HTTP 431 Request Header Fields Too Large vs 526 Invalid SSL Certificate
HTTP 431 (Request Header Fields Too Large) is a 4xx Client Error response, while 526 (Invalid SSL Certificate) is a 5xx Server Error response. 431 indicates that the server refuses to process the request because an individual header field or all headers collectively are too large. In contrast, 526 means that cloudflare-specific. The origin's SSL certificate could not be validated.
Description
The server refuses to process the request because an individual header field or all headers collectively are too large.
When You See It
When cookies accumulate to an excessive size, or a single header (like Authorization) is too long.
How to Fix
Reduce header sizes. Clear excessive cookies. Use shorter tokens.
Description
Cloudflare-specific. The origin's SSL certificate could not be validated.
When You See It
When using Cloudflare Full (Strict) mode with an invalid origin certificate.
How to Fix
Install a valid SSL certificate on the origin, or use Cloudflare Origin CA certificate.
Key Differences
431 is a 4xx Client Error response, while 526 is a 5xx Server Error response.
HTTP 431: The server refuses to process the request because an individual header field or all headers collectively are too large.
HTTP 526: Cloudflare-specific. The origin's SSL certificate could not be validated.
You encounter 431 when when cookies accumulate to an excessive size, or a single header (like Authorization) is too long.
You encounter 526 when when using Cloudflare Full (Strict) mode with an invalid origin certificate.
When to Use Which
For 431 (Request Header Fields Too Large): Reduce header sizes. Clear excessive cookies. Use shorter tokens. For 526 (Invalid SSL Certificate): Install a valid SSL certificate on the origin, or use Cloudflare Origin CA certificate.