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TI-2026-67477 Aliases CVE-2026-71890GHSA-jc55-7frr-fv5r

MLS external commit can remove an arbitrary group member

Priority MEDIUM 43CWE-863Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Attack requirements
None
Impact type
Security bypass
Confidentiality · integrity · availability
None · High · None
Fix status
Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA. Impact type from CWE-863.

Summary

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00249 16th percentileScored 2026-11-07
Fix Fixed BC-JAVA (Legion of the Bouncy Castle Inc.): 1.86

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
Yes
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
Legion of the Bouncy Castle Inc.BC-JAVA1.73 to before 1.861.86Patch availableCNA

CPE names Threatist (inferred) · 1

Inferred from how NVD named this product before; not given by a source.

CPEVersionsRoleVendor
cpe:2.3:a:bouncycastle:bc-java:*:*:*:*:*:*:*:*——Legion of the Bouncy Castle Inc.

Remediation

  • Fix · CNA BC-JAVA (Legion of the Bouncy Castle Inc.) Fixed in 1.86

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

  • Release ·

    Legion of the Bouncy Castle BC-JAVA 1.86

    11 records Highest priority Priority MEDIUM 4 MEDIUM, 7 LOW

    Other records, highest priority first

    • TI-2026-67476 PKIXCertPathReviewer does not apply X.509 name constraints to the target certificatePriority MEDIUM 43
    • TI-2026-67475 CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absentPriority MEDIUM 42
    • TI-2026-67472 MLS X.509 credential not bound to the LeafNode signature keyPriority MEDIUM 41
    Show all 11 records →

Timeline

  1. 2026-10-03 Threatist record created from CVE-2026-71890
  2. 2026-10-03 Alias added: GHSA-jc55-7frr-fv5r
  3. 2026-10-04 Priority changed: MEDIUM → LOW
  4. 2026-10-05 CISA SSVC decision points changed
  5. 2026-10-05 Priority changed: LOW → MEDIUM

References

Patch 1

Advisory 1

  • TI-2018-15967 Legion of the Bouncy Castle Legion of the Bouncy Castle Java Cryptography APIs 1.58 up to… CVSS 3.1CRITICAL 9.8
  • TI-2017-09736 BouncyCastle JCE TLS Bleichenbacher/ROBOT CVSS 3.0HIGH 7.5
  • TI-2026-42545 MTI/A0 DH agreement exponentiates unvalidated peer value CVSS 4.0CRITICAL 9.3
  • TI-2026-42533 JSSE hostname verifier CN-fallback enabled by default despite documented opt-in CVSS 4.0CRITICAL 9.3
  • TI-2026-41816 Stapled OCSP response accepted without binding to the checked certificate CVSS 4.0CRITICAL 9.3
  • TI-2026-07701 Name Constraints bypass via trailing dot in rfc822Name and URI CVSS 4.0CRITICAL 9.3
See all 109 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences