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CVE-2026-59638 is an improper certificate hostname-validation vulnerability in Bouncy Castle TLS/JSSE libraries for Java. The JSSE hostname verifier enables fallback to the certificate Common Name (CN) by default, despite CN fallback being documented as opt-in. This can cause hostname verification to accept a certificate based on its CN where Subject Alternative Name-based identity validation should be required. Affected releases are Bouncy Castle for Java before 1.85, Bouncy Castle for Java LTS before 2.73.12, and BC-FJA bctls-fips before 1.0.24, 2.0.24, and 2.1.24 in their respective release series.
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A Bouncy Castle Java JSSE hostname-verification flaw in which common-name (CN) fallback is enabled by default despite being documented as opt-in. This can weaken intended hostname-verification behavior.
A critical certificate hostname validation flaw in Bouncy Castle Java TLS/JSSE components where CN-fallback was enabled by default despite being documented as opt-in, potentially allowing improper certificate validation with host mismatch.
One of a cluster of Bouncy Castle vulnerabilities in a sustained campaign affecting security boundaries across Java, LTS, and FIPS lines; exact flaw details are not provided in the content.
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Cross-references every affected SKU, including bundled OEM variants.
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