Bouncy Castle is a widely used Java crypto library; MLS ratchet flaw can affect many apps and secure messaging, risking large-scale message compromise or service disruption.
CVE-2026-59644
Bouncy Castle MLS ratchet accepts arbitrary 32-bit generation counters.
Is CVE-2026-59644 being exploited?
Not confirmed. CVE-2026-59644 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.31% probability of exploitation in the next 30 days.
How severe is CVE-2026-59644?
CVE-2026-59644 is rated High with a CVSS score of 8.7. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-59644?
Yes. A fix has been recorded for CVE-2026-59644. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-59644 affect?
CVE-2026-59644 affects Bouncy Castle for Java < 1.85, Java applications using Bouncy Castle MLS implementation, Secure messaging services using affected builds. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-59644?
Upgrade Bouncy Castle to >= 1.85 and validate MLS inputs
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Attackers can set arbitrary generation counters to desynchronize MLS ratchet, enabling message replay/injection, loss of forward secrecy, or DoS.
Patch when possible
- affected>= 1.73 and < 1.85
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.