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
CVE-2026-71890
Primary source · Revision 1
Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.
Security context
Source reference: cve:CVE-2026-71890
BC-JAVA
Source reference: /containers/cna/affected/0/product
- Affected versions
- Not stated in this record
- Fixed versions
- Not stated in this record
- Prerequisites
- Not stated in this record
Structured affected-version data
Original package identities and version rules from this source.
[
{
"repo": "https://github.com/bcgit/bc-java",
"vendor": "Legion of the Bouncy Castle Inc.",
"modules": [
"mls"
],
"product": "BC-JAVA",
"versions": [
{
"status": "affected",
"version": "1.73",
"lessThan": "1.86",
"versionType": "maven"
}
],
"platforms": [
"all"
],
"packageURL": "pkg:maven/org.bouncycastle/bcmls-jdk18on",
"packageName": "bcmls",
"programFiles": [
"Group"
],
"collectionURL": "https://www.bouncycastle.org/download/bouncy-castle-java/",
"defaultStatus": "unaffected"
}
]- Related CVEs
- CVE-2026-71890
Source reference:
/cveMetadata/cveId
Publisher severity
high
Source reference: /containers/cna/metrics
- Event
- #8774 · Revision 1
- Added to Heist Feed
- Publisher date