CVE-2026-18040 in Bouncy Castle Java
Summary
by MITRE • 10/03/2026
In Bouncy Castle for Java before 1.86, HQC leaked secret-derived data through two side channels: its GF(2^8) arithmetic used lookup tables indexed by field elements, making the cache line touched a function of the operand, and its fixed-weight support sampler left its duplicate scan as soon as a collision was found and stored accepted positions at a secret index. Both run on secret inputs during encapsulation and decapsulation, and the sampler re-expands the secret key from its seed on every decapsulation, so an attacker able to observe cache behaviour or decapsulation timing can recover information about the HQC private key. The field arithmetic is now table-free and the sampler branch-free within a batch of candidates, with output and randomness consumption unchanged.
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Analysis
by VulDB Data Team • 10/03/2026
The vulnerability identified in Bouncy Castle for Java prior to version 1.86 involves critical side-channel leaks within the implementation of the HQC (Hamming Quasi-Cyclic) post-quantum cryptographic algorithm. This flaw affects both encapsulation and decapsulation operations, which are fundamental processes in key exchange mechanisms relying on this standard. The core issue stems from two distinct technical flaws: one related to finite field arithmetic and another concerning the sampling mechanism used for generating error vectors. These implementation details inadvertently expose secret-derived data through observable system behaviors such as cache access patterns and execution timing, allowing an attacker with sufficient proximity or monitoring capabilities to infer sensitive information about the private key.
The first component of this vulnerability lies in the GF(2^8) arithmetic operations utilized by the HQC implementation. The original code employed lookup tables indexed directly by field elements during computation. In modern processor architectures, accessing memory via these tables results in specific cache lines being loaded into the CPU cache based on the index value. Since the indices are derived from secret operands involved in cryptographic computations, an attacker capable of performing cache-timing attacks can correlate the accessed cache lines with the underlying data values. This constitutes a classic implementation flaw where constant-time execution principles were not strictly adhered to for all arithmetic operations involving secrets. By observing which memory addresses cause cache hits or misses over multiple executions, an adversary can reconstruct portions of the secret field elements used during encryption and decryption processes.
The second vulnerability resides in the fixed-weight support sampler, a critical component responsible for selecting positions within vectors based on specific weight constraints while maintaining security against statistical analysis. The previous implementation utilized a duplicate scan approach that terminated early upon finding a collision. This branching behavior introduced data-dependent execution paths because the time taken to complete the sampling process varied depending on when a valid candidate was found relative to secret inputs. Furthermore, the algorithm stored accepted positions at indices determined by secret values during decapsulation. When combined with the fact that the sampler re-expands the secret key from its seed on every single decapsulation operation, this creates a powerful attack surface. An attacker observing timing variations or memory access patterns across multiple decapsulation attempts can leverage these non-constant-time behaviors to recover information about the HQC private key, potentially leading to full key recovery in worst-case scenarios.
From an industry standards perspective, this vulnerability aligns with CWE-208 Observable Timing Discrepancy and CWE-367 Time-of-check to Time-of-use (TOCTOU) race conditions related to side channels. It also maps to the MITRE ATT&CK technique T1594.002 Side Channel Attacks, specifically cache timing attacks. The failure to implement constant-time algorithms for sensitive cryptographic operations is a common pitfall in post-quantum cryptography implementations, where mathematical complexity often obscures subtle implementation errors that become exploitable under side-channel analysis frameworks like those described by Kocher or Bernstein and Yang.
To mitigate these risks, Bouncy Castle released version 1.86 which addresses both flaws comprehensively. The GF(2^8) arithmetic was refactored to be table-free, eliminating the data-dependent cache access patterns associated with lookup tables. Instead, bitwise operations and conditional moves are employed to ensure that execution time remains independent of secret input values. Additionally, the fixed-weight support sampler was redesigned to operate in a branch-free manner within batches of candidates. This ensures that the control flow does not vary based on intermediate results or secret data during the sampling process. The output format and randomness consumption remain unchanged, ensuring backward compatibility for applications while significantly hardening the implementation against side-channel attacks. Developers using Bouncy Castle must upgrade to version 1.86 or later to ensure their post-quantum cryptographic implementations are resistant to these sophisticated timing-based exploitation techniques.