# CVE-2026-71885: MLS X.509 credential not bound to the LeafNode signature key

> An attacker can impersonate a victim in MLS groups by bypassing X.509 credential binding, allowing message decryption and identity spoofing in deployments using external commits.

- **Severity:** Critical 9.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N/U:Amber`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 3 Oct 2026
- **Updated:** 5 Oct 2026
- **Also known as:** CLEANSTART-2026-PP12728, DEBIAN-CVE-2026-71885, GHSA-jch3-5j66-88vx, UBUNTU-CVE-2026-71885

## What is CVE-2026-71885?

CVE-2026-71885 is a critical authentication bypass vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 3 Oct 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 1.86, the Messaging Layer Security (MLS, RFC 9420) implementation did not bind an X.509 credential to a LeafNode's signature_key. LeafNode.verify() checked a leaf's signature against the signature_key carried in the leaf itself, while the credential's X.509 certificate chain was stored but never parsed or validated, so the end-entity certificate's public key was never required to match signature_key as RFC 9420 sec. 5.3 requires. A party could therefore present another party's certificate as its credential while signing the leaf, and the enclosing KeyPackage, with an unrelated key, and be accepted under that other party's identity through KeyPackage.verify() and the Group leaf-validation path. In a deployment that admits external commits without an independent credential-admission check, an unauthenticated attacker could be admitted under a victim's X.509 identity, evict the victim (resynchronization compares whole credentials rather than signing keys), derive the current epoch, decrypt subsequent group messages, and send messages accepted as the victim. TreeKEM.LeafNode now requires the end-entity certificate's subject public key, in the cipher suite's signature encoding, to equal signature_key for an X.509 credential and rejects the leaf otherwise, including an empty chain or a certificate whose key type does not match the cipher suite; certificate-chain and identity validation to a trust anchor remain the application's responsibility per RFC 9420 sec. 5.3.1. Deployments using only basic credentials are unaffected.

Weakness: CWE-295 (Improper Certificate Validation), CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-71885?


## How do I fix CVE-2026-71885?

Upgrade to a fixed version.

## Is CVE-2026-71885 being exploited?

There is no known exploitation of CVE-2026-71885 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-71885 and fix it if it is affected.

What it is: CVE-2026-71885 is a critical authentication bypass vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 3 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-71885 in the wild and no public exploit code.

Affected product: Legion of the Bouncy Castle Inc. BC-JAVA. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-71885/ (Markdown: https://insights.sbomify.com/CVE-2026-71885/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-71885
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-71885
- OSV: https://osv.dev/vulnerability/CVE-2026-71885

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-71885/
Summaries are generated and may contain mistakes; the official description above is the record.