The flaw in Bouncy Castle for Java allows the MLS hash‑ratchet to accept an arbitrary 32‑bit generation counter supplied by the sender, an issue classified as CWE‑834. This undermines the integrity expectations of the MLS protocol, allowing a malicious actor to replay or fabricate messages that may be accepted as authentic, potentially exposing encrypted traffic or enabling further attacks.

Legion of the Bouncy Castle Inc.’s BC‑Java library is affected in all releases prior to version 1.85.

The CVSS score of 8.7 indicates high severity, and although the EPSS score is not available, the vulnerability is not listed in CISA’s KEV catalog. The likely attack vector is a network‑based attacker who can inject or modify MLS packets; by setting a deceptive generation counter the attacker can bypass counter checks and cause later messages to be incorrectly authenticated. Once exploited, the attacker could tamper with or intercept confidential data being protected by the affected library.