Resolving a Mutual TLS session resumption vulnerability
On January 23, 2025, Cloudflare was notified via its Bug Bounty Program of a vulnerability in Cloudflare’s Mutual TLS (mTLS) implementation.
The vulnerability affected customers who were using mTLS and involved a flaw in our session resumption handling. Cloudflare’s investigation revealed no evidence that the vulnerability was being actively exploited. And tracked as CVE-2025-23419, Cloudflare mitigated the vulnerability within 32 hours after being notified. Customers who were using Cloudflare’s API shield in conjunction with WAF custom rules that validated the issuer's Subject Key Identifier (SKI) were not vulnerable. Access policies such as identity verification, IP address restrictions, and device posture assessments were also not vulnerable.
The bug bounty report detailed that a client with a valid mTLS certificate for one Cloudflare zone could use the same certificate to resume a TLS session with another Cloudflare zone using mTLS, without having to authenticate the certificate with the second zone.
Cloudflare customers can implement mTLS through Cloudflare API Shield with Custom Firewall Rules and the Cloudflare Zero Trust product suite. Cloudflare establishes the TLS session with the client and forwards the client certificate to Cloudflare’s Firewall or Zero Trust products, where customer policies are enforced.
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