mTLS client certificate revocation vulnerability with TLS Session Resumption


On December 16, 2022, Cloudflare discovered a bug where, in limited circumstances, some users with revoked certificates may not have been blocked by Cloudflare firewall settings. Specifically, Cloudflare’s Firewall Rules solution did not block some users with revoked certificates from resuming a session via mutual transport layer security (mTLS), even if the customer had configured Firewall Rules to do so. This bug has been mitigated, and we have no evidence of this being exploited. We notified any customers that may have been impacted in an abundance of caution, so they can check their own logs to determine if an mTLS protected resource was accessed by entities holding a revoked certificate.

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