A somewhat common error on campus is for users to see error 400: Request Header Or Cookie Too Large when trying to load campus websites.

The error happens most often to users such as deans, senior staff, and faculty who have administrative/authenticated access to a large number of U of I websites, but it can happen to anyone.

The error occurs because cookies are stored on a per domain basis, and for U of I websites, the domain illinois.edu contains a large number of websites.

The HTTP cookie protocol only officially supports a maximum of 4096 bytes (4k) per cookie, which can easily be exceeded for U of I users. We have seen some U of I library proxy sites store upwards of 50MB in the cookie file. Once the cookie file has grown very large, it's likely that you will encounter a web server that is configured with a smaller cookie limit such as 4k or 8k, and it will start rejecting your requests with error 400.

Unfortunately, there is no permanent fix for this issue. The best temporary fix is to clear the browser cookie for the illinois.edu domain. This will mean the user will be logged out of all illinois.edu websites. Note that it is not necessary to clear all browser cookies. (Although that will work as a last resort).

How you clear cookies for one domain differs in each browser. Here are the steps for Google Chrome:

1. In the upper-right corner, click the three dots, and then click Settings
2. Click Privacy and security
3. Click Third-party cookies
   
4. Click See all site data and permissions
   
   
5. Type illinois into the search box
6. Click the trash can next to the illinois.edu domain cookie