CVE-2016-9243: HKDF in cryptography before 1.5.2 returns an empty byte-string if used with a length less than algorithm.di...

Security readout for executives and security teams

This flaw affects the Python cryptography library's HKDF key-derivation behavior before version 1.5.2. Under a specific short-output request, HKDF could return an empty byte string instead of derived key material. Business risk depends on whether applications used this exact HKDF pattern in security-sensitive flows. Exposure is most likely in Python applications or packaged systems using pyca cryptography versions before 1.5.2 and calling HKDF with an output length below the digest size. Systems not using HKDF, or using later versions, are unlikely to be affected. Treat this as a targeted dependency remediation item, not an emergency, unless critical systems rely on affected HKDF behavior. Prioritize inventory, upgrade planning, and validation in applications handling authentication, encryption, or key exchange. Mitigation focus: Identify installed pyca cryptography versions across applications and base images.; Upgrade cryptography to version 1.5.2 or later where affected versions are present.; Review Fedora and Ubuntu advisories for distribution-specific package updates..

Prepared

Jun 13, 2026, 05:45 UTC (UTC+00:00)

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.