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 before 1.85. 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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