CVE-2026-59638: JSSE hostname verifier CN-fallback enabled by default despite documented opt-in

August 3, 2026 (updated September 16, 2026)

Affects Bouncy Castle for Java LTS before 2.73.12. HostnameUtil gates the fallback that matches the certificate CN when no dNSName SAN is present on a property whose unset default the code treats as enabled, contrary to its documentation. Because name-constraint processing only constrains SAN entries of the constrained type, a leaf with no dNSName SAN passes a dNSName-constrained chain, and the verifier then matches the attacker-chosen CN against the target hostname.

References

Code Behaviors & Features

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