Description
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.
Published: 2026-10-03
Score: 8.7 High
EPSS: n/a
KEV: No
Impact: Removal of an arbitrary group member can lead to impersonation and loss of group security
Action: Immediate Patch
AI Analysis

Impact

The vulnerability resides in Bouncy Castle for Java prior to version 1.86, where validation of an MLS external commit's proposal list fails to check that the removed leaf matches the joiner's own new leaf. This allows any party holding the group's public GroupInfo, the data an external joiner receives, to craft a commit that removes any member's LeafIndex. The affected members will apply this commit, evicting the targeted member and allowing the attacker to take over the slot. As a result, the attacker can gain unauthorized participation in the group, undermining confidentiality and integrity.

Affected Systems

Legion of the Bouncy Castle Inc.'s Bouncy Castle for Java (BC-JAVA) library, any installation using a version earlier than 1.86.

Risk and Exploitability

The CVSS base score of 8.7 classifies this flaw as high severity. No EPSS score is publicly available, but the flaw can be exploited by any entity that can supply a crafted external commit, which is essentially an authenticated or unauthenticated network call to Group.externalJoin or Group.handle. The lack of credential validation in these APIs eliminates a critical guard, meaning the attack can be executed without privileged credentials. Because the vulnerability is not listed in KEV, it is not known to be actively exploited in the wild yet, but it presents a significant risk for any deployment relying on the exposed APIs.

Generated by OpenCVE AI on October 3, 2026 at 10:20 UTC.

Remediation

No solution or workaround provided in the CVE record.

OpenCVE Recommended Actions

  • Upgrade to Bouncy Castle for Java version 1.86 or later.
  • Verify that any uses of Group.externalJoin and Group.handle are restricted to trusted, authenticated parties only.
  • If upgrading is delayed, revoke or rotate the group's public GroupInfo to prevent external joiners from submitting malicious commits until the patch is applied.

Generated by OpenCVE AI on October 3, 2026 at 10:20 UTC.

Tracking

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Advisories

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History

Sat, 03 Oct 2026 09:15:00 +0000

Type Values Removed Values Added
Description 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.
Title MLS external commit can remove an arbitrary group member
Weaknesses CWE-863
References
Metrics cvssV4_0

{'score': 8.7, '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'}

Subscriptions

No data.

cve-icon MITRE

Status: PUBLISHED

Assigner: bcorg

Published:

Updated: 2026-10-03T08:57:00.965Z

Reserved: 2026-08-08T00:06:07.401Z

Link: CVE-2026-71890

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-10-03T09:17:05.607

Modified: 2026-10-03T09:17:05.607

Link: CVE-2026-71890

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-10-03T10:30:19Z

Weaknesses