HTTP 203 Non-Authoritative Information — Proxy Modified Response

Overview

HTTP 203 Non-Authoritative Information indicates the response was modified by a proxy or intermediate server before reaching the client.

Key Details

• The origin server returned 200 but a proxy modified the response
• Common with transforming proxies that compress or modify content
• The client should be aware the response may differ from the origin
• Rarely seen in practice — most proxies do not change the status code
• Defined in RFC 7231 Section 6.3.4

Common Causes

• Transparent proxy modifying response headers or body
• CDN applying content transformation (minification, image optimization)
• Corporate proxy injecting content into the page
• Caching proxy returning a modified version of the resource

Steps

1. 1No fix typically needed — the response was successful
2. 2If content appears modified, bypass the proxy to get the original response
3. 3Check response headers for Via: or X-Cache: to identify the modifying proxy
4. 4Use HTTPS to prevent proxy modification (proxies cannot modify encrypted content)
5. 5Configure CDN to return 200 instead of 203 if the transformation is expected

Tags

More in 2xx Success

Frequently Asked Questions