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

Quick assessment

Affected
Legion of the Bouncy Castle Inc. BC-JAVA
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在 Bouncy Castle for Java 1.86 版本之前，对 MLS（RFC 9420）外部加入（external commit）中提案列表的验证存在缺陷。具体而言， 方法仅按类型统计提案数量并对已移除叶节点索引进行边界检查，但从未验证该被移除的叶节点是否与加入者（joiner）相关。 根据 RFC 9420 第 12.2 节的规定，外部加入（external commit）中最多允许包含一个“移除”（Remove）提案，该提案用于加入者移除其自身的一个旧版本身份；并且当存在此类移除提案时，commit

CVSS 8.7 · High EPSS 0.25% · P15

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 1

VendorProduct Version RangeStatus
Legion of the Bouncy Castle Inc. BC-JAVA 1.73< 1.86 affected
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I. Basic Information for CVE-2026-71890

Vulnerability Information
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Vulnerability Title
MLS external commit can remove an arbitrary group member
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
授权机制不正确
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Legion of the Bouncy Castle Inc. BC-JAVA 1.73 ~ 1.86 -

II. Public POCs for CVE-2026-71890

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III. Intelligence Information for CVE-2026-71890

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Patches & Fixes for CVE-2026-71890 (1)

Other References for CVE-2026-71890 (1)

Same Patch Batch · Legion of the Bouncy Castle Inc. · 2026-10-03 · 12 CVEs total

CVE-2026-71885 9.2 CRITICAL MLS X.509 credential not bound to the LeafNode signature key
CVE-2026-71888 8.7 HIGH CMS AuthenticatedData exposes attacker-inserted authAttrs when digestAlgorithm is absent
CVE-2026-71889 8.7 HIGH PKIXCertPathReviewer does not apply X.509 name constraints to the target certificate
CVE-2026-85515 8.2 HIGH OpenPGP message truncation not reported, bypassing the SEIPDv1 integrity check
CVE-2026-71887 8.2 HIGH OpenPGP data signature accepted from a signing subkey without cross-certification
CVE-2026-71883 8.2 HIGH Native AES packet cipher returns the raw AES key on an alias
CVE-2026-71886 8.2 HIGH OpenPGP certification accepted from a subkey without certification authority
CVE-2026-71891 7.1 HIGH BLS12-381 key validation accepts a public key built on a foreign curve
CVE-2026-71892 6.9 MEDIUM CMS key-transport recipient key-size validation never runs for RFC 9709 HKDF-derived keys
CVE-2026-18040 5.9 MEDIUM HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-depen
CVE-2026-97873 5.3 MEDIUM Legacy PBES1 and PKCS#12 PBE iteration count honoured unbounded in the raw JCA provider

IV. Related Vulnerabilities

Same product: BC-JAVA
Same vendor: Legion of the Bouncy Castle Inc.
Same weakness: CWE-863

V. Comments for CVE-2026-71890

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