CVE-2016-9243 in cryptography
Summary
by MITRE
HKDF in cryptography before 1.5.2 returns an empty byte-string if used with a length less than algorithm.digest_size.
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Analysis
by VulDB Data Team • 09/17/2024
The vulnerability identified as CVE-2016-9243 resides within the HKDF (HMAC-based Extract-and-Expand Key Derivation Function) implementation in cryptographic libraries prior to version 1.5.2. This flaw represents a critical weakness in key derivation processes that can significantly compromise the security of cryptographic operations relying on proper key material generation. The issue manifests when HKDF is invoked with a requested output length that is smaller than the underlying hash algorithm's digest size, causing the function to return an empty byte-string instead of the expected key material. This behavior violates fundamental cryptographic principles where key derivation functions should consistently produce output of the requested length regardless of input parameters.
The technical nature of this vulnerability stems from improper boundary condition handling within the HKDF implementation. When a cryptographic library processes key derivation requests, it must ensure that the output length matches the requested parameters regardless of the relationship between the requested length and the underlying algorithm's digest size. The flaw occurs because the implementation fails to properly validate or handle cases where the requested key length is less than the digest size, leading to the return of empty output rather than the appropriate truncated key material. This behavior creates a potential attack surface where adversaries could exploit the predictable empty return to bypass key derivation processes or manipulate cryptographic protocols that depend on HKDF for secure key generation.
From an operational perspective, this vulnerability can have severe consequences across multiple security domains including secure communications, authentication systems, and data encryption protocols. When HKDF returns empty byte-strings instead of properly derived keys, it effectively neutralizes the cryptographic protection mechanisms that depend on these functions. Applications using vulnerable cryptographic libraries may experience authentication failures, encryption breakdowns, or complete bypass of security controls that rely on proper key derivation. The vulnerability is particularly dangerous in protocols where HKDF is used to derive session keys, encryption keys, or authentication tokens, as the empty output could lead to predictable key material or complete failure of the cryptographic operation, potentially allowing attackers to establish unauthorized access or decrypt sensitive information.
The impact of CVE-2016-9243 aligns with CWE-209, which addresses information exposure through improper error handling, and can be categorized under ATT&CK technique T1552.001 for unsecured cryptographic storage. This vulnerability demonstrates how seemingly minor implementation flaws in cryptographic primitives can lead to fundamental security breaches throughout the entire system architecture. Organizations implementing cryptographic solutions must ensure that all key derivation functions properly handle edge cases and maintain consistent output behavior regardless of input parameters. The vulnerability also relates to CWE-330, which addresses insufficient randomness in cryptographic operations, as the empty output could lead to predictable cryptographic states that reduce overall entropy in security protocols.
Mitigation strategies for CVE-2016-9243 primarily involve upgrading to cryptographic libraries version 1.5.2 or later where the implementation has been corrected to properly handle cases where requested key lengths are smaller than the digest size. System administrators should conduct comprehensive vulnerability assessments to identify all applications and services using affected cryptographic libraries, particularly those implementing HKDF for key derivation. Additionally, organizations should implement monitoring solutions to detect anomalous cryptographic behavior patterns that might indicate exploitation attempts. The fix typically involves ensuring that HKDF implementations return properly truncated key material rather than empty strings when the requested output length is less than the algorithm's digest size. Security teams should also consider implementing additional cryptographic validation checks to verify that key derivation functions produce output of expected lengths and that no empty or null values are returned during critical cryptographic operations.