CVE-2026-12803 KCCMBlockCipher MAC does not bind nonce when AAD is absent (cross-nonce AEAD forgery)
Attackers can forge authenticated encryption messages by exploiting a nonce binding flaw in the KCCMBlockCipher MAC implementation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-12803?
CVE-2026-12803 is a high-severity vulnerability in Bouncycastle Bc-Java, published on 3 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-354
Which versions are affected by CVE-2026-12803?
It affects 2 products.
Products include Bouncycastle Bc-Java, Bouncycastle Bouncy Castle for Java Lts.
How do I fix CVE-2026-12803?
Upgrade to a fixed version.
Is CVE-2026-12803 being exploited?
There is no known exploitation of CVE-2026-12803 in the wild and no public exploit code.
Other vulnerabilities in Bouncycastle Bc-Java
- CVE-2007-6721 Legion of the Bouncy Castle Java Cryptography API Bleichenbacher Oracle Vulnerability
- CVE-2018-1000613 Deserialization of Untrusted Data in Bouncy castle
- CVE-2026-58062 Stapled OCSP response accepted without binding to the checked certificate
- CVE-2026-59638 JSSE hostname verifier CN-fallback enabled by default despite documented opt-in
- CVE-2026-59650 MTI/A0 DH agreement exponentiates unvalidated peer value
Sources
Also known as CLEANSTART-2026-JR63274, CLEANSTART-2026-MX96340, CLEANSTART-2026-RY52259, CLEANSTART-2026-SU76739, CLEANSTART-2026-WF89427, DEBIAN-CVE-2026-12803 and 2 more.