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iOS client certificate authentication or iOS13.5 – the real important fix

“Apple Safari before 6.2.5, 7.x before 7.1.5, and 8.x before 8.0.5 does not properly select X.509 client certificates, which makes it easier for remote attackers to track users via a crafted web site.”

https://nvd.nist.gov/vuln/detail/CVE-2015-1129#VulnChangeHistoryDiv

Impact: Users may be tracked by malicious websites using client certificates

Description: An issue existed in Safari’s client certificate matching for SSL authentication. This issue was addressed through improved matching of valid client certificates.

security content of iOS 9 @apple