CVE-2026-12852

This is a memory allocation vulnerability in Bouncy Castle's Java implementation of the MLS (Messaging Layer Security) wire decoder. The decoder accepts an attacker-controlled length value from the wire format and immediately allocates a buffer before validating whether that length is reasonable. This is a pre-validation allocation flaw — different from a buffer overflow, but capable of enabling resource exhaustion or denial-of-service if an attacker declares an excessively large length value. The vulnerability is patched in version 1.85. The CVSS score of 8.7 reflects the theoretical severity of uncontrolled allocation. The EPSS score of 0.00263 suggests low opportunistic exploitation likelihood. For this CVE, the EPSS is likely accurate — MLS is not ubiquitous like TLS, and this bug class doesn't readily enable code execution, making it less attractive for automated exploitation. However, the EPSS metric measures external scanning probability, not the actual concentration of risk in deployed systems. The more important question is the blast radius. MLS (RFC 9420) is deployed in encrypted messaging systems where the protocol manages group key state across participants. If this allocation vulnerability triggers a DoS, the failure isn't just a connection dropping — it's potentially making an entire conversation tree inaccessible until key state is restored. This is a protocol-level integrity concern, not merely a parsing crash. Patch propagation is the real exposure vector. Bouncy Castle is a reference library that gets embedded and shaded into downstream Java artifacts — messaging platforms, enterprise applications, SDKs. A version scan of your direct dependencies won't reveal whether a shaded copy of Bouncy Castle 1.84 is bundled inside a third-party JAR you're depending on. You should verify whether any MLS-enabled components bundle Bouncy Castle and confirm they're on 1.85 or later. If you're using a messaging platform or secure communication product that integrates MLS, check with your vendor about their Bouncy Castle version — the patch exists, but propagation into embedded dependencies lags significantly behind the upstream release.

Reviewed through automated stages and approved by a human before publication.