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CWE-354Weakness · CWE-354

CVE-2026-12803

HIGH · 8.7 CVSS v4.0 Published 2026-08-03
Patch available
A vendor patch is available. No clean upgrade release — apply the published patch.
See remediation →
96/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click Patch available 3 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The KCCMBlockCipher in Bouncy Castle Java versions before 1.85 (and LTS before 2.73.12) fails to properly bind the nonce to the MAC when Additional Authenticated Data (AAD) is absent. This allows cross-nonce AEAD forgery, enabling an attacker to potentially forge valid MACs for different messages using different nonces when AAD is not used.

MitigationUpgrade Bouncy Castle for Java to version 1.85 or later, or LTS version 2.73.12 or later, then verify all cryptographic operations using KCCMBlockCipher continue to function correctly.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
Authentication
X
User interaction
None
Scope
X

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Bouncy Castle Java library presence
    Locate the bouncycastle JAR or dependency in your project (e.g., check classpath, build files like pom.xml or build.gradle, or application lib directories)
    Affected if Bouncy Castle for Java library is present in the environment
  2. Determine Bouncy Castle version
    Inspect the JAR file name, MANIFEST.MF inside the JAR, or check the dependency declaration in your build configuration. Compare the version number to the affected ranges: versions before 1.85, or LTS versions before 2.73.12
    Affected if The installed version is before 1.85, or is an LTS version before 2.73.12
  3. Identify KCCMBlockCipher usage
    Search codebase for imports and usage of org.bouncycastle.crypto.modes.KCCMBlockCipher or references to KCCM in encryption/decryption code
    Affected if KCCMBlockCipher is used in the application code
  4. Check for AAD in KCCM operations
    Review the KCCMBlockCipher usage code to determine if AAD (Associated Authenticated Data) is provided via updateAAD() calls. If the cipher is initialized or used without any AAD input, the vulnerability applies
    Affected if KCCMBlockCipher is used without providing any AAD data to the cipher operations

You are affected if Bouncy Castle for Java version is before 1.85 (or LTS before 2.73.12) AND KCCMBlockCipher is used in your code without providing AAD to the cipher operations.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Patch available Apply the vendor patch
Vendor patch github.com →
Interim mitigation

Upgrade Bouncy Castle for Java to version 1.85 or later, or LTS version 2.73.12 or later, then verify all cryptographic operations using KCCMBlockCipher continue to function correctly.

Recommended fix High confidence

1.85 (main line) or 2.73.12 (LTS)

  1. Locate the Bouncy Castle dependency in your project's build configuration (pom.xml, build.gradle, or equivalent)
  2. Update the Bouncy Castle version to 1.85 or later for the main line, or 2.73.12 or later for the LTS line
  3. Run your build to verify the dependency resolves correctly
  4. Run your test suite to ensure the upgrade does not break existing functionality
  5. If using a build tool with dependency locking (e.g., Gradle with lockfiles, Maven with dependencyManagement), update the lockfile as well
Caveat Review the Bouncy Castle release notes for 1.85 and 2.73.12 for any compatibility notes; minor version upgrades typically do not introduce breaking changes

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,720
Get the patch applied

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,752.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-12803 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data