CVE-2026-97873

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.

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.

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.

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.

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.

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.

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.