In Bouncy Castle for Java before 1.85, JSSE hostname verifier CN-fallback enabled by default despite documented opt-in. (CVE-2026-59638)

CVE-2026-59638 describes a vulnerability in Bouncy Castle for Java before version 1.85, where the JSSE hostname verifier's Common Name (CN) fallback is enabled by default despite documentation stating it should be opt-in. This behavior also affects Bouncy Castle for Java LTS versions before 2.73.12 and Bouncy Castle for Java FIPS (BC-FJA) versions before bctls-fips 1.0.24 (1.0.X series), 2.0.24 (2.0.X series), and 2.1.24 (2.1.X series). The issue is classified under CWE-297 (Improper Validation of Certificate with Host Mismatch). The CVSS v3.1 score is 8.6 (critical), indicating network attack vector, low attack complexity, no privileges or user interaction required, and impacts confidentiality, integrity, and availability.

The vulnerability allows improper hostname verification due to the CN-fallback being enabled by default, which can lead to potential man-in-the-middle attacks or other attacks compromising confidentiality, integrity, and availability of communications relying on Bouncy Castle's JSSE hostname verification.

No explicit patch links or vendor advisory content are provided in the input data. Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Users should monitor official Bouncy Castle advisories for updates and apply patches once available. Until then, consider disabling CN-fallback manually if possible or using updated versions once released.