Hello Duc Dinh Minh,
Thank you for the post, When client authentication is enabled on the web server, client needs to present their certificate Since the browser may have multiple client certificates installed, it will ask the user to select the appropriate client certificate for the specific website they are accessing." below is the link on enabling client cert authentication on WAF,
https://docs.imperva.com/bundle/v15.2-web-application-firewall-user-guide/page/2621.htm
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Syed Noor Fazal
Product Support Engineer
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