Bouncycastle: In Bouncy Castle for Java before 1.85, MLS hash-ratchet honours arbitrary 32-bit generation counter from sender. (CVE-2026-59644)

The vulnerability identified as CVE-2026-59644 affects Bouncy Castle for Java versions prior to 1.85. The issue lies in the MLS (Message Layer Security) hash-ratchet mechanism, which improperly accepts an arbitrary 32-bit generation counter value from the sender. This behavior can be exploited to cause denial of service impacts. The CVSS 3.1 vector indicates the attack can be performed remotely without privileges or user interaction, with low attack complexity, and results in availability impact only.

An attacker can remotely trigger a denial of service condition by exploiting the arbitrary 32-bit generation counter acceptance in the MLS hash-ratchet. There is no impact on confidentiality or integrity according to the CVSS vector. No known exploits in the wild have been reported.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Since the vulnerability affects versions before 1.85, upgrading to version 1.85 or later is likely the intended remediation once available. Until then, monitor vendor communications for official fixes.