← All Status Codes
505
HTTP Version Not Supported
🔴 Server Error
Low Risk
📖 What Is HTTP 505?
The server does not support the HTTP version used in the request. Most servers support HTTP/1.1 and HTTP/2.
🛡️ Security Implications
Downgrade attacks may force HTTP/1.0 to bypass security features only available in HTTP/1.1+. Enforce minimum HTTP version.
🔍 Common Causes
Client using an outdated or unsupported HTTP version. Misconfigured proxy sending wrong protocol version.
🔧 How to Fix
Update client software. Ensure proxy servers forward the correct HTTP version. Enable HTTP/2 support on the server.
🖥️ How to Check
curl -I -o /dev/null -w "%{http_code}" https://example.com