Error guide / HTTP 400 Bad Request
HTTP 400 Bad Request Error
HTTP 400 Bad Request means the server received a request it considers malformed and refused to process it. Unlike a 500 error (the server's fault) or a 404 (the page is missing), a 400 says the request itself is broken: the browser sent something the server could not parse. Most often the cause is on the visitor's side (corrupted cookies, an oversized header, a malformed URL); less often it is a misconfigured proxy, CDN or a server-side rule.
Check your site
What causes it
The most common cause is corrupted or oversized cookies for that domain: LiteSpeed and Nginx return '400 Bad Request - Request Header Or Cookie Too Large'. Next come a malformed URL with illegal characters or broken %-encoding, and request headers that grow too large and hit the server's limits. Less often the culprit is an intermediary: a misconfigured proxy or CDN (such as Cloudflare) between the visitor and the site, or an .htaccess or mod_security rule that rejects the request.
How to fix it
- Reload the page and open the same URL in incognito or private mode, or in another browser. This bypasses cookies and extensions, so if it works there the problem is local, on your side.
- Clear cookies and site data for that specific domain (not your whole history). Corrupted or bloated cookies are the number one cause of a 400, especially after a site is changed or moved.
- Check the URL for typos, illegal characters and broken %-encoding; type the address by hand instead of following a saved or pasted link.
- Confirm the site actually responds and is not down for everyone by running the availability check below. If the site is fully unreachable, the problem is not your request.
- Disable browser extensions that rewrite requests (VPN, proxy, ad-blockers) and try from another network or device, to rule out an intermediary adding bad headers.
- If the site is yours: review your CDN or proxy settings (such as Cloudflare) and any recent changes to .htaccess or mod_security rules; raise the header-size limits in LiteSpeed or Apache if needed.