Understanding the Cloudflare Browser Integrity Check
The Cloudflare Browser Integrity Check (BIC) operates similar to Bad Behavior and looks for common HTTP headers abused most commonly by spammers and denies access to your page. It also challenges visitors without a user agent or with a non-standard user agent such as commonly used by abusive bots, crawlers, or visitors.
BIC is enabled by default, and you can configure it in Security > Settings. You can disable the BIC using a firewall bypass rule. Also, use a Page Rule to selectively enable or disable this feature for certain sections of your website. For example, disable BIC for your API traffic.