← All Status Codes
503
Service Unavailable
🔴 Server Error
Critical Risk
📖 What Is HTTP 503?
The server is currently unable to handle the request due to temporary overloading or scheduled maintenance. Should include a Retry-After header.
🛡️ Security Implications
Application-layer DDoS attacks aim to cause 503 errors. Implement rate limiting, WAF rules, and auto-scaling to maintain availability.
🔍 Common Causes
Server overloaded, maintenance mode enabled, resource limits reached (max connections, memory, CPU), database connection pool exhausted.
🔧 How to Fix
Wait and retry. Check server resources (CPU, memory, connections). Scale up or add more servers. Enable maintenance mode page during updates.
🖥️ How to Check
curl -I -o /dev/null -w "%{http_code}" https://example.com