Vulnerabilities

Bouncy Castle for Java versions 1.73 through before 1.78 has an observable timing discrepancy (CWE-208) — the KyberSlash1 and KyberSlash2 flaws — in its ML-KEM (CRYSTALS-Kyber) routines Poly.toMsg, Poly.compressPoly, and PolyVec.compressPolyVec, which divide secret-derived polynomial coefficients by the modulus q. An attacker who can initiate or observe a large number of ML-KEM decapsulation operations performed with the same long-term private key and measure their timing can use the secret-dependent division timing to recover that key. Key recovery defeats the confidentiality of the post-quantum key exchange (CVSS 4.0 rates this 8.2 High with high attack complexity and High confidentiality impact); encapsulation-side compression is unaffected because it only operates on values that become the public ciphertext. Any application or service using affected bc-java releases for ML-KEM/Kyber decapsulation is affected, particularly deployments that reuse static ML-KEM keys across many exchanges. No in-the-wild exploitation is known (EPSS 0.3%, not in CISA KEV), but public proof-of-concept timing attacks are published at kyberslash.cr.yp.to.

Do: Upgrade Bouncy Castle for Java to 1.78 or later, which removes the secret-dependent divisions in the ML-KEM routines. Inventory applications bundling bc-java jars in the 1.73–1.77 range and prioritize those performing ML-KEM/Kyber decapsulation with long-term (reused) keys, rotating such keys after patching. Where an immediate upgrade is not possible, treat high-volume, network-observable ML-KEM decapsulation paths as the highest risk.