400 Bad Request Error: Guide to Understanding and Resolving

Error 400 Bad Request

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The 400 Bad Request HTTP error code is a common issue that users can encounter when browsing the web. In this article, we will explore the causes of this error and provide you with some tips on how to troubleshoot and fix it. By the end of this post, you should have a better understanding of what causes the 400 Bad Request error and how to resolve it.

Table of Contents

What Is The 400 Bad Request Error?

This error occurs on your browser or “client side” when something is wrong with your request that the server cannot process.

As mentioned in the RFC 9110 HTTP Semantics by the IETF HTTP Working Group, the 400 (Bad Request) status code indicates that the server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).

Different 400 Error Messages

Depending on the web server, or browser, you may find different descriptive messages for this error:

  • 400 Bad Request.
  • Bad Request.
  • This page isn’t working. If the problem continues, contact the site owner. HTTP ERROR 400
  • This page isn’t working at the moment. If the problem persists, contact the site owner. HTTP ERROR 400
HTTP 400 Bad Request Error Page in Chrome
Default php 400 error message displayed in a Chrome browser

The default 400 Bad Request Error displayed in OSX Safari looks like a blank page:

Safari Blank page on HTTP Error 400
Safari default 400 error message.

On Microsoft Edge, the Error will show this:

HTTP Error 400 Page in OSX Microsoft Edge
OSX Microsoft Edge, HTTP Error 400 message.

Some hosting servers can provide their own error page, like our Wetopi Load Balancer 400 Error:

Wetopi 400 Bad Request Error Page
Wetopi personalized 400 error page.

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400 Bad Request Causes

As per its definition earlier, a 400 Bad Request Error occurs when the web server can’t process something on your end. But don’t let a 400 Bad Request Error ruin your browsing experience! Check out the most common reasons behind this error:

1. 400 Error In The URL String Syntax

A 400 Bad Request error can occur if the URL is typed incorrectly, contains malformed syntax, or includes illegal characters. It may happen by mistake if we manually type the address, in particular if the URL has encoded chars (the ones that start with the %)

For example, you may reproduce this error by typing a wrong encoded char at the end of an url. Try typing https://wetopi.com followed by /%%0/

In this URL, the double %%0 is a wrong encoded char!

2. 400 Error In the Request Headers

For example, one of the HTTP headers is malformed, or its size exceeds the maximum limit allowed by the server.

An HTTP header contains information about the request, such as the browser type, the URL, and the type of data sent. If the header is incorrect or too large, the server won’t be able to process the request and will return a 400 error code.

3. 400 Error In the Cookies

Cookies are a way for websites to store data on a user computer to remember their preferences, login information, and other details. However, like HTTP headers, cookies can also cause a “400 bad request” error if they are formatted incorrectly or exceed the server size limits. Some examples are:

  • If a website sets a cookie with a name that contains spaces or special characters, it may cause a “400 bad request” error if the cookie is not properly encoded or formatted. For example, a cookie with the name “my cookie” should be encoded as “my%20cookie” to be valid.
  • Another example of a malformed cookie is when it contains invalid characters or values. For instance, if a cookie value contains a semicolon or a comma, it can confuse the server cookie parsing algorithm and cause a “400 bad request” error.
  • Finally, cookies can exceed the server size limits, causing a “400 bad request” error.

4. 400 Error In the request Body content

When submitting content to a website, users may encounter a “400 Bad Request” error if the data sent to the server has an error or exceeds a limit. For example, if a user tries to upload a file to a website, and it exceeds the server file size limit, the server will reject the request and return the “400 Bad Request” error.

Similarly, if the user enters invalid or incomplete data in a form, the server may also reject the request and return the error message.

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How To Fix The 400 Bad Request Error Code

Be sure to follow the next steps to fix the 400 error:

1. Verify the URL

The most common cause of a 400 Bad Request error is a mistake in the URL entered in the browser address bar. It could be due to typos, missing or incorrect characters, or an outdated or invalid URL. Double-check the URL, spelling, and formatting, especially if the address has encoded chars.

If the error persists, the next step is to clear the session storage and cookies in relation to the domain you are browsing. Keep reading:

2. Clear the Session Storage

When we remove all the Session Storage information for the page showing the error, we remove cookies, cache, and locally stored information.

How to clear the Session Storage on a Chromium-based (Google Chrome, Microsoft Edge, or any other browser that uses the Chromium engine):

  1. Open the browser and click the three dots icon in the top-right corner.
  2. Click “More tools” and then “Developer tools” or use the keyboard shortcut “Ctrl+Shift+I” (Windows) or “Cmd+Option+I” (Mac).

    Open the developer tools
  3. Select the “Application” tab and then click “Storage” in the left-hand panel:

    Developer Tools, clear site data
  4. In the right-hand panel click the “Clear site data” button.

At that stage, just reload the page and see if things are working again.

3. When Uploading a file, check the size

If you’re experiencing the 400 Bad Request Error while uploading a file, as a test, try using a much smaller file to see if the error goes away.

If this previous test works, then you know that the 400 Error is related to the Upload File Size limits of the server.

4. Inspect the logs

If the previous fixes do not solve the problem and you encounter the “400 Bad Request” error on your website, it could indicate a general problem with the server.

You can inspect the server logs for more information. Server logs should be accessible for clients in any hosting company, although sometimes they may be hard to locate or access.

If your WordPress site is hosted on a Wetopi server, you can easily access the logs with just two clicks.

  1. Open the Options Menu
  2. Click Show logs:

    Show Logs in Options Menu on wetopi server


The 400 Bad Request HTTP error code occurs when there is something wrong with the request made by the client side that the server cannot process.

This could be due to various causes:

  • an error in the URL string syntax
  • an error in the header request, cookies, or body content request.

In this article, we have suggested ways to fix this error by:

  • verifying the URL entered,
  • clearing the session storage,
  • or inspecting the logs.

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All HTTP Status Codes

200 OK

201 Created

202 Accepted

203 Non-Authoritative Information

204 No Content

205 Reset Content

206 Partial Content

207 Multi-Status

208 Already Reported

226 IM Used

300 Multiple Choices

301 Moved Permanently

302 Found

303 See Other

304 Not Modified

305 Use Proxy

307 Temporary Redirect

308 Permanent Redirect

400 Bad Request

401 Unauthorized

402 Payment Required

403 Forbidden

404 Not Found

405 Method Not Allowed

406 Not Acceptable

407 Proxy Authentication Required

408 Request Timeout

409 Conflict

410 Gone

411 Length Required

412 Precondition Failed

413 Payload Too Large

414 Request-URI Too Long

415 Unsupported Media Type

416 Requested Range Not Satisfiable

417 Expectation Failed

418 I’m A Teapot

421 Misdirected Request

422 Unprocessable Entity

423 Locked

424 Failed Dependency

426 Upgrade Required

428 Precondition Required

429 Too Many Requests

431 Request Header Fields Too Large

444 Connection Closed Without Response

451 Unavailable For Legal Reasons

499 Client Closed Request

500 Internal Server Error

501 Not Implemented

502 Bad Gateway

503 Service Unavailable

504 Gateway Timeout

505 HTTP Version Not Supported

506 Variant Also Negotiates

507 Insufficient Storage

508 Loop Detected

510 Not Extended

511 Network Authentication Required

599 Network Connect Timeout Error

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