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

github.com/bcgit/bc-java/commit/5ac55351cd1a8a7184d41c96a7ee87df0770240a
github.com/bcgit/bc-java/commit/799bd15320a6310a447863638aa3df64acef829b
github.com/bcgit/bc-java/wiki/CVE-2026-59638
nvd.nist.gov/vuln/detail/CVE-2026-59638

Code Behaviors & Features

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