The internet runs on connections. When one of those connections breaks—especially between a visitor and a website—the result is an error page that leaves users frustrated and website owners scrambling.
Among the many HTTP errors that can disrupt your browsing experience, Cloudflare Error 520 stands out as one of the most perplexing. It is a generic, catch-all message that offers little immediate insight into what went wrong. Unlike a 404 “Not Found” or a 503 “Service Unavailable,” the 520 error doesn’t point to a specific, easily identifiable problem.
This guide explains everything you need to know about Cloudflare Error 520 in 2026—what it is, why it happens, how to fix it, and how to prevent it from recurring.
Part 1: What Is Cloudflare Error 520?
Cloudflare Error 520—often displayed as “Web Server Is Returning an Unknown Error”—is a Cloudflare-specific status code that occurs when the origin web server returns an empty, malformed, or otherwise unexpected response to Cloudflare’s request.
To understand this error, it helps to understand how Cloudflare works. When a visitor opens a Cloudflare-protected site, the request first goes to Cloudflare, which then forwards it to the origin server. Cloudflare expects the origin web server to return a valid HTTP response. When that response is missing, broken, or non-HTTP, Cloudflare cannot pass it through and instead shows a 520 page.
Think of Error 520 as a placeholder for when something goes wrong with the connection between Cloudflare and the origin server. The connection between the user’s browser and Cloudflare servers is fine—the issues are on the origin web server side.
How Error 520 Differs from Other HTTP Errors
Unlike other 5xx error codes, which are defined in the HTTP standards and can be generated by any server or proxy, Error 520 is specific to Cloudflare. Other providers use different messages to indicate an empty response or rely on standard HTTP response headers in their error logs.
This distinction is important: Error 520 does not mean the website is down for everyone. It means the connection between Cloudflare and the origin server has failed for a specific reason that Cloudflare cannot interpret.
Part 2: Common Causes of Cloudflare Error 520
Generally, Error 520 indicates an issue between Cloudflare and the origin server. The exact reasons aren’t always in the server logs, but they can be broken down into six major causes.
1. Server Crashes or Resource Overload
Origin web servers may crash or kill processes mid-request, often due to running out of resource capacity, such as CPU or RAM. When a server is overwhelmed, it may respond improperly, crash, or fail to complete requests. Cloudflare then receives malformed or no response, resulting in a 520 “unknown error” instead of a normal connection or another 5xx code.
This is one of the most common causes of Error 520.
2. Firewall or Security Tools Blocking Cloudflare
Server-side firewall settings, rate limiters, or security plugins may mistakenly block or limit Cloudflare IP addresses. When the connection between the origin server and Cloudflare is tampered with, there is a risk of unexpected or broken responses, which creates recurring 520 errors.
This is often an oversight: website administrators may configure security tools without realizing they need to whitelist Cloudflare’s IP ranges.
3. Malformed or Empty HTTP Responses
When the server sends an empty or malformed HTTP response header, Cloudflare treats it as an unknown error. This can be caused by a variety of crashes, misconfigurations, or bugs. In many cases, server logs reflect the related causes for malformed HTTP responses.
4. Oversized Cookies or Headers
Oversized headers, especially cookie headers, may cause Error 520. Such headers become malformed and may be rejected or responded to incorrectly. Currently, Cloudflare supports headers of up to 128 KB. If the header size exceeds this limit, the request may fail.
5. SSL/TLS or HTTP/2 Misconfigurations
Problems in the HTTPS layer, like mismatched settings and incomplete certifications, may cause connections between Cloudflare and the origin server to reset or fail mid-response. In such cases, a 520 error is sometimes displayed as a catch-all error in the Cloudflare dashboard and for visitors.
6. Application or CMS-Level Errors
Bugs in application code or badly written plugins and CMS extensions are common causes of the web server crashing before sending a full response to Cloudflare. This results in malformed or empty replies that Cloudflare treats as an unexpected response labeled as a 520 error.
Part 3: How to Fix Cloudflare Error 520
Your ability to fix this error depends on whether you have access to the origin web server settings. If you are a visitor, your options are limited. If you are the website administrator, you have more tools at your disposal.
For Website Visitors
If you are simply browsing the web and encounter Error 520, your options are:
Retry the connection – The issue may be temporary. Wait a few moments and refresh the page.
Clear cookies and cache – Oversized cookies or corrupted cache can sometimes trigger the error.
Connect with a different IP address – If the issue is related to IP blocking or rate limiting, switching networks (e.g., from Wi-Fi to mobile data) may help.
Contact the website owner – If the error persists, reach out to the site administrator and report the issue.
