What Exactly Is Error 520?

Error 520, also known as the "Web Server Returned an Unknown Error" message, is a server-side HTTP status code specific to Cloudflare’s content delivery network (CDN) and security platform. Unlike generic 5xx server errors that are sent directly from origin servers, Error 520 is generated exclusively by Cloudflare when it is unable to receive a valid, standards-compliant HTTP response from the origin server that hosts the website you are trying to access. This error falls into the 5xx category of HTTP status codes, which all indicate issues related to server-side operations rather than user-side client errors (which fall into the 4xx category, such as 404 Not Found or 403 Forbidden).

To understand Error 520 clearly, it is helpful to distinguish it from other common Cloudflare-specific and generic 5xx errors. For example, Error 502 Bad Gateway occurs when Cloudflare receives an invalid response from the origin server that is still recognizable as an HTTP response, while Error 503 Service Unavailable indicates the origin server is intentionally offline or overloaded. Error 504 Gateway Timeout happens when Cloudflare waits too long for a response from the origin server and cuts the connection. Error 520, by contrast, is triggered when the response from the origin server is either completely empty, malformed to the point that it cannot be recognized as an HTTP response, or violates Cloudflare’s platform limits in a way that cannot be mapped to a more specific status code.

Error 520 can create significant disruptions for both end users and website owners. For regular visitors, the error blocks access to the content, services, or products they are trying to reach, forcing them to either abandon the site or spend time troubleshooting. For website owners and administrators, frequent Error 520 occurrences can lead to sharp drops in organic traffic, reduced conversion rates, damage to brand reputation, and even negative impacts on search engine rankings, as search engine crawlers will treat repeated 5xx errors as a sign of an unreliable site. It is not uncommon for e-commerce sites to lose thousands of dollars in revenue during extended Error 520 outages, especially during peak traffic periods such as holiday sales or product launches.

Error 520 can appear in a wide range of use cases. You may encounter it when trying to access a small business website, a SaaS platform, a media streaming site, or even a personal blog. In some cases, the error will appear for all visitors to the site, indicating a widespread issue with the origin server or its configuration. In other cases, the error will only appear for visitors in specific geographic regions, or for visitors using certain ISPs, indicating a regional routing issue, geographic access restriction, or IP-based block that is interfering with Cloudflare’s ability to communicate with the origin server. For users who frequently access content hosted in other countries, region-specific Error 520 messages are particularly common, as many website owners implement geographic IP blocks to restrict access to content for licensing, security, or compliance reasons. If you regularly encounter region-locked Error 520 messages, using a reliable proxy service to switch your IP address to a region that is allowed to access the site can resolve the issue quickly.

Common Root Causes of Error 520

To resolve Error 520 effectively, you first need to identify its underlying root cause. The causes can be broadly grouped into four categories: origin server issues, security tool interference, invalid HTTP responses, and network routing problems. We will break down each category in detail below, including common scenarios and how to recognize them.

1. Origin Server Unavailability or Crash

The most common cause of Error 520 is a fully or partially unresponsive origin server. When the origin server crashes, runs out of critical system resources, or is taken offline for maintenance, it cannot respond to Cloudflare’s requests at all, or returns a signal that is not recognized as a valid HTTP response. Common triggers for origin server unavailability include: excessive CPU or memory usage caused by unexpected traffic spikes, DDoS attacks, or poorly optimized website code; full disk storage that prevents the server from writing temporary files or logging data; hardware failures on the physical server hosting the site; downtime caused by the web hosting provider; and crashed web server software (such as Nginx, Apache, or LiteSpeed) or application runtime environments (such as PHP, Python, or Node.js). If the origin server is completely unresponsive, Error 520 will appear for all visitors to the site, regardless of their location or device.

