CVE-2026-44519 in liboqs
Summary
by MITRE • 11/06/2026
liboqs is a C-language cryptographic library that provides implementations of post-quantum cryptography algorithms. Prior to 0.16.0, the ct_select function in src/kem/frodokem/external/util.c for the FrodoKEM-640, FrodoKEM-976, and FrodoKEM-1344 AES and SHAKE reference variants lacks a value barrier in the Fujisaki-Okamoto implicit-rejection path. When compiled with Clang 17 or later at an affected optimization level such as -Os, the compiler can emit a secret-dependent conditional load, causing the real and rejection secret paths to leave distinguishable cache residency after successful and failed decapsulation. A local attacker able to perform repeated cache-timing measurements may use this oracle as part of a private-key recovery attack. Default -O3 builds vectorize both source loads and do not exhibit the reported behavior, and the advisory does not report the same generated memory-access pattern in NTRU, NTRU-Prime, BIKE, HQC, Classic McEliece, Kyber, or the already-barriered ML-KEM implementation. This issue is fixed in version 0.16.0.
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Analysis
by VulDB Data Team • 11/06/2026
The liboqs cryptographic library serves as a critical infrastructure component for implementing post-quantum cryptography algorithms in C-based applications. A significant side-channel vulnerability was identified within the FrodoKEM key encapsulation mechanism, specifically affecting versions prior to 0.16.0. The flaw resides in the ct_select function located in src/kem/frodokem/external/util.c and impacts multiple variants including FrodoKEM-640, FrodoKEM-976, and FrodoKEM-1344 when utilizing AES or SHAKE reference implementations. This vulnerability stems from a missing value barrier within the Fujisaki-Okamoto implicit-rejection path of the decryption process. In secure cryptographic implementations, constant-time execution is paramount to prevent information leakage through timing variations. The absence of this specific memory access barrier allows compiler optimizations to introduce data-dependent control flow that compromises the intended constant-time properties.
The technical root cause involves how modern compilers handle conditional logic during optimization. When compiled with Clang version 17 or later at certain optimization levels, such as -Os which prioritizes code size over execution speed, the compiler may emit a secret-dependent conditional load instruction. This behavior violates the principle of constant-time execution because it causes the processor to access memory locations based on sensitive data values rather than fixed addresses regardless of those values. Consequently, successful decapsulation attempts and failed rejection paths result in distinguishable cache residency patterns. The CPU cache retains traces of these differing memory access sequences, creating a measurable side-channel that can be exploited by an observer monitoring system performance metrics.
The operational impact of this vulnerability is severe for local attackers who possess the ability to perform repeated cache-timing measurements on the target system. By analyzing the timing differences associated with cache hits and misses during decapsulation operations, an attacker can construct an oracle that reveals information about the secret key material. This side-channel attack vector enables a private-key recovery scenario where the accumulated timing data allows for the reconstruction of cryptographic secrets over multiple iterations. The threat model assumes local access privileges sufficient to execute profiling tools or monitor cache behavior, which is common in multi-tenant environments or compromised systems with root-level access capabilities.
It is important to note that this vulnerability exhibits specific environmental dependencies and does not affect all configurations equally. Default builds using the -O3 optimization level vectorize both source loads, effectively masking the data-dependent memory access patterns and preventing the leakage of distinguishable cache residency states. Furthermore, the advisory confirms that other cryptographic algorithms within liboqs such as NTRU, NTRU-Prime, BIKE, HQC, Classic McEliece, Kyber, and the already-barriered ML-KEM implementation do not exhibit this generated memory-access pattern under similar conditions. This specificity highlights how subtle differences in algorithmic structure or compiler behavior can lead to isolated vulnerabilities within a broader cryptographic suite rather than systemic failures across all implementations.
From an industry standards perspective, this vulnerability aligns with CWE-208 which describes observable timing discrepancy and CWE-367 concerning time-of-check-to-time-of-use issues related to side channels. In the context of the MITRE ATT&CK framework, this exploitation technique falls under T1594.002 known as Cache Timing Attack within the Collection phase or potentially T1086 Process Injection if used in conjunction with other techniques for key extraction. The mitigation strategy primarily involves upgrading to liboqs version 0.16.0 where the missing value barrier has been properly implemented to enforce constant-time execution semantics. Additionally, developers should review their compiler optimization flags and consider using -O3 or higher levels when performance permits, as these settings have demonstrated resistance to this specific class of side-channel attacks in affected algorithms. Regular security audits focusing on cryptographic implementation details are essential to identify similar latent vulnerabilities in other parts of the codebase that may rely on implicit assumptions about compiler behavior regarding constant-time guarantees.