For Website Administrators
If you have access to the origin web server or Cloudflare dashboard, follow these steps:
Step 1: Check Your Origin Server’s Status
The first thing to do is to confirm whether the origin web server is functioning as expected. Restart web services and check the CPU, RAM, and other resource usage. If the server crashes, hangs, or restarts under load, stabilize it before further troubleshooting.
Step 2: Bypass Cloudflare to Isolate the Problem
If all seems well with the origin server, temporarily switch DNS records to DNS only or pause the proxy so traffic goes directly to your server.
- If the error persists when Cloudflare services are turned off, the issue is the origin web server.
- If the website fails only when Cloudflare services are turned on, the likely problems are with the proxy, DNS, or Cloudflare edge-specific settings.
Step 3: Review Server Error Logs
Investigate website error logs more carefully, looking for crashes, other 5xx errors, or stack traces around the time the 520 error occurred. Fixing underlying app or server issues usually converts the 520 error into some standard HTTP status code or enables a successful connection.
Step 4: Update Firewall, WAF, or Security Plugin Settings
Ensure all Cloudflare IP address ranges are allowed by your firewall settings, including Web Application Firewalls (WAF) and various security plugins. An effective strategy might be to start looking into recently made changes or newly installed security tools.
Step 5: Reduce or Correct Problematic Headers/Cookies
Check the size of your headers and determine which ones might be problematic, considering the 128 KB limit Cloudflare imposes. Completely clearing or at least trimming tracking cookies, session data, and plugin-added headers is likely to help.
Step 6: Verify SSL/TLS and HTTP Settings
SSL mismatches or expired certificates can sometimes trigger 520 errors if the origin server drops the connection. Faulty HTTP/2 or TLS configurations can also be a cause. As a diagnostic step, temporarily disable HTTP/2 in the Cloudflare dashboard to see whether protocol-level issues are the cause.
Step 7: Disable Conflicting Plugins, Themes, or Custom Code
Test the website by temporarily disabling non-essential CMS plugins, themes, and custom middleware. Disable each of them in turn and test the connection with enough load to recreate the problem. Once the problematic code is found, patch, replace, or reconfigure it.
If none of these steps resolve the issue, contact your hosting provider for troubleshooting assistance. Make sure to share the specific error details, such as the full error code, the time when the problem occurred, and the URL that resulted in the error.
Part 4: How to Prevent Cloudflare Error 520
Prevention is always better than cure. For website administrators, proactive measures can significantly reduce the likelihood of encountering Error 520.
For Website Administrators
Keep server resources stable – Monitor the origin server load, especially CPU and RAM. If your website is seeing more traffic, implement optimization measures or increase resources to prevent the site from crashing and prematurely disconnecting from Cloudflare.
Maintain compatible SSL/TLS and HTTP settings – Set up SSL mode correctly in the Cloudflare dashboard, keep certificates valid, and ensure HTTP/2 and TLS settings are compatible.
Avoid excessive cookies or header bloat – Limit the size and number of cookies and custom headers, so requests always fall within the safe header size limits of 128 KB. Double-check your header size before adding new plugins or making other changes that might increase the header size.
Test updates in staging – New CMS settings, plugins, themes, server updates, or other changes must be tested in a staging environment before deployment. This ensures that potential crashes or other issues that can produce 520 errors are spotted early in error logs.
Allow Cloudflare IP ranges – Blocked IPs might not show up clearly in the error logs; it’s best to do it manually. Whitelist Cloudflare’s IP addresses in firewalls, WAFs, and other security tools so that Cloudflare services wouldn’t be blocked.
For Web Scraping and Automation Workflows
Web scraping or automation setups can implement rate limiting and automatic request delays to mitigate possible 520 errors. This is helpful in cases when empty response issues happen due to server overload.
Part 5: Frequently Asked Questions
Can an ISP issue cause Cloudflare Error 520?
Yes, Internet Service Provider (ISP) issues can contribute to Cloudflare 520 error, but only indirectly. Connectivity problems with the origin server or intermediary ISPs may contribute to abnormal response headers that cause a 520 error.
Yes, clearing cookies and cache can sometimes help fix the 520 error, but it’s not a guaranteed or permanent fix. It helps in cases of request headers exceeding Cloudflare limits. However, most commonly, the fix lies with the website owner addressing origin server issues like misconfigurations, firewalls, or too large or missing response headers.
Can server overload cause a 520 error?
Yes, origin web server overload is a common cause of the Cloudflare 520 error. When the origin server gets too busy, it may respond improperly, crash, or fail to complete requests. Cloudflare sees such unexpected responses and returns a 520 unknown error instead of a normal HTTP response header.
Can DNS problems lead to Error 520?
End-users’ local DNS problems usually affect whether the visitor reaches Cloudflare at all, so the 520 error doesn’t appear. However, misconfigured, incorrect, or outdated DNS on the origin server can cause Cloudflare’s 520 Error. Wrong DNS records may cause the Cloudflare server to connect to incorrect or non-responsive servers that return unexpected, invalid responses.
