CVE-2026-71890: MLS external commit can remove an arbitrary group member
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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Bouncy Castle for Javato a version that resolves this vulnerability.Fixed in 1.86
Event History
Frequently Asked Questions
Which deployments are exposed?
Bouncy Castle for Java versions before 1.86 are affected when they process MLS external commits. Groups that distribute public GroupInfo for external joining are exposed because possession of that information is sufficient for an attacker to construct the malicious commit.
Does an attacker need to be an existing group member?
No. Any party holding the group's public GroupInfo could submit an external commit that names an arbitrary member's LeafIndex for removal.
What is the practical impact of successful exploitation?
The targeted group member can be evicted, and the attacker can take over that member's slot in the ratchet tree. Other group members apply the malicious removal.
What should teams do to remediate this issue?
Upgrade Bouncy Castle for Java to version 1.86 or later. Until upgraded, limit access to group public GroupInfo where possible to reduce who can create external commits.
How can I identify potentially affected groups?
Identify systems using Bouncy Castle for Java before 1.86 that accept MLS external commits or provide GroupInfo to external joiners. Those groups should be treated as potentially vulnerable if their GroupInfo may be available to untrusted parties.