2. Firewall or Security Tool Interference

Another extremely common cause of Error 520 is interference from security tools running on the origin server, which block Cloudflare’s IP addresses from communicating with the server. Cloudflare routes all traffic to the origin server through its own global network of data centers, so all requests reaching the origin server will come from one of Cloudflare’s official IP ranges. If the origin server’s web application firewall (WAF), iptables rules, security plugin, or server-side security software is not configured to whitelist Cloudflare’s IP ranges, it will flag Cloudflare’s requests as suspicious and block them, either by dropping the connection entirely or returning a non-standard response that triggers Error 520. In addition to server-side security tools, geographic access restrictions implemented by the website owner can also trigger Error 520 for visitors in blocked regions. For example, a streaming service that only holds distribution rights for content in the United States may block all IP addresses from outside the US, including Cloudflare’s non-US nodes, leading to Error 520 for visitors trying to access the service from Europe, Asia, or other regions. Many users first attempt to resolve this type of region-locked error by using a free proxy to test access from an allowed region, but free proxies often suffer from slow speeds, frequent disconnections, and IP addresses that are already blocked by most major sites, leading to even more frequent Error 520 messages. For reliable, consistent access to region-locked content, a premium proxy service like OwlProxy is a far better choice, as it offers 50M+ dynamic proxies and 10M+ static proxies covering 200+ countries and regions, so you can easily find an IP address in the region you need to access.

3. Invalid or Malformed HTTP Responses

Error 520 is also triggered when the origin server returns a response that does not comply with the official HTTP specification, or exceeds Cloudflare’s platform limits. The most common examples of invalid responses include: empty responses, where the origin server closes the connection without returning any data at all; responses with missing or invalid HTTP status codes, which Cloudflare cannot map to a standard error message; responses with excessively large headers, which exceed Cloudflare’s maximum allowed header size of 16KB; responses with corrupted or malformed body content; and responses generated by misconfigured CGI scripts or server-side applications that print debugging information or non-HTTP content before the official HTTP headers. For example, if a WordPress plugin is misconfigured to print PHP debug messages before the site’s HTTP headers are sent, the resulting response will be malformed and trigger Error 520 for all visitors. Similarly, if a site stores too much data in user cookies, the combined size of the Cookie header and other response headers can exceed the 16KB limit, leading to Error 520 for returning visitors with large cookie files.

4. Network and Routing Issues

The final category of Error 520 causes is network and routing issues between Cloudflare’s data centers and the origin server. Even if the origin server is running correctly and security tools are properly configured, problems with the intermediate network can prevent Cloudflare from receiving a valid response. Common network-related causes include: routing errors on the public internet that cause Cloudflare’s requests to be routed to the wrong server or dropped entirely; ISP-level network outages or throttling that disrupt the connection between Cloudflare and the origin server; TCP reset attacks or misconfigured network devices that close the connection between Cloudflare and the origin server mid-request; DNS resolution errors that cause Cloudflare to send requests to the wrong IP address for the origin server; and SSL/TLS handshake failures caused by expired origin server SSL certificates, incompatible cipher suites, or misconfigured SSL settings on either Cloudflare or the origin server. For example, if the origin server’s SSL certificate expires, Cloudflare will be unable to establish a secure encrypted connection to the server, leading to Error 520 for all visitors. Similarly, if Cloudflare’s SSL mode is set to "Full (Strict)" but the origin server is using a self-signed SSL certificate that is not trusted by Cloudflare, the handshake will fail and trigger the error.

Step-by-Step Troubleshooting to Fix Error 520

The troubleshooting steps for Error 520 vary depending on whether you are a regular visitor trying to access a site, or a website owner/administrator responsible for the site that is throwing the error. We will cover both sets of steps in detail below, so you can resolve the error regardless of your role.

Troubleshooting for Regular Visitors

If you encounter Error 520 when trying to access a website you do not own, follow these steps to identify and resolve the issue:

• Step 1: Refresh the page and test basic browser fixes. The first and simplest step is to refresh the page by pressing F5 or clicking the refresh button in your browser. In some cases, the error is caused by a temporary network glitch that resolves itself within a few seconds. If refreshing does not work, try accessing the site in incognito or private browsing mode, which disables browser extensions and uses a fresh cache and cookie set. If the site loads in incognito mode, the issue is likely caused by a corrupted browser cache, outdated cookie, or conflicting browser extension. To fix this permanently, clear your browser’s cache and cookies, and disable any extensions that may be interfering with website access, such as ad blockers, VPN extensions, or security plugins.

• Step 2: Test your local network connection. Next, test whether the issue is specific to your local network. Try accessing the site using a different device connected to the same network, such as your smartphone or tablet. If the site does not load on any device on your network, try restarting your router or modem to reset your network connection. You can also try switching to a different network, such as your mobile data plan, to see if the site loads. If the site loads on mobile data but not on your home or office network, the issue is likely related to your ISP, or your network’s IP address has been blocked by the site’s security tools.

