CVE-2026-71890
HighCVSS 8.7Exploitation Probability (EPSS)
Low risk15th percentile - higher than 15% of all known CVEs
Summary
In Bouncy Castle for Java before 1.86, validation of an MLS (RFC 9420) external commit's proposal list failed to verify that the removed leaf actually belonged to the joiner. This missing check allowed any party holding the group's public GroupInfo to send a commit with a Remove proposal naming any member, causing that member to be evicted and their slot in the ratchet tree to be taken over. The fix requires the removed leaf's credential to match the joiner's own new leaf credential on both the sending and receiving side.
Risk Assessment
An attacker who knows the group's public GroupInfo can remove any member and take over their position in the ratchet tree, compromising the integrity and confidentiality of group communication. This affects all applications using the public Group.externalJoin and Group.handle APIs.
Recommendation
Upgrade Bouncy Castle for Java to version 1.86 or later. If upgrading is not possible, restrict access to MLS group public GroupInfo to trusted recipients only.
Other vulnerabilities in Bouncy Castle for Java
See all- CVE-2026-97873Medium
In Bouncy Castle for Java before 1.86 (and LTS before 2.73.13), the legacy PBES1 and PKCS#12 PBE families used an unbounded iteration count from untrusted input, allowing a small input to dictate arbitrary work. The fix rejects negative or over-limit counts (default 10,000,000) via the org.bouncycastle.pbe.max_iteration_count property.
- CVE-2026-71888High
In Bouncy Castle for Java before 1.86, the streaming CMS AuthenticatedData parser accepted messages where digestAlgorithm and authAttrs disagreed about whether authenticated attributes were present. The parser chose based on digestAlgorithm alone, so it could verify the content MAC and then return attributes via getAuthAttrs() as though they had been authenticated, even though the MAC never covered them. This also affects LTS before 2.73.13 and FIPS releases (BC-FJA) before the corresponding bcpkix-fips and bcutil-fips versions.
- CVE-2026-71887High
In Bouncy Castle for Java before 1.86, the high-level OpenPGP API accepted a data signature made by a signing subkey whose Subkey Binding signature carried no embedded Primary Key Binding (cross-certification) signature when that binding omitted a Key Flags subpacket. isSigningKey() inherited SIGN_DATA from the primary key via getKeyFlags() and getApplyingSubpacket(), while verifyEmbeddedPrimaryKeyBinding() read only the binding signature's own hashed subpackets and treated the subkey as non-signing, so the same subkey was both signing-capable and exempt from cross-certification.
- CVE-2026-18040Medium
In Bouncy Castle for Java before 1.86, the HQC implementation leaked secret-derived data through two side channels: GF(2^8) arithmetic used lookup tables indexed by field elements, and the fixed-weight support sampler stopped scanning on collision and stored positions at a secret index. An attacker observing cache behavior or decapsulation timing can recover information about the HQC private key.
- CVE-2026-18036High
A side-channel vulnerability exists in the NTRU implementation of Bouncy Castle for Java before version 1.86. The reduction of secret values using the % operator in three helper functions causes execution time to depend on the secret operand (division by a variable), potentially allowing an attacker to recover information about the NTRU private key through timing measurements.
- CVE-2026-17507High
In the MLS implementation (RFC 9420) of Bouncy Castle for Java before 1.86, the uint32 leaf_index is stored as a signed int, allowing a wire value with the top bit set to decode as a negative number. Incorrect comparison in GroupKeySet.SecretTree.hasLeaf and Group.validateRemove allows bypassing membership checks, potentially leading to a DoS attack via unbounded growth of the node list and JVM heap exhaustion.
- CVE-2023-33201Medium
Bouncy Castle for Java before version 1.74 is affected by an LDAP injection vulnerability. This vulnerability affects applications that use an LDAP CertStore from Bouncy Castle to validate X.509 certificates, where the certificate's Subject Name is inserted into an LDAP search filter without proper escaping.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS