In Bouncy Castle for Java before 1.85, KCCMBlockCipher MAC does not bind nonce when AAD is absent (cross-nonce AEAD forgery). (CVE-2026-12803)
Bouncy Castle for Java versions before 1.85 and LTS versions before 2.73.12 have a vulnerability in the KCCMBlockCipher MAC implementation where the nonce is not bound when Additional Authenticated Data (AAD) is absent. This flaw allows a cross-nonce AEAD forgery attack, undermining the integrity guarantees of the cipher.
AI Analysis
Technical Summary
The vulnerability CVE-2026-12803 affects Bouncy Castle for Java prior to version 1.85 and its LTS versions before 2.73.12. The issue lies in the KCCMBlockCipher MAC, which fails to bind the nonce when no Additional Authenticated Data (AAD) is provided. This cryptographic flaw enables an attacker to perform a cross-nonce AEAD forgery, potentially allowing unauthorized message forgery across different nonces. The weakness corresponds to CWE-354 (Improper Validation of Integrity Check Value).
Potential Impact
The vulnerability compromises the integrity protection of authenticated encryption using KCCMBlockCipher in affected Bouncy Castle versions. An attacker can forge valid authentication tags across different nonces when AAD is absent, potentially leading to acceptance of forged messages. This undermines the security guarantees of the cryptographic protocol relying on this cipher.
Mitigation Recommendations
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 any released patches or updates that address this issue. Until a fix is available, avoid using KCCMBlockCipher without AAD or consider alternative cryptographic constructions.
In Bouncy Castle for Java before 1.85, KCCMBlockCipher MAC does not bind nonce when AAD is absent (cross-nonce AEAD forgery). (CVE-2026-12803)
Description
Bouncy Castle for Java versions before 1.85 and LTS versions before 2.73.12 have a vulnerability in the KCCMBlockCipher MAC implementation where the nonce is not bound when Additional Authenticated Data (AAD) is absent. This flaw allows a cross-nonce AEAD forgery attack, undermining the integrity guarantees of the cipher.
CVSS v4.0
Affected software
pkg:maven/org.bouncycastle/bcprov-jdk15onpkg:maven/org.bouncycastle/bcprov-jdk15on-ltsRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability CVE-2026-12803 affects Bouncy Castle for Java prior to version 1.85 and its LTS versions before 2.73.12. The issue lies in the KCCMBlockCipher MAC, which fails to bind the nonce when no Additional Authenticated Data (AAD) is provided. This cryptographic flaw enables an attacker to perform a cross-nonce AEAD forgery, potentially allowing unauthorized message forgery across different nonces. The weakness corresponds to CWE-354 (Improper Validation of Integrity Check Value).
Potential Impact
The vulnerability compromises the integrity protection of authenticated encryption using KCCMBlockCipher in affected Bouncy Castle versions. An attacker can forge valid authentication tags across different nonces when AAD is absent, potentially leading to acceptance of forged messages. This undermines the security guarantees of the cryptographic protocol relying on this cipher.
Mitigation Recommendations
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 any released patches or updates that address this issue. Until a fix is available, avoid using KCCMBlockCipher without AAD or consider alternative cryptographic constructions.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- GHSA-4r33-j73q-cw77
- Osv Schema Version
- 1.4.0
- Aliases
- ["CVE-2026-12803"]
- Database Specific Severity
- HIGH
- Cvss Version
- 4.0
Threat ID: 6a710683bf32cb7a343985c8
Added to database: 08/03/2026, 21:22:11 UTC
Last enriched: 08/03/2026, 21:41:20 UTC
Last updated: 09/17/2026, 22:01:33 UTC
Views: 64
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