• Step 3: Test access from a different geographic region. If you suspect the error is caused by a geographic access restriction, you can test this by using a proxy service to switch your IP address to a region that is allowed to access the site. OwlProxy is an ideal choice for this use case, as it supports SOCKS5, HTTP, and HTTPS protocols, and offers IP addresses in 200+ countries and regions, so you can easily find a working IP for the region you need. Unlike free proxies, OwlProxy’s IP addresses are regularly updated and are rarely blocked by major sites, so you can access region-locked content without encountering repeated Error 520 messages.

• Step 4: Check if the site is down for everyone. If none of the above steps work, the issue is likely on the website owner’s end. You can confirm this by using a site status tool such as DownDetector or IsItDownRightNow, which checks if a site is down for users around the world. If the tool shows the site is down for everyone, you will need to wait for the website owner to resolve the issue. If the tool shows the site is up for most users but you still cannot access it, you can try using OwlProxy to connect through a different IP address and region to bypass the block.

Troubleshooting for Website Owners and Administrators

If you own or manage the website that is throwing Error 520, follow these detailed steps to identify and resolve the root cause:

• Step 1: Bypass Cloudflare to test the origin server directly. The first step in troubleshooting is to confirm whether the issue is with the origin server itself, or with the connection between Cloudflare and the origin server. To do this, log into your Cloudflare dashboard, navigate to the DNS settings for your domain, and change the proxy status for your root domain and www subdomain from "Proxied" to "DNS only". This will route traffic directly to your origin server, bypassing Cloudflare entirely. Wait a few minutes for the DNS changes to propagate, then try accessing your site directly. If you still see an error when accessing the origin server directly, the issue is with your origin server or its configuration, and you can proceed to the next steps. If the site loads correctly when bypassing Cloudflare, the issue is with the connection between Cloudflare and your origin server, and you can skip to Step 3.

• Step 2: Troubleshoot origin server issues. If the origin server is throwing an error when accessed directly, start by checking the server’s resource usage. Log into your server’s control panel or SSH into the server, and use tools like top, htop, or df to check CPU, memory, and disk usage. If CPU or memory usage is at 100%, identify the process that is consuming the most resources and restart it, or upgrade your server plan to handle higher traffic loads. If disk usage is at 100%, delete unnecessary files such as old logs, backups, or cached content to free up space. Next, check if your web server software is running correctly. Use commands like systemctl status nginx or systemctl status apache2 to confirm the service is active and running. If the service is stopped, restart it and check the error logs for any messages that indicate why it crashed. You should also check your application runtime environment, such as PHP-FPM or Node.js, to ensure it is running correctly and not throwing fatal errors.

• Step 3: Whitelist Cloudflare’s IP ranges in all security tools. If the origin server works correctly when accessed directly, the most likely cause of Error 520 is that your security tools are blocking Cloudflare’s IP addresses. First, you need to get the latest list of Cloudflare’s official IP ranges from the Cloudflare website. Then, add these IP ranges to the whitelist of all security tools running on your server, including your WAF, iptables rules, server-side security software, and any CMS security plugins (such as Wordfence for WordPress, or Security Suite for Drupal). You should also check if you have any geographic access restrictions enabled, and ensure that Cloudflare’s IP ranges from all regions are allowed to access your server. Once you have added the whitelist entries, switch your Cloudflare DNS settings back to "Proxied" and test if the Error 520 is resolved.

• Step 4: Check for invalid HTTP responses. If whitelisting Cloudflare’s IPs does not resolve the issue, check if your origin server is returning invalid or malformed HTTP responses. First, use the curl command to send a request directly to your origin server and check the response. For example, run curl -v -H "Host: yourdomain.com" http://your-origin-server-ip to see the full response headers and body. Look for any obvious issues, such as empty responses, missing status codes, or debug content printed before the headers. You should also check the size of your response headers: Cloudflare limits response headers to 16KB, so if the combined size of your headers is larger than that, you will need to reduce their size. Common ways to reduce header size include removing unnecessary custom headers, limiting the size of user cookies, and disabling debug headers that are only needed for development. Next, check your server’s error logs for any messages that indicate malformed responses. For Nginx, the error log is usually located at /var/log/nginx/error.log, and for Apache it is at /var/log/apache2/error.log. Look for messages related to CGI errors, application crashes, or invalid output.

