CVE-2026-71890
Received Received - Intake

Improper External Commit Validation in Bouncy Castle for Java

Publication date: 2026-10-03

Last updated on: 2026-10-03

Assigner: bcorg

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.

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Meta Information

Published
2026-10-03
Last Modified
2026-10-03
Generated
2026-10-03
AI Q&A
2026-10-03
EPSS Evaluated
N/A
NVD
CVE-2026-71890
EUVD
EUVD-2026-92006

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
bouncy_castle bc_java to 1.86 (exc)
org.bouncycastle bouncy_castle 1.86

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-863 The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check.

Attack-Flow Graph

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Executive Summary

CVE-2026-71890 is a flaw in Bouncy Castle for Java before version 1.86 affecting the Messaging Layer Security (MLS) protocol. It allows an attacker with access to a group's public GroupInfo to remove any group member during an external commit by including a Remove proposal targeting that member's leaf index. The vulnerability exists because the validation process did not verify that the removed leaf belonged to the joiner, as required by RFC 9420. The fix ensures that an external commit with a Remove proposal is only accepted if the removed leaf's credential matches the joiner's new leaf credential.

Detection Guidance

To detect this vulnerability, check if your Bouncy Castle Java library version is below 1.86. Use commands like 'mvn dependency:tree' or 'gradle dependencies' to inspect the library version in your project. If using a JAR file, inspect the manifest or run 'java -jar your-application.jar' to check the version.

Impact Analysis

This vulnerability could allow an attacker to evict legitimate group members and take over their positions in the ratchet tree. If you use Bouncy Castle's MLS implementation in Java before version 1.86, an attacker with access to your group's public GroupInfo could remove you or other members from the group without detection. This could lead to unauthorized access, data breaches, or disruption of secure communications within the group.

Compliance Impact

This vulnerability could potentially impact compliance with GDPR and HIPAA by enabling unauthorized removal of group members in secure communication systems. GDPR requires strict access controls and data protection, while HIPAA mandates secure handling of sensitive health information. The flaw allows attackers to evict legitimate users, disrupting authorized access and potentially leading to unauthorized data exposure or manipulation.

Mitigation Strategies

Upgrade Bouncy Castle for Java to version 1.86 or later to address the MLS external commit validation flaw that allows unauthorized member removal.

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