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

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-03 · updated 2026-08-28
• Weakness: CWE-297

Description

In Bouncy Castle for Java before 1.85, JSSE hostname verifier CN-fallback enabled by default despite documented opt-in. This issue also affects Bouncy Castle for Java LTS before 2.73.12, and Bouncy Castle for Java FIPS (BC-FJA) 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).

Affected products

• Legion of the Bouncy Castle Inc. BC-JAVA: before 1.85 — fixed in 1.85
• Bouncycastle Bctls-Fips: from 1.0.0 before 1.0.24; from 2.0.19 before 2.0.24; from 2.1.20 before 2.1.24 — fixed in 1.0.24, 2.0.24, 2.1.24
• Legion of the Bouncy Castle Inc. Bouncy Castle for Java LTS: through 2.73.11

How to fix

Upgrade to a fixed version: Legion of the Bouncy Castle Inc. BC-JAVA 1.85; Bouncycastle Bctls-Fips 1.0.24, 2.0.24, 2.1.24. Check the vendor advisory for mitigations.

References

• https://github.com/bcgit/bc-java/commit/5ac55351cd1a8a7184d41c96a7ee87df0770240a
• https://github.com/bcgit/bc-java/commit/799bd15320a6310a447863638aa3df64acef829b
• https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9059638

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