• Step 5: Check SSL/TLS configuration. Another common cause of Error 520 is misconfigured SSL/TLS settings. First, log into your Cloudflare dashboard and navigate to the SSL/TLS settings page. Confirm that your SSL mode matches the configuration of your origin server: if your origin server has a valid SSL certificate signed by a trusted CA, use "Full" or "Full (Strict)" mode; if your origin server does not have an SSL certificate, use "Flexible" mode. Next, check that your origin server’s SSL certificate is not expired, and that it is valid for your domain. You can test this by running openssl s_client -connect your-origin-server-ip:443 -servername yourdomain.com to check the certificate details. If the certificate is expired, renew it and restart your web server. You should also check that your origin server supports the cipher suites used by Cloudflare: Cloudflare publishes a list of supported cipher suites on its website, and you should ensure that your server’s SSL configuration includes these suites to avoid handshake failures.

• Step 6: Check network and routing issues. If none of the above steps resolve the issue, check for network and routing problems between Cloudflare and your origin server. First, run a traceroute or MTR test from your server to Cloudflare’s IP addresses to check for packet loss or routing errors. You can also use Cloudflare’s own diagnostic tools in the Cloudflare dashboard to test the connection between Cloudflare’s data centers and your origin server. If the test shows packet loss or routing issues, contact your hosting provider to resolve the network problem. You should also check your server’s firewall rules to ensure that outgoing traffic to Cloudflare’s IP ranges is allowed, and that there are no rules that are dropping or resetting connections from Cloudflare.

• Step 7: Contact support. If you have followed all the above steps and still cannot resolve the Error 520, contact Cloudflare support and your hosting provider’s support team for further assistance. Be sure to provide them with copies of your server’s error logs, the results of your curl and traceroute tests, and details of the steps you have already taken to troubleshoot, so they can identify the issue quickly.

If you regularly need to test access to your site from different regions to identify regional Error 520 issues, using a reliable proxy service can save you a significant amount of time and effort. The table below compares OwlProxy to other popular proxy services on the market, so you can choose the best option for your needs:

As you can see, OwlProxy offers a far larger IP pool, wider regional coverage, and more flexible usage terms than most competing proxy services, making it the ideal choice for both regular users and website owners looking to resolve and prevent Error 520 issues.

How to Prevent Error 520 Long-Term

Once you have resolved the immediate Error 520 issue, taking proactive steps to prevent it from recurring in the future can save you from costly downtime and frustrated visitors. Below are long-term prevention strategies for both website owners and regular users.

Prevention Strategies for Website Owners

For website owners, the key to preventing Error 520 is to ensure your origin server is reliable, your configuration is correct, and you can identify and resolve issues before they affect visitors. Follow these steps:
1. Implement continuous server monitoring: Use monitoring tools such as Prometheus, Grafana, or Nagios to track your server’s CPU, memory, disk, and network usage 24/7. Set up alerting rules to notify you via email, SMS, or Slack when resource usage exceeds safe thresholds, so you can resolve issues before they cause the server to crash. You should also monitor your web server and application runtime health, and set up alerts for crashed services or high error rates. Many hosting providers are rolling out enhanced DDoS protection features in 2025 that can help reduce origin server crashes that trigger error 520, so consider upgrading to a plan that includes these features if you are frequently targeted by DDoS attacks.
2. Permanently whitelist Cloudflare’s IP ranges: Make sure Cloudflare’s IP ranges are permanently whitelisted in all your security tools, and update the whitelist regularly whenever Cloudflare announces new IP ranges. Most security tools allow you to import IP range lists automatically, so set up a regular task to update the whitelist to avoid accidental blocks after Cloudflare expands its IP pool.
3. Optimize your HTTP response configuration: Enforce a limit on the size of your response headers to ensure they never exceed Cloudflare’s 16KB limit. Remove any unnecessary custom headers, limit the amount of data stored in user cookies, and disable debug headers in production environments. You should also test your application regularly to ensure it never returns empty or malformed responses, even when encountering errors. Implement custom error pages that return valid HTTP status codes and well-formed responses, so even if your application encounters an error, it will not trigger Error 520.
4. Regularly audit your SSL/TLS configuration: Set up alerts to notify you 30 days before your SSL certificate expires, so you can renew it in time and avoid handshake failures. Regularly audit your SSL cipher suite configuration to ensure it is compatible with Cloudflare’s supported suites, and keep your web server software updated to the latest version to benefit from security patches and improved SSL compatibility.
5. Use Cloudflare’s caching and optimization features: Reduce the load on your origin server by enabling Cloudflare’s caching features for static content such as images, CSS, JavaScript, and HTML files. You can also use Cloudflare’s Auto Minify feature to reduce the size of your static files, and Argo Smart Routing to improve the reliability of the connection between Cloudflare and your origin server. By reducing the number of requests that reach your origin server, you can lower the risk of server overload and crashes that trigger Error 520.
6. Regularly test regional access: Use a proxy service like OwlProxy to test access to your site from different regions around the world on a regular basis, to identify regional blocks, routing issues, or configuration errors that cause Error 520 for users in those regions, before they report the issue to you. OwlProxy’s wide regional coverage makes it easy to simulate user access from any country or region, so you can resolve issues before they affect a large number of visitors.

