Integrate monitoring, logging, and collaborative workflows with your hosting provider and Cloudflare so that 520 incidents become rare, brief, and well-understood. If you're building data pipelines that need to work reliably across thousands of domains, consider letting Olostep handle the infrastructure complexity so your team can focus on what the data actually means.