High 8.7Published Oct 3, 2026

CVE-2026-71890MLS external commit can remove an arbitrary group member

Updated under an hour ago (2026-10-19 08:50 UTC)

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It affects Legion of the Bouncy Castle Inc. BC-JAVA. Add what you run to find out whether that includes you.

EPSS puts the chance of exploitation in the next 30 days at 0.25%, higher than 15% of scored CVEs.

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Every CVE for Legion of the Bouncy Castle Inc. BC-JAVA

Exploited in the last 30 days

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.

CVSS: 8.7 HIGH (v4.0, CNA)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
Weakness (CWE): CWE-863
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): 0.25% (percentile 15)
Assigned by: bcorg
NVD analysis: Awaiting Analysis
Record last updated: Oct 5, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Legion of the Bouncy Castle Inc. BC-JAVA legion_of_the_bouncy_castle:bc_java (from the name)	from 1.73 before 1.86	CNA record

References

https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071890vendor-advisory
https://github.com/bcgit/bc-java/commit/7e8bb10eb90baddf3f10b8679f627462b9522d24patch

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.