CVE-2026-12803: KCCMBlockCipher MAC does not bind nonce when AAD is absent (cross-nonce AEAD forgery)

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 2026-09-02
Weakness: CWE-354

Description

In Bouncy Castle for Java before 1.85, KCCMBlockCipher MAC does not bind nonce when AAD is absent (cross-nonce AEAD forgery). This issue also affects Bouncy Castle for Java LTS before 2.73.12.

Affected products

Legion of the Bouncy Castle Inc. BC-JAVA: before 1.85 — fixed in 1.85
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. Check the vendor advisory for mitigations.

References

https://github.com/bcgit/bc-java/commit/697794413ebf7bc5e3fce609a707826ba52981af
https://github.com/bcgit/bc-java/commit/7d79aa76e984da85f2a541cae8ba2ae56e1713bc
https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9012803

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