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CVE-2026-59639: CMS verifySignatures returns true for SignedData with zero signers

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% 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-347

Description

In Bouncy Castle for Java before 1.85, CMS verifySignatures returns true for SignedData with zero signers. This issue also affects Bouncy Castle for Java LTS before 2.73.12, and Bouncy Castle for Java FIPS (BC-FJA) before bcpkix-fips 1.0.12 (1.0.X series), 2.0.12 (2.0.X series) and 2.1.12 (2.1.X series).

Affected products

• Legion of the Bouncy Castle Inc. BC-JAVA: before 1.85 — fixed in 1.85
• Legion of the Bouncy Castle Inc. BCPKIX-FIPS: before 1.0.12; from 2.0.7 before 2.0.12; from 2.1.8 before 2.1.12 — fixed in 1.0.12, 2.0.12, 2.1.12
• 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; Legion of the Bouncy Castle Inc. BCPKIX-FIPS 1.0.12, 2.0.12, 2.1.12. Check the vendor advisory for mitigations.

References

• https://github.com/bcgit/bc-java/commit/99ddc6dcc6782e6a76b0dd587c77e62eb7096ad0
• https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9059639

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