431 Request Header Fields Too Large HTTP Status Code

Description

HTTP Status Code 431 indicates "Request Header Fields Too Large." This response status code is sent by a server when the client's request contains header fields that are too large for the server to process. It suggests that the size of the request's header fields exceeds the server's capacity to handle them, possibly because an individual header field, or all the header fields collectively, are too large.

This issue might arise when a client sends a request with too many headers or with one or more headers that are excessively large, potentially as a result of a malicious attack or a misconfigured client. To resolve this issue, the client may need to reduce the size or number of header fields it sends in its requests.

Unlike many other HTTP response codes, which can indicate problems with the server, a 431 status code specifically points to an issue with the request made by the client. It implies that the request should not be repeated without modifications, particularly to the size or quantity of the request's header fields.

Status

HTTP

431 Request Header Fields Too Large

Common Causes

Here are some common causes for a 431 status code:

Excessive Cookies: If a website sets too many cookies, or if the cookies are too large, the cumulative size of the request headers can become too large. This is one of the most common reasons for this error, as cookies are included in every request to the domain that set them.
Extremely Long URLs: Although technically URLs are part of the request line, not the headers, some servers include URLs when calculating the size of the header. Very long URLs, especially those with lots of query parameters, can contribute to this problem.
Large Amounts of Query Parameters: When a request contains an unusually high number of query parameters, and these are appended to the URL, they can contribute to an oversized request header.
Custom Headers: Custom headers added by applications, especially if they are verbose or numerous, can also contribute to the size of the request header exceeding the server's limit.
User-Agent Strings: Some browsers or bots may use excessively long User-Agent strings, which, along with other headers, can push the request size over the limit.
Misconfiguration: Sometimes, the server might be misconfigured with an unusually low limit for header size. This might not be an issue with the client or the request per se but with how the server is set up to handle incoming requests.
Security Software or Extensions: Some security software, browser extensions, or other middleware might add additional headers to requests, which can inadvertently increase the size of the request headers.

To resolve this issue, the causes need to be identified and addressed, which might involve clearing cookies, reducing the number of custom headers, or configuring the server to accept larger headers.