Prevention Strategies for Regular Users

For regular users who frequently encounter Error 520 when accessing region-locked or IP-blocked sites, follow these steps to prevent the error from recurring:
1. Use a reliable premium proxy service: Instead of relying on unstable free proxies that frequently trigger Error 520, invest in a premium proxy service like OwlProxy. OwlProxy offers flexible pricing plans to suit different use cases: static proxies are charged by subscription period with unlimited traffic, making them ideal for users who need to access sites from a fixed region for extended periods of time. Dynamic proxies are charged by traffic, with no expiration date for purchased traffic, making them ideal for users who only need to switch IP addresses occasionally. You can choose the plan that best fits your usage pattern on the OwlProxy purchase page.
2. Choose the right proxy type for your use case: If you are accessing streaming services or social media sites that require a consistent IP address, use a static ISP proxy or static IPv4/IPv6 proxy from OwlProxy. If you are web scraping or accessing sites that frequently block IP addresses, use a dynamic residential proxy to rotate your IP address automatically and avoid blocks. OwlProxy supports switching between proxy protocols at any time, so you can easily adjust your configuration to suit different sites and use cases.
3. Keep your browser and network configuration optimized: Regularly clear your browser cache and cookies to avoid corrupted files that can trigger Error 520. Disable unnecessary browser extensions that can interfere with website access, and ensure your local network configuration is correct to avoid routing issues.

Frequently Asked Questions

Q: Can a proxy service really help me fix Error 520 when accessing blocked sites?

A: Yes, absolutely. A large percentage of Error 520 messages encountered by regular users are caused by geographic access restrictions or IP-based blocks that prevent Cloudflare from routing your request to the origin server correctly. By using a proxy service to switch your IP address to a region that is allowed to access the site, you can bypass these blocks and access the site without encountering Error 520. OwlProxy’s extensive IP pool covering 200+ countries and regions makes it easy to find an IP address that works for the site you are trying to access, and its reliable network ensures you will not encounter frequent disconnections or errors that are common with free proxies.

Q: How do I know if Error 520 is caused by my local network/IP or the website itself?

A: You can test this by trying to access the site from a different device and a different network, such as your mobile data plan. If the site loads correctly on the other network, the issue is likely with your local IP address or network configuration, and using a proxy will resolve the issue. If the site does not load on any network, the issue is on the website owner’s end, and you will need to wait for them to resolve it. You can also use a free proxy to do a quick initial test to confirm if an IP switch resolves the issue, before upgrading to a premium proxy service for long-term use.

Q: As a website owner, how can I use a proxy service to prevent Error 520 on my site?

A: For website owners, proxy services are an invaluable tool for testing regional access to your site. By using OwlProxy to simulate access from different countries and regions, you can identify regional blocks, routing issues, or configuration errors that cause Error 520 for users in those regions, before they report the issue to you. This allows you to proactively resolve issues and ensure a consistent experience for users around the world. OwlProxy’s unlimited dynamic route extraction allows you to test as many regions and IP addresses as you need, with no extra fees beyond your initial traffic purchase.