How to Fix Your Client Has Issued a Malformed or Illegal Request

What does this mean: 400. that’s an error. your client has issued a malformed or illegal request. that’s all we know? While the causes can vary from an incorrect URL to stale cookies, error 400 bad request basically prevents you from reaching a webpage.

Instead of accessing the app’s engine, Google’s servers return this message. YouTube, for instance, is commonly impacted by this but it shouldn’t be a surprise to anyone given the number of issues constantly plaguing the video-sharing site.

See YouTube videos lagging in Chrome or recurrent YouTube glitches in Firefox, just to give a couple of quick examples.

However, this pesky anomaly can be encountered on various platforms or sites, as follows:

• Safari, Firefox, Chrome your client has issued a malformed or illegal request
• Your client has issued a malformed or illegal request. Postman, Python, Curl, Firebase
• Your client has issued a malformed or illegal request. that’s all we know. Google Translate, Google Maps, Discord

As you can see, the error is not exclusive to a particular domain, app, or OS. So what does an illegal request mean? To answer this, we must understand the causes and triggers behind error 400 bad request. Here we go.

What does your client has issued a malformed or illegal request mean?

Malformed URL	Authentification	Cookies/Extensions	Browser cache/ DNS
→ A common cause of the malformed error is a wrong (or malformed) URL format, such as whitespace – extra spaces/characters, missing leading or trailing slashes, etc.	→ A request for secured pages (HTTPS) without proper credentials can cause this error. To avoid this, primary identity should be authenticated prior to accessing any websites.	→ Stale cookies that need to be cleared, especially cookies shared with other extensions can issue malformed or illegal requests.	→ Outdated browser cache or stored DNS records are also common causes.

The YouTube case is a bit more particular with the error being mainly caused by shared cookies between YouTube and other extensions such as The Great Suspender. So, you might want to disable add-ons.

If this action doesn’t solve the problem, you need to clear the specific faulty cookies from Developer tools or use Incognito mode.

Even if you may not have thought about it, switching to a different browser might be the best option for you.

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Finally, the malformed or illegal request error 400 can also be caused by issues with the server. In this case, we’re talking about occasional glitches that you simply have to wait out.

How do you fix your client has issued a malformed or illegal request that’s all we know?

• 1. Double-check the URL
• 2. Try the Incognito mode
• 3. Clear YouTube’s cookies from Chrome
  • 3.1 Clear Youtube’s cookies using Developer Tools
• 4. Clear Chrome’s cache
• 5. Clear DNS cache
• 6. Reinstall Chrome

1. Double-check the URL

A wrong or simply put, a malformed URL is among the most widespread causes triggering a malformed or illegal request error 400.

To rule out this possibility, we suggest you take a closer look at the requested URL and correct any spelling or syntax errors, as follows:

• identify and remove any unsupported symbols from your hyperlinks – [])(«<>
• make sure your input is correct (backslashes are where they should be and so on)
• make sure the domain name is correct and avoid using capslock
• don’t leave whitespace before or after the URL
• watch out for extra or missing characters

2. Try the Incognito mode

1. Hit the Windows key, type Chrome, then open the browser.
2. Use the following keyboard shortcut to open the Incognito mode: Ctrl + Shift + N.
3. Now try accessing YouTube and check if the error is gone.

Chrome’s private mode will clear any site data and cookies after ending a session. However, you can enable third-party cookies in Incognito mode if you choose to do so.

And speaking of cookies, here’s how to clear them in case this solution does not solve the problem.

3. Clear YouTube’s cookies from Chrome

1. Hit the Windows key, type Chrome, then open the browser.
2. In the search bar, input the following location, then hit Enter: chrome://settings/siteData
3. There, type youtube, in the search bar from the top-right of the page.
4. All the cookies related to YouTube should appear. Click on the Clear all data button.

After doing that, check if a Malformed or illegal request message still appears.

3.1 Clear Youtube’s cookies using Developer Tools

1. Press on the Windows key, type Chrome, then click on the first result.
2. Navigate to YouTube.
3. In the main YouTube window, use the following keyboard shortcut: Ctrl + Shift + I.
4. Now, the Developer Tools should open on the right side of the window.
5. There, click on the two arrows icon located next to Sources, then choose the Application option.
6. In the left panel, under Storage, expand Cookies.
7. All the cookies should appear.
8. Delete the gsScrollPos-[4-digit number].

After following these steps, check if error 400: Your client has issued a malformed or illegal request message is gone. If not, make sure you keep on reading.

4. Clear Chrome’s cache

1. Press on the Windows key, type Chrome, then click on the first result.
2. Use the following keyboard shortcut to open the Clear browsing data page: Ctrl + Shift + Delete.
3. In the Time range section, use the drop-down menu, and choose All Time.
4. Now, checkmark the Cached images and files option.
5. Click on the Clear data button.

5. Clear DNS cache

1. Hit the Windows key, type cmd, then open Command Prompt.
2. Now, input the following command and press Enter: ipconfig /flushdns
3. This will delete the DNS cache.
4. Then, check if the issue persists.

6. Reinstall Chrome

1. Use the following keyboard shortcut to open Windows Settings: Windows + I.
2. Go to the Apps section.
3. In the opened window, in the search bar located in the right side, type Chrome.
4. Click on the found result, then choose to Uninstall the browser.
5. After the process is completed, navigate to the Chrome download page.
6. There, click on the Download Chrome button.
7. Follow the on-screen instructions to reinstall the browser.

You can also try these easy and general fixes that fixed error 400 for some users.

These solutions are relatively simple and you should have no problem solving error 400: your client has issued a malformed or illegal request. Remember that The Great Suspender extension is the main cause, but it’s not the only one.

We’d like to know your thoughts on this subject so feel free to share them in the comments section below.