Cloudanix VulnDB

CVE-2026-71890

None

CVSS

—

EPSS

0.2%

Published

Oct 3, 2026

CISA due date

—

Cloudanix analysis

Next patch cycle SSVC: Track

A cryptography library bug let outsiders remove people from secure group conversations and take their spot without permission.

Exploitation
Remote Unauthenticated
Attacker needs
None
Complexity
Low
Impact
Privilege Gain
Component
Library
Class
Authorization Bypass
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

Validation of MLS external commit proposal lists failed to enforce that a Remove proposal targets the joiner's own prior leaf and that the new LeafNode credential matches the removed participant, allowing an arbitrary member eviction and ratchet tree slot takeover via public GroupInfo.

Why it matters to the business

Attackers can silently evict legitimate participants from secure messaging or collaboration groups and assume their identity or position, disrupting operations and enabling unauthorized access in any system relying on the vulnerable library for MLS.

How an attack unfolds

An attacker obtains the group's public GroupInfo, crafts an external commit containing a Remove proposal naming any LeafIndex, and submits it through the externalJoin or handle APIs; the missing credential check allows the proposal to be accepted, evicting the target and installing the attacker in their tree position.

How to fix it

Upgrade Bouncy Castle for Java to version 1.86 or later; the updated validation now requires the removed leaf credential to match the joiner's new leaf on both send and receive paths.

How to confirm the fix worked

Confirm library version is 1.86 or newer; test that external commits with mismatched Remove proposals are rejected and that only matching-credential self-removes are accepted.

Typical attack path

  1. Acquire public GroupInfo for the target MLS group
  2. Construct external commit carrying a Remove proposal for any chosen LeafIndex
  3. Submit commit via Group.externalJoin or Group.handle without matching credential
  4. Evict target member and occupy their ratchet tree slot

How to detect it

Inspect MLS external commit logs for Remove proposals; flag any where the removed leaf credential does not match the joiner's supplied leaf credential.

What your security team should do

  • Upgrade all Bouncy Castle deployments to 1.86 immediately
  • Audit code paths using Group.externalJoin and Group.handle for MLS usage
  • Monitor MLS group membership events for unexpected evictions
  • Review any downstream dependencies on the affected library

The record

In Bouncy Castle for Java before 1.86, validation of an MLS (RFC 9420) external commit's proposal list, org.bouncycastle.mls.protocol.Group.validateExternalCachedProposals, counted the proposals by type and bounded the removed leaf index but never established that the removed leaf had anything to do with the joiner. RFC 9420 sec. 12.2 permits at most one Remove proposal in an external commit, with which the joiner removes an old version of themselves, and requires that where one is present the LeafNode in the commit's path field meet the criteria it would have to meet in an Update for the removed leaf, in particular that its credential present identifiers acceptable for the removed participant. The ordinary proposal-list validator's self-remove rule is deliberately not applied on this path, because a resync commit legitimately removes a leaf the joiner owns, but nothing was put in its place. Any party holding the group's public GroupInfo, which is precisely what an external joiner is meant to be given, could therefore commit a Remove naming any member's LeafIndex and have every member apply it, evicting that member and taking over their slot in the ratchet tree. The credential check that should have prevented this existed only in the gRPC interop harness and so protected no other caller of the public Group.externalJoin and Group.handle API. An external commit carrying a Remove is now accepted only when the removed leaf's credential is identical to the one in the joiner's own new leaf, on both the sending and the receiving side.

Affected packages

Ecosystem Package Introduced Fixed

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