What is the 400 Bad Request: Request Header or Cookie too Large and How Do You Fix it? Comments Off on What is the 400 Bad Request: Request Header or Cookie too Large and How Do You Fix it?

Table of Contents

What is a 400 Bad Request Error?
What Triggers a 400 Bad Request Error
How Cookies Add to Header Bloat
The Difference between Header Errors and Cookie Errors
Common Causes of Excessive Request Sizes
Clearing Specific Site Data vs. Full Cache Wipe
Identifying and Disabling Problematic Extensions
Best Practices for Preventing Future Header Bloat

We recently had a client working on the back end of their WordPress website when they were suddenly hit with an error message that read “400 Bad Request: Request Header or Cookie too Large”.  Naturally, they contacted us to ask what the heck was going on.

What is a 400 Bad Request Error?

In a nutshell, a 400 bad request error is a generic client HTTP error that is returned when a request sent to the web server cannot be understood. This is usually due to a URL string syntax error, expired or invalid browser cookies, DNS lookup cache issue, or the request size to the server being too large for it to handle.

What Triggers a 400 Bad Request Error?

Web servers like Apache or Nginx set strict limits on how much data they will accept in a single request header. These “caps” prevent overloads that could slow things down or open up security holes.

The 400 Bad Request is thus triggered when your browser sends more than that the server can handle. It is not the full web page size that is the problem, rather it is just the header chunk at the top. Since cookies are stored inside this header, they pile up, sometime leading to the whole request gets flagged as oversized.

A fair analogy might be to think of headers as the envelope containing a letter or document. If you stuff too many letters or documents inside of the envelope, the post office may send it back to you. Web servers act like the post office and thus, basically do the same to keep web traffic flowing.

How Cookies Add to Header Bloat

Cookies are included with every page load from a website. They store pieces of information such as your shopping cart or login status. Over time, websites add even more cookies for tracking purposes, which bloats the header.

For example, imagine visiting a popular technology blog. As such, it may load with a lot of ads. Each ad drops its own cookie for targeting purposes. Inevitably, your request may end up carrying dozens, pushing past the web server’s limit and triggering the 400 bad request error.

Session tools often worsen this. Some apps stuff full user details into one cookie. That single item can tip the scale, even if others stay small.

The Difference between Header Errors and Cookie Errors

Not every 400 error are solely caused by issues with cookies. Sometimes, malformed headers and a particular browser extension can also cause problems. The 400 error message often lumps these together, but tools like browser dev consoles can help you identify the culprit. Check if deleting/clearing cookies fixes it, or if headers from extensions are the real issue.

Keep in mind that headers handle one-off data, while cookies persist across visits. Mixing them up leads to wrong fixes, so test both to determine the true source.

Common Causes of Excessive Request Sizes

• Ads and analytics from Google or Facebook, leave cookies everywhere. Each script adds its own, tracking clicks and views.
• Frequent logins pile on more. APIs spit out long tokens for security, stored right in cookies. If you shop or bank often, these can also build up quickly.
• Browser extensions like ad blockers or VPNs tweak headers on the fly. Some add extra fields for privacy, pushing sizes over the edge.
• Corporate firewalls can also be problematic in this regard as they inject security headers that bulk up requests. If you are at work, this might explain why you might suddenly experience this type of error.

Clearing Specific Site Data vs. Full Cache Wipe

Start small to avoid losing everything. In Chrome, hit Ctrl+Shift+Delete, pick “Cookies and other site data,” and filter by the troubled site.

Firefox works the same: “Menu” then to “Settings” then “Privacy” then “Cookies and Site Data”. Search the domain and remove. It takes seconds and often solves the 400 Bad Request right away.

Full cache clears are last resort as they log you out everywhere.

1. Open browser settings.
2. Find privacy or clear data section.
3. Select only cookies for that URL.
4. Restart and reload the page.

Identifying and Disabling Problematic Extensions

Use Incognito Mode

It is extension-free by default. If the website loads fine using incognito or private mode, your problem likely lies in an add-on.

Manually Inspecting and Deleting Large Cookies

In Chrome, right-click page then “Inspect”, then “Application” tab, then “Cookies”. Sort by size.

Firefox: “Tools” then to “Web Developer”, then “Storage” then “Cookies”. Note any over 4KB should be deleted manually.

Best Practices for Preventing Future Header Bloat

Implement Efficient Cookie Management Strategies

• Keep cookies lean, use IDs that point to server-stored data. No need to send user bios every request. Set short expirations for temp ones, like 30 minutes.
• Rotate sessions often. This clears old bloat automatically.

If you would like to learn more about the 400 Bad Request: Request Header or Cookie too Large error, contact our expert web design team at Prominent Web Design.