Part 6: How IPFLY Supports Reliable Web Access
While Error 520 is typically a server-side issue that website administrators must resolve, users accessing websites through proxies or automation tools can also experience this error due to network-related factors. IPFLY provides the residential proxy infrastructure that enables consistent, reliable web access.
Why Network Quality Matters
Error 520 can be triggered indirectly by network issues. If you are using a low-quality proxy or a shared IP address, the origin server may treat your traffic as suspicious, potentially leading to connection resets or malformed responses. Additionally, rate limiting and request throttling can cause server overload that manifests as 520 errors.
The solution lies in using clean, dedicated residential IPs. By routing your traffic through a real ISP-assigned residential IP, you appear to websites as a genuine user, dramatically reducing the chances of triggering server-side issues that could lead to 520 errors.
IPFLY Residential Proxy Solutions
IPFLY’s residential proxies route your traffic through real consumer IPs, making your browsing activity indistinguishable from normal internet traffic.
Key advantages for reliable web access:
- Real residential IPs – From a pool of residential addresses, ensuring high availability and low detection risk
- ISP-registered authenticity – IPs appear as genuine consumer connections, not datacenter infrastructure
- Stable connections – Consistent, reliable access without the downtime of free proxies
- Clean IP reputation – No history of abuse or suspicious activity
- Full protocol support – HTTP, HTTPS, and SOCKS5
Residential proxies are ideal for users who need reliable, long-term access to websites for browsing, automation, or data collection.
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IPFLY Static Residential Proxies (ISP Proxies)
For users who need consistent, long-term access—such as running automation workflows or managing multiple accounts—IPFLY’s Static ISP proxies provide unparalleled stability.
Key advantages:
- 100% dedicated IP – Exclusive use with no sharing
- ISP-registered authenticity – IPs appear as genuine residential connections
- IP will not change – Consistent identity for long-term access
- Unlimited bandwidth – No traffic restrictions
- Full protocol support – HTTP, HTTPS, and SOCKS5
Static ISP proxies are perfect for professionals who need consistent, reliable web access without interruptions.
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IPFLY Datacenter Proxies
For speed-critical operations where residential authenticity is less important, IPFLY’s dedicated datacenter proxies offer high-performance connectivity.
👉 Explore IPFLY Datacenter Proxies
How IPFLY Compares to Free Solutions
| Feature | Free Proxy / Shared IP | IPFLY Residential Proxy |
| IP Type | Datacenter or shared | Real residential ISP IPs |
| Detection Risk | High – easily flagged | Low – appears as normal traffic |
| Privacy | Unknown logging, ads | No logging, private browsing |
| Security | Malware and tracking risks | Secure, vetted infrastructure |
| Speed | Slow, throttled | Fast, stable connections |
| Reliability | Unpredictable, frequently down | Consistent, long-term access |
Understanding and Overcoming Error 520
Cloudflare Error 520 is a generic, catch-all message that indicates a failure in the connection between Cloudflare and the origin web server. It can be caused by server crashes, firewall blocks, oversized headers, SSL/TLS misconfigurations, or application errors.
Key takeaways:
- Error 520 is Cloudflare-specific – It occurs only when a website uses Cloudflare’s infrastructure.
- The issue is on the origin server side – The connection between the user and Cloudflare is fine; the problem is between Cloudflare and the origin server.
- Common causes include server overload, blocked IPs, oversized headers, and misconfigurations – Each requires a different approach to resolve.
- Visitors have limited options – Retry, clear cookies, or switch networks.
- Administrators have more tools – Check server status, bypass Cloudflare, review logs, update firewall settings, and verify SSL/TLS configurations.
- Prevention is key – Monitor server resources, maintain compatible settings, and test changes in staging environments.
By understanding the causes and solutions for Error 520, both website administrators and visitors can navigate this frustrating error more effectively.
Build Reliable Web Access with IPFLY
Error 520 is often triggered by server-side issues, but network quality plays a role too. IPFLY provides the residential proxy infrastructure that enables consistent, reliable web access—whether you are browsing, automating, or collecting data.
IPFLY offers flexible proxy solutions for every use case:
- Residential Proxies – Real consumer IPs with stable sessions. Ideal for reliable web access without triggering server-side issues.
- Static Residential Proxies – 100% dedicated, ISP-registered IPs with consistent identity. Perfect for long-term automation and account management.
- Datacenter Proxies – High-performance IPs for speed-critical operations.
Get started today: Register for an IPFLY account and explore the full product lineup on the IPFLY homepage. Access the web with confidence, knowing your connection is stable, private